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Cerebrospinal fluid alkalosis during high-altitude sojourn in unanesthetized ponies
Respirution
(1975) 25, 23337; North-Holluntl
Physiology
CEREBROSPINAL
FLUID ALKALOSIS
SOJOURN
Publishing
DURING
IN UNANESTHETIZED
Madison,
of
Veterinary
Science
Wisconsin, 53706
of
Milwukee.
Abstract.
Unanesthetized
to 3400 m (N=6) P,,,
hypoxia.
arterial
fluid (CSF)
Cerebrospinal
blood
Enoironmental
HIGH-ALTITUDE
withdrawn
were studied
altitude.
conditions
from
the cisterna
decreased
slightly
after
further
after I-5 days at high altitude
1 hr of hypoxia
4300 m) and then increased
(APa,,,
significantly
of’
Wiscowin,
qf
Wisconsin.
near sea level (250 m) and during
under
decreased
College
The pH, Pco,, and PO2 of arterial
were measured an indwelling
Uniuersit)
Medicd
U.S.A.
fluid was sampled from
D. D. BUSS,
Medicine,
Medicine.
Wiscor~.sir~53226,
adult female ponies
and 4300 m (N=7)
of cerebrospinal
days)
of Prerentil:e
and Depurtment
und Department
Anmterdnnl
PONIES
J. A. ORR, G. E. BISGARD, H. V. FORSTER, J. A. DEMPSEY and J. A. WILL Department
Company,
sojourns
and pH and
(1 hr) and chronic
of acute magna
In both
blood
of the awake
arterial
catheter.
groups
= -0.6
mm Hg at 3400 m; -3.9
(APa,,,
= -7.2
pony
of animals,
(l-45 and Pa,,>
mm Hg at 4300 m),
mm Hg at 3400 m; - 12.3 mm Hg at
after 6 days of chronic
hypoxia
(APa,oZ=
+4.1
mm Hg at
3400 m; +4.7 mm Hg at 4300 m). Although Pa,, decreased markedly during acute hypoxia, subsequent changes m Pa,,, at high altitude did not alter Pa,, from that observed during acute hypoxia (Pa,,=52
mm
hypoxia
(ApH=
Hg at 3400 m; 41 mm +0.013
l-2 days at high altitude remained
alkaline
Hg at 4300
unit at 3400 m; (ApH=
to control
f0.031
unit at 3400 m: +0.064
values throughout
hypoxia, CSF pH was imperfectly regulated than (4300 m) arterial blood. Furthermore, arterial
[HCO,]
local ‘CSF-specific’
during
chronic
hypoxia
mechanisms Acid-base Control
sojourn.
status fluid
Under
and regulated the similarity
may indicate
as previously
Cerebrospinal
m). The
pH
of CSF
+Q.O33 unit at 4300 m) and
increased
became
more
during
acute
alkaline
,after
unit at 4300 m). At 4300 m, CSF pH these conditions
of chronic
hypocapnic
in a magnitude equal to (3400 m) or less of relative changes in CSF [HCO;] and
a passive
regulation
of CSF [HCO;]
rather
than
proposed. High altitude
acclimatization
Hypoxia
of breathing
Conflicting views exist concerning the regulation of cerebrospinal fluid (CSF) pH in man during chronic hypocapnic hypoxia. In 1963, Severinghaus et al. presented data from 4 subjects sojourning at 3880 m that indicated near complete compensation of lumbar spinal fluid (LSF) after 2-8 days at this altitude. The return of CSF pH to near normoxic levels in spite of a sustained arterial alkalosis was postulated to be due to mechanisms specific for local regulation of brain extracellular fluid (ECF) [HCO;] and pH. Possible mechanisms included active transport of H+ from capillary blood to brain ECF (Severinghaus et al., 1963) or increased Accepted
for
publication
2 June 1975. 23
24
J. A. ORR
production
et (I/.
of lactic acid by brain tissue (Sorensen,
of these data, Severinghaus
et al. (1963) proposed
H+ played a key role in the stimulation chronic hypoxia.
1971). Furthermore, that the relative
of ventilation
during
on the basis
increase
in CSF
acclimatization
to
More recently, Dempsey et al. (I 974) and Forster et al. (1975) studied 7 subjects during IO-day to 34week sojourns at 3100 and 4300 m altitude. During the sojourn LSF pH remained significantly alkaline to normoxic measurements and was compensated to a similar extent as was arterial blood pH. Consequently no local mechanism for CSF pH regulation need be postulated as changes in CSF [HCO;] seemed to be passively following decreases in plasma [HCO; 1. Furthermore the data were inconsistent with the concept of a positive role for CSF H+ as a mediator of ventilatory acclimatization. The primary problem of interest pertains to the regulation of [HCO;] and [H ‘1 in brain ECF, and in this regard studies in humans are limited by the required use of the lumbar sampling site. It has been shown that lumbar and cisternal CSF aciddbase status are not identical in the steady state (Plum and Price, 1973). It is
commonly presumed that long-term those
in cisternal
and brain
changes in lumbar CSF acid-base status parallel extracellular fluid (Plum and Price, 1973); but this
assumption has not been adequately tested. A further limitation in the human studies described above is the relative absence of data obtained during the time course of the acclimatization process. We felt that most of these limitations would be avoided. and therefore the question of CSF [H+] regulation in chronic hypocapnic hypoxia could be better answered. if studies were carried out on experimental animals. The pony is well suited for these studies: this animal is highly trainable. can be studied in the unanesthetized state, has a large CSF volume permitting repeated sampling of CSF during the acclimatization process, and the pony is a non-ruminant so that possible changes in CO, elimination via the rumen are not a complicating factor. In addition, the data collected could provide unique information on an animal species not previously utilized in high-altitude respiratory studies. For these reasons, two studies in consecutive years were designed to study the regulation of acid-base status in blood and CSF during acclimatization to high altitude. Each study consisted of data collected 2) during acute hypoxia and 3) during sojourns 4300 m altitude.
1) near sea level during normoxia, of up to 45 days at either 3400 or
Materials and methods ANIMALS
STUDIED
Two groups of 6 and 7 female ponies were selected for these studies. The average weight of these ponies was 200 kg (range: 150-260 kg) and did not vary significantly throughout the course of the studies. The animals were purchased in Wisconsin and were free of any clinical signs of disease. The ponies were trained to stand in a
ACID-BASE
wooden
stanchion
STATUS
restrained
experiments
were the animals
COLLECTION
OF DATA
OF THE PONY
only by a halter anesthetized
DURING
HYPOXIA
and at no time during
25 any of these
or tranquilized.
A standardized protocol was used to study these animals in each experiment. The basic protocol consisted of 1) initially collecting several arterial blood samples, 2) simultaneous sampling of blood and cisternal CSF and 3) measurement of resting ventilation. At the beginning of each experiment the pony was led into the stanchion and a teflon catheter was placed percutaneously in the descending aorta (Will and Bisgard, 1972). After the placement of the catheter, several blood samples were collected anaerobically and the acid-base status of each sample was measured. If Paco2 and Pao2 did not fluctuate more than 1.0 mm Hg on these consecutive samples, the pony was judged to be in a steady state and preparations were made for CSF collection. After local anesthesia (27: lidocaine) of the skin, CSF was sampled from the cisterna magna of the pony. A 20-ga, 85-mm, thin-walled needle was advanced into the cistern and 10 ml of CSF was withdrawn anaerobically. Although necessary only in 2 instances, samples contaminated with air bubbles or blood were discarded. The fluid was then immediately analyzed for Pco, and pH. During the collection of CSF, blood was simultaneously withdrawn from the aorta for similar acid-base determinations. After collections of arterial blood and CSF, \j~ was measured in a manner similar to that previously described for calves (Bisgard et al., 1974). A plastic mask was fitted over the animal’s nose and mouth and taped securely in place. A low resistance three-way valve fitted to the mask allowed the volume of expired air to be measured on a Parkinson Cowan gasmeter (Instrumentation Associates, New York, N.Y.). Temperature of expired air was read directly after each measurement and ‘irk was corrected to BTPS. The equipment and procedure used for the measurement of ‘?E did not disturb the animal’s breathing as evidenced by the close agreement of Pacoz and Pao, of blood samples collected with and without the breathing mask and valve system attached. Resting VE was expressed as ‘?E per square meter body surface area (BSA). BSA(m’)= 10.5.weight(g)“/lOOOO (Spector, 1956). SOJOURN
TO 3400
m
ALTITUDE
Control studies were completed in Madison, WI (altitude 250 m, PB = 740 mm Hg) while the animals breathed ambient air. Control measurements were made on 3 separate occasions, but because the acid-base measurements of blood and CSF did not differ between studies (P >0.05), the data were pooled and normoxic measurements represent a mean of 3 determinations. Following the control study, simulation of 3400 m was carried out for 1 hour during which the ponies breathed 14.7_tO.l% 0, from a large neoprene balloon. Arterial blood was sampled every 15 min in order to follow the time course of
J. A. ORR
26 acid-base
changes
in blood.
A duration
et d.
of 1 hr of hypoxia
was selected
because
PacoZ remained relatively stable between 30 and 60 min of hypoxia. Arterial blood and CSF were simultaneously collected during the last several minutes of acute hypoxia. The acute hypoxic exposure was repeated on the six ponies studied, but since no differences
between
studies were found (P > 0.05) the data presented
in the results
represent a mean of two determinations on each pony. After the completion of the sea-level studies the ponies were transported by truck to Climax, Colorado ( PB = 510 mm Hg). Studies were completed after 2, 5, 10 and 45 days at 3400 m altitude. The protocol was similar to the control studies, except \jE was not measured on day 2 at 3400 m. In addition to the above measurements the ventilatory response to acute hypercapnia was determined for these six ponies at 250 m and after 5. 10 and 45 days at 3400 m. The ponies were given 40x, 0, to breathe and the concentration of CO, was increased stepwise while ii~ was continuously recorded. Levels of CO, added to the inspired gas were approximately 0, 2, 4, 5 and 6’5,. Each level of COZ concentration was breathed for 3 min after which \ir;: was measured and a blood sample withdrawn for P+o, determinations. At 3400 m the ponies were given 60:‘:, O2 to breathe to approximate breathing 40’;; 0, at 250 m. A further explanation of this method is offered by Bisgard et ul. ( 1973) in a recent report on calves. SOJOURN
TO
4300 m
ALTITUDE
Studies near sea level were similar to the control and acute hypoxia (FIEF= 12.0f 0.17; 0,) studies previously described. However, unlike the previous year, only 1 control and 1 acute hypoxia study was completed on consecutive days in the 7 ponies included in these studies. In addition to the 60-min acute hypoxic exposure, 4 ponies were selected for an additional experiment. These 4 ponies breathed 12% O2 for 6 hr and at the end of this time. CSF and arterial blood were collected simultaneously and the acid-base status of each was determined. Exposure to chronic hypoxia was done at Mt. Evans, Colo. The seven ponies were trucked to 4300 m and studied after at this altitude. Minute ventilation was only measured on day Unfortunately, insufficient time and equipment space prevented responses in this group of ponies. However, blood proteinized and frozen for later lactate analysis. ANALYSIS
and
CSF
(Pe=450 mm Hg). 1, 4. 6 and 9 days 2 and 8 at 4300 m. the testing of CO, samples
were
de-
TECHNIQUES
with conArterial Po,, Pco, and pH and CSF PcoZ and pH were measured ventional Radiometer electrodes (The London Co., Cleveland, Ohio) maintained at 38 “C. In addition to conventional calibration procedures using humidified gases, blood from several ponies was tonometered at a known PcoJ and P,, to check and correct the electrode measurements made on arterial blood. Artificial CSF with a known [HCO;] was prepared and tonometered at known Pco,‘s to check and The accuracy and reproducibility correct CSF PcoZ and pH electrode calibrations.
ACID-BASE STATUS OF THE PONY DURING HYPOXIA
27
of these methods have been published previously (Dempsey el al., 1974). In addition, measurement of total CO, (CT& in the Van Slyke apparatus was completed on several CSF samples and close agreement was found between calculations of CSF [HCO;] based on CT,,, USelectrode measurements of Pco, and pH similar to our previous reports (Dempsey et al., 1974). In two instances the rectal temperature of the pony was 39 “C rather than 38 ‘C. In these cases acid-base measurements were corrected to 38 ‘C using temperature coefficients published for humans (Severinghaus, 1966; Mitchell et nl., 1965). Bicarbonate concentrations in blood and CSF were calculated from measured PcoL and pH using solubility coefficients and dissociation constants previously reported for human blood (Severinghaus, 1965) and CSF (Mitchell et al., 1965). To compare the compensation of pH in blood and CSF, a method offered by Siesjo ( 1971) was used to calculate the per cent of pH compensation for the respiratory alkalosis occurring at high altitude. Lactate concentration in CSF and blood was determined by a modified calorimetric technique (Barker and Summerson, 1941). Statistical analysis of the data was done by use of an analysis of variance or Student’s t test (Steel and Torrie, 1960). Differences of P -=c 0.05were considered significant. Results STuDtEs AT 3400 m ALTITUDE The mean Pa,-o, and Pa,, data of the six ponies studied at 250 m and after sojourn at 3400 m are shown in fig. 1. Arterial Po, fell from 99.3 mm Hg during normoxia to 52.1 mm Hg during acute hypoxia and did not differ by more than 2.5 mm Hg from this value during any of the measurements at 3400 m altitude. The response of Pat,,, to acute and chronic hypoxia was triphasic in nature. Control Paco2 (39.3 mm Hg) decreased 0.6 mm Hg during acute hypoxia and fell an additional 6.6 mm Hg the first 2-5 days at 3400 m. After 10 days, Pacoz increased 4-5 mm Hg and remained at this level on the 45th day of sojourn. At 250 m, mean pHa was 7.403. During acute hypoxia, pHa increased 0.03 units and remained unchanged through the first 10 days at 3400 m. After 45 days mean pHa had returned to near the control value (7.398). Arterial [HCO;] increased from 24.9 meq/l during normoxia to 26.3 meq/l during acute hypoxia, and after 2 and 5 days at 3400 m, was reduced to 21.9 meq/l. On day 10 and 45 at 3400 m, [HCO;]a had returned to control levels. Changes in CSF Pco2 p aralleled previously described changes in Pacoz (fig. 1). Cerebrospinal fluid Pea, decreased from a mean normoxic measurement of 47.9 mm Hg to 46.0 mm Hg during acute hypoxia. The Pcoz of CSF then decreased 7.0-8.6 mm Hg after 2 and 5 days at 3400 m, and increased 4-5 mm Hg after 10 and 45 days. Mean cerebrospinal fluid pH of these ponies was 7.324 at 250 m. During acute hypoxia CSF pH increased 0.013 units. On the 2nd day at 3400 m, CSF pH was 7.339 and by 10 days CSF was not different from measurements at 250 m.
7.44
28
,,~---c-___r______~___--------~.
.\\(
~ ‘\
fPHa
‘\
I
I
/-+-
1
I”
E
3
2
$
22
E
I 18
mo2
99.2
52.1
NOIMOX,A 1 hr
52.0
54.6 ’
2da
5da 3400
Fig. 1. Meanskstandard 3400-m
altitude.
error
of the mean (SEM)
In all cases N=6
except
during
m
of arterial the j-day
blood
and CSF data
(N=4)
and
lo-day
during
sojourn
at
(N= 5) measurements.
PaC~2(mmw AT INTERCEPT 9
n
10.0
35
.
A
0%
-P OAO
6.0
T
6.0
0 f
AA
30
A
.
2.0
l
Y!!!!lT 0
4.0
n
t
0
+
.
A
.
A
n
-
25
Sea Level
5D +
3400m
Fig. 2. Individual
IOD
45D
b
Sea Level
Altitude
and mean (kSEM)
data obtained
to characterize
-
50 u
.
IOD
450
3400 m Altitude ventilatory response
of ponies
to
CO,. The Aii~/AParo~ values, that is, the slope of the CO, response curve, were obtained using the least_squares method to determine the line of best fit. The Pa,,, at the intercept is the point on the Pa,,, axis that the line of best tit intersects when it is extended downward.
ACID-BASE
mo,
Fig.
Bf8
STATUS OF THE PONY DURING
41.2, 43.7 “1 lhr Ida N*n*OXII
44.8 1 2da
41.7 1 4du 4300
45.4 I 6Q
HYPOXIA
43.4 8do
29
42.6 I 9do
m
3. MeansiSEM of acid-base data of arterial blood and CSF during sojourn at 4300-m altitude. In all cases N=7 except during the I-hr (N=6), 2-day (N=6) and 4-day (N=S) measurements.
were similar to [HCO;]a changes except during acute Changes in CSF[HCO;] hypoxia. While [HCO;]a rose 1.4 meq/l during acute hypoxia, CSF[HCO;] remained unchanged from normoxic concentrations (23.8 meq/l). The mean X&/m2 (BTPS), respiratory rate, and VT (BTPS) are listed in table 1. Changes in $‘E did not parallel previously described changes in Paco2. Minute ventilation increased 4.0 I/min/m2 during acute hypoxia, but ii~, respiratory rate and VT did not change s~gni~cantly from control during exposure to chronic hypoxia. Changes in the ventilatory response of these ponies to CO2 is shown in fig. 2. On the 5th day at 3400 m the slope of the COz response was increased as evidenced by a 1350/, greater increase in \j~ for each mm Hg increase in Pa,,,. This increased sensitivity to COz was transitory; however, as on the 10th day at 3400 m Aii~lAPa,-,, was only 48% greater than at 250 m. By 45 days at 3400 m the slope of the COz response curve was not different from that measured at 250 m. Changes in the intercept of the COz response paralleled changes in the slope of the response. STUDIES AT
4300 m
The acid-base status and Paoz of blood during sojourn at 4300 m are presented
Normoxia Acute hypoxia (1 hr) 2 days 5 days 8 days 10 days 45 days
-
13.511.6 12.7+1.8 12.5kO.8
4
5 6
10.6+2.7 14.5f3.7
6 6
-
N OEjrll~ (1 mPs~min~~‘~m~2)
34Wm
1923.2 21F2.7
21 +2.4 2924.7 _ 221 1.0
-.______-
~. _.~_. f (breaths~min)
1.9420.25 1.94+0.16 1.96f0.24
1.74t0.10 1.84+0.10
.-
VT (l/breath)
~_
______
7
7 6 5
-._
_I_~
6.9rtO.S -
7.51_ 1.2 13.2k2.1 6.8&0.8
_
~:\I iiE/rnZ (1 BTPS~min-‘~m~m2)
4300 m
_
2.57;tO.28 9,1.7
-
2 T--
. --.-______-
1.72-&0.23 2.09+0.26 2.39kO.35 -
?
15+2.3 22i 5.4 to+ 1.6
RTPS.
? 0 $
.__~
to
VT (l/breath) __
f (br~dths~min)
_--.
Resting ventilatnry data measured at Madison, Wisconsin and after indicated exposures to 3400. and 4300-m altitude. Volumes were corrected Values are means&standard error of the mean (SEM). ~-_---. ~ __ .~___ --
TABLE 1
ACID-BASE
STATUS OF THE PONY DURING
in fig. 3. Arterial Par fell from 85.8 mm Hg during during acute hypoxia and as at 3400 m was statistically throughout
the sojourn
at 4300 m. The response
31
HYPOXIA
normoxia to 41.3 mm Hg unchanged from this value
of PacoL to hypoxia
was again
triphasic with respect to time. The first phase was a reduction of PacOz of 3.9 mm Hg from the control normoxic measurement (39.6 mm Hg). The 2nd phase was a further reduction in PacOz after 14 days at 4300 m at which time Pacoz was 6.9 to 8.3 mm Hg lower than the Pacoz measured during acute hypoxia. After 6 days at 4300 m PacOz increased 4.7 mm Hg and remained at this level through day 9. The mean normoxic pHa of the 7 ponies included in this study was 7.409. During acute hypoxia pHa increased 0.07 units above the normoxic mean pH. After 1 day at 4300 m mean pHa was 7.452 or only 0.04 units above the mean normoxic pHa. Arterial pH remained at this level until day 8 when it was no longer different from the mean pHa at 250 m. Arterial [HCO;] remained unchanged during acute hypoxia from the control measurements at 250 m (24.5 meq/l). After 1 day at 4300 m [HCO;]a fell 5.0 meq/l and remained 2.7 to 3.4 meq/l below normoxic measurements for the entire 9-day sojourn. Changes in CSF Pco2 again paralleled changes in PaCOL (fig. 2). The Pco, of CSF decreased from a mean normoxic measurement 47.8 mm Hg to 41.7 mm Hg during acute hypoxia. The PcoL of CSF then decreased 9.3 to 12.0 mm Hg from day 1 through day 4 at 4300 m and increased 34 mm Hg after 6 and 9 days. CSF pH remained alkaline as compared to control measurements during the entire sojourn at 4300 m. The mean pH of CSF increased from 7.331 at 250 m to 7.364 during acute hypoxia and increased again to 7.395 after 1 day at 4300 m. On day 4,6 and 9 CSF pH was not different from the pH measured during acute hypoxia. Changes in CSF [HCO;] were again similar to changes in [HCO;]a except during acute hypoxia, during which CSF[HCO;] decreased 1.4 meq/l while [HCO;]a remained unchanged. Lactic acid concentrations in blood and CSF during sojourn at 4300 m are shown in table 2. The lactic acid concentration of blood did not change during altitude sojourn
while
CSF
lactic
acid concentration
TABLE Mean lactic acid concentrations
( f SEM) in blood (reported
increased
0.52 meq/l
during
acute
2
and CSF during
normoxia
and sojourn
at 4300 m
as meqil) ~~_______
N
CSF
Arterial
blood
Normoxia
7
2.71kO.21
2.03 f 0.28
1 hr (4300 m)
6
3.23 + 0.22
2.43 k 0.49
1 day (4300 m) 4 days (4300 m)
7 5
3.70 & 0.06 3.40 f 0.09
I .99 * 0.28
6 days (4300 m)
7
2.95 + 0.09
1.33kO.07
9 days (4300 m)
7
2.91 +O.lO
1.46i_O.l5
1.65+0.15
_.~
32
J. A. ORR el al.
hypoxia and remained elevated for the first four days at 4300 m. As at 3400 m, VE did not increase during the I-9-day sojourn at 4300 m (table 1). Minute ventilation did increase during acute hypoxia, but returned to control measurements during chronic hypoxia. Respiratory rate was increased during acute hypoxia while VT did not change throughout altitude sojourn. As previously mentioned, in order to determine if 1 hr was sufficient time to attain a steady state, 4 ponies were given 12% O2 to breathe for 6 hr. The acid-base status of blood and CSF during normoxia and after I and 6 hr of hypoxia is shown in table 3. After 6 hr of hypoxia P+,,, decreased 0.7 mm Hg and [HCd;]a increased 1.1 meq/l relative to values obtained after 1 hr of hypoxia. Cerebrospinal fluid PC-,, decreased 2.6 mm Hg and [HCO,] decreased 1.0 meq/l relative to measurements after 1 hr of hypoxia. CSF pH was not appreciably altered by the longer duration of hypoxia.
TABLE 3 Acid-base status of CSF and arterial blood during normoxia and after 1 and 6 hr of 12% Flo*. Values are means+ SEM (N = 4). -._ --._ PH
Pcoi (mm W
__~_
Wosl
Lactate
(mew? --
(me@) -_-
Normoxia
CSF arterial blood
7.334+0.002 7.424kO.010
47.6 f I .O 38.7 + 1.0
24.7 + 0.5 24.5kO.3
2.89kO.36 2.21 io.50
1 hr (12% FI,,)
CSF arterial blood
7.375 i 0.004 7.470*0.003
40.9i: 1.3 33.5+0.8
22.4kO.7 23.7iO.7
3.36i 0.30 2.36rtO.53
6 hr
CSF arterial blood
7.370 + 0.007 7.495 + 0.015
38.3 k 0.2 32.8 _t 0.9
21.4+0.2 24.8 k 1.4
3.84f0.16 2.14kO.42 .-~
(12%Fb,)
cohmimo~
OF
3400-m
AND
4300-m DATA
The similarity of the changes in the variables studied in blood and CSF during sojourn to these two altitudes is evident. At both 3400 and 4300 m, Paoz decreased during acute hypoxia and remained essentially unchanged during the altitude sojourns. Likewise, the response of Pace, to hypoxia was similar at both altitudes, differing only in the magnitude of the response. Arterial Pcoz decreased slightly during acute hypoxia, decreased further after several days, and began to rise after 6 days at high altitude. Changes in CSF pH and pHa were also similar at the 2 altitudes studied. Arterial pH was most alkaline during acute hypoxia and became progressively compensated with time at high altitude. Cerebrospinal fluid pH increased during acute hypoxia, became more alkaline as the hypocapnia progressed over the first several days, and then returned to levels measured during acute hypoxia.
33
ACID-BASE STATUS OF THE PONY DURING HYPOXIA
,
x
8
[HCo;l~:q, 1
t
’
I
’
1
-4.0
-6.0 1
Fig. 4. Relative changes in [HCO;] from normoxia during sojourn to 3400-m (closed circles) and 4300-m (open circles) altitudes. Values were calculated from mean [HCO;] in arterial blood and CSF.
0
10do l I
2do 04do
0 hr
bda
l 45da
93 l&
0
9 da
V
I
0
I
20 %
4’0 PM,
I
60
1 80
Id0
compenroG3n
Fig. 5. Compensation of pH in arterial blood and CSF during sojourn to 3400-m (closed circles) and 4300-m(open circles)altitude. Mean pH compensation was calculated from mean pH and [HCO;] values.
J. A. ORR
34
et Cd.
Discussion Present
findings
is accompanied
have shown that in healthy, by a compensation
awake ponies,
sojourn
of CSF pH, which is incomplete
to high altitude and for the most
part, parallel to that in arterial blood. The pertinent comparisons between blood and CSF are summarized in figs. 4 and 5. After 1 hr of hypoxia, with mild hypocapnia, the reduction in [HCO,] and hence. degree of pH compensation, in CSF exceeded that in blood. However, as hypocapnia progressed over the ensuing l-5 days of hypoxia, the decrease in [HCO;] and pH compensation in the CSF was equal to (at 3400 m) or less than (at 4300 m) that in arterial blood. Finally, beyond 5 days of sojourn. the Pcoz of the pony rose gradually in blood and CSF. and the pH and [HCO;] of each fluid also returned toward sea-level control values. the rate of return being somewhat greater in blood than CSF at 4300 m. The equivalence of pH compensation observed between blood and CSF suggests that the regulation of CSF [HCO;] during chronic exposure to a relatively wide range of hypoxemic hypocapnia is governed by passive exchange processes. Furthermore, recent evidence obtained during short-term hypocapnia in the anesthetized dog (Pelligrino et ul., 1974) suggests that the close correspondence presently observed in pH compensation between CSF and blood reflects a critical dependence of CSF [HCO;] on corresponding changes in plasma [HCO;]. Obviously the findings on these ponies during chronic hypoxic hypocapnia do not support local ‘CSFspecific’ mechanisms involved in the precise regulation of CSF [HCO;] or pH. However, a clear contribution from local mechanisms to CSF [HCO,] regulation is evident in the acute stages of the hypoxic exposure. The [HCO;] of blood does not change during acute hypoxia, while CSF [HCO;] decreases slightly after l-6 hr at 4300 m. This may be attributed in part to a corresponding increase in CSF lactic acid (table 2) secondary to a selective cerebral hypoxia. Although we are unable to determine if this mechanism continued to effect a reduction in CSF [HCO;] beyond 6 hr of hypoxia, the close correspondence in [HCO;] changes in CSF and blood suggest that increases in cerebral lactic acid had a negligible effect on CSF [HCO;] re g u 1a t’ton during chronic hypoxia. We are confident that the above discussion of CSF acid-base status accurately reflects changes in brain ECF Hf throughout sojourn at high altitude. Fencl er al. (1966) indicated that a steady state between CSF and brain ECF is reached in goats after several days of sustained metabolic alkalosis or acidosis. One could assume the same to be true during chronic hypocapnic hypoxia. As mentioned previously, the data collected from 4 ponies after 6 hours of hypoxia are similar to the results obtained after 1 hour of hypoxia (table 3). This may indicate that a steady state is reached between CSF H ’ and brain ECF H + after only 1 hr of hypoxia. If CSF H ’ is an accurate measure of cerebral ECF H + then it is obvious that the ECF H + of these ponies was imperfectly regulated during chronic hypocapnic hypoxia. As outlined in the introduction, these data on the pony are relevant to two basic problems previously investigated in the human sojourner to high altitude. First, as previously discussed, these data support previous data from our laboratory on man
ACID-BASE
STATUS OF THE PONY DURING HYPOXIA
35
sojourning at 3100 and 4300 m altitude in that pH compensation in CSF is incomplete and does not exceed that in blood during chronic hypocapnic hypoxia. Secondly, these data have limited implications concerning the regulation of respiration during altitude sojourn in man. Before discussing the regulation of respiration in the pony during high-altitude sojourn, the question arises as to whether or not the pony actually exhibits ventilatory acclimatization to high altitude. Two points of evidence support the contention that this animal species shows an increased ventilatory drive during chronic hypoxia. First, similar to man (Forster et al., 1971), these animals exhibited a positive interaction between acute hypercapnia and chronic hypoxia. That is, the slope as well as the extrapolated intercept of the CO, response curves were increased after several days at 3400 m compared to CO* responses tested at 250 m. Secondly, Pacoz decreases in the pony during progression from acute (1 hr) to chronic (1-5 days) hypoxia similar to responses in men taken to similar altitudes (Dempsey et al., 1974; Forster et al., 1975). However while Pa,,, of man remains lowered throughout sojourn, the Pace, of the pony increases after several weeks’ exposure to hypoxia. The transient nature of this response is similar to reports on goats (Lahiri et al., 1971) and Hereford steers (Grover et al., 1963). Although increases in the acute response to hypercapnia during chronic hypoxia and the decrease in Paco2 during the first week at high altitude relative to acute hypoxia support ventilatory acclimatization in the pony, changes in VE do not. In man, decreases in Pacoz during ventilatory acclimatization are associated with . increases m VE. In the pony, Paco2 is lower during the first few days at high altitude than at 250 m, but VE is not statistically elevated above control measurements. In other words, changes in PacOz are poorly correlated with changes in QE. The reason for failure of \j~ to increase in the pony while Pacoz decreases is not known. Although a decrease in tissue CO, production in the pony during altitude sojourn may explain this phenomenon, it seems unlikely since \jco2 does not change in man at high altitude under a constant workload (Lenfant and Sullivan, 1971). Consequently if we assume that \jco2 did not change in these ponies, a decrease in Pace, at a constant \j~ may indicate an increase in VA with a corresponding decrease in VD. Until these data become available on the pony, no conclusions can be made regarding ‘?A in the pony at high altitude. If, however, PacOz changes do reflect increases in VA, the changes in the measured chemical stimuli cannot explain this increase in VA. For example, Pacoz was significantly lower after several days at either elevation compared to the PacOz after 1 hr exposure to the same PIN,. Yet in examining the known chemical stimuli to breathing in either instance, one sees that CSF pH rose, Pao, remained constant and arterial pH either remained constant or decreased slightly (0.03 units at 4300 m). Thus the proposed increase in ventilatory drive at high altitude in the pony cannot be explained by relative decreases in CSF pH and probably not by peripheral chemical stimuli such as Pao2 or pHa. In summary, the pony appears to acclimatize to high altitude in a different way
36
J. A. ORR et a/.
from man. The pony does not increase VE at high altitude but may increase There may be other mechanisms operating with respect to gas exchange in the pony since Paoz remained essentially constant during chronic hypoxia (> 6 days) while Pa,,, was increasing. VA.
Acknowledgements
This study was supported by grants from PHS HL (No. 13154, 15473 and 15469902) U.S. Army Research and Development Command (No. 17-74-C-4020), the American Lung Association, and the College of Agricultural and Life Sciences, University of Wisconsin, Madison. The authors wish to extend thanks to all who provided technical assistance including Mr. Gordon Johnson, Ms. Monique Wanner, Ms. Jean Vaughn, Mr. Michael Madalon, Ms. Chris Alexander, Ms. Rita Leczynski, Mr. John Pope, Ms. Gail Jamieson, Mr. William Bayer and to Ms. Karen Griffin. We are also indebted to Dr. Mario Iona and the University of Denver, Department of Physics who provided the laboratory facilities at Mt. Evans, Colorado and the Climax Molybdenum Mine for their assistance at Climax, Colorado. References Barker.
S. B. and W. H. Summerson
material. Bisgard,
(1941). The calorimetric
determination
of lactic acid in biological
J. Biol. Chem. 35: 5355554.
G. E., A. V. Ruiz, R. F. Grover
and J. A. Will (1973). Ventilatory
control
in the Hereford
calf.
J. Appl. Physiol. 35: 220-226. Bisgard,
G. E., A. V. Ruiz, R. F. Grover
meters altitude Dempsey,
J. A., H. V. Forster
hypoxemia Fencl,
in the Hereford and
G. A. doPico
(1974).
Ventilatory
and
of acid-base
J. R. Pappenheimer balance.
(1966).
with deductions
Studies
concerning
interstitial fluid. Am. J. Physiol. 210: 459472. Forster. H. V., J. A. Dempsey, M. L. Birnbaum, W. G. Reddan, (1971).
Effect of chronic
J. Appl Physiol. Forster, Grover,
exposure
acclimatization
to 3400
acclimatization
to moderate
to hypoxia
on
the
respiratory
response
the ionic composition
J. Thoden,
on ventilatory
R. F. Grover
response
to
of cerebral and J. Rankin
to CO,
and
hypoxia.
3 1: 586-592.
H. V.. J. A. Dempsey
during
Ventilatory
21: 271-296.
in man. The role of spinal fluid [H+]. J. Clin. Incest. 53: 1091-l 100.
V., T. B. Miller
disturbances
and J. A. Will (1974).
Calf. Respir. Physiol.
acclimatization
and L. W. Chosy
to high altitude
(1975). Incomplete
R. F., J. T. Reeves, D. H. Will and S. G. Blount,
steers at high altitude. Lahiri, S., N. S. Cherniack, goat and its adaptation
Jr. (1963).
J. Appl. Physiol. 18: 567-574. N. H. Edelman and A. P. Fishman to chronic
compensation
of CSF Hi
in man
(4300 m). J. Appl. Physiol. (In press).
and life-long
hypoxia.
Pulmonary
(1971). Regulation Respir. Physiol.
vasoconstriction of respiration
in in the
12: 388403.
Lenfant, C. and K. Sullivan (1971). Adaptation to high altitude. New Engl. J. Med. 284: 129881308. Mitchell, R. A., D. A. Herbert and C. T. Carman (1965). Acid-base constants and temperature coefficients Pelligrino.
for cerebrospinal
fluid. J. Appl. Physiol.
D. A., C. G. Irvin, S. M. Mastenbrook
regulation
on plasma
[HCO,]
in respiratory
Plum, F. and R. W. Price (1973). Acid-base N. Enyl. J. Med. 289: 134661351. Severinghaus,
J. W.. R. A. Mitchell,
20: 27-30.
and J. A. Dempsey alkalosis balance
B. W. Richardson
(1974). Dependence
and hypoxemia. of cisternal
and
Federation lumbar
and M. M. Singer (1963).
of CSF HC
Proc. 33: 1369.
cerebrospinal Respiratory
fluid. control
ACID-BASE
at high altitude Severinghaus.
suggesting
STATUS
OF THE PONY
active transport
Vol. II, edited by H. Rahn and W. 0. Fenn.
37
HYPOXIA
of CSF pH. J. Appl. Physiol.
regulation
J. W. (1965). Blood gas concentrations.
DURING
In: Handbook Washington,
of Physiology.
D.C., American
18: 115551166.
Section 3. Respiration. Physiology
Society,
pp.
1475-1487. Severinghaus,
J. W. (1966). Blood gas calculator.
Siesjo, B. K. (1971). Quantification
J. Appl. Physiol. 21: 110881116.
of pH regulation
in hypercapnia
and hypocapnia.
Stand.
J. Clin.
Lab. Invest. 28: 113-119 Sorensen, Spector,
S. C. (1971). The chemical
control
W. S., editor (1956). Handbook
Steel, R. G. D. and J. H. Torrie
(1960).
of ventilation.
of Biological Principles
Data.
and
Actu. Physiol. Philadelphia,
Procedures
Stand. Suppl. 361, 72~. W. B. Saunders,
of Statistics.
Co., p. 175.
New York,
McGraw
Hill Book Co., pp. 194-23 1. Will, J. A. and animals.
G.
E. Bisgard
J. Appl. Physiol.
(1972).
33: 400401.
Cardiac
catheterization
of unanesthetized
large
domestic
(1975) 25, 23337; North-Holluntl
Physiology
CEREBROSPINAL
FLUID ALKALOSIS
SOJOURN
Publishing
DURING
IN UNANESTHETIZED
Madison,
of
Veterinary
Science
Wisconsin, 53706
of
Milwukee.
Abstract.
Unanesthetized
to 3400 m (N=6) P,,,
hypoxia.
arterial
fluid (CSF)
Cerebrospinal
blood
Enoironmental
HIGH-ALTITUDE
withdrawn
were studied
altitude.
conditions
from
the cisterna
decreased
slightly
after
further
after I-5 days at high altitude
1 hr of hypoxia
4300 m) and then increased
(APa,,,
significantly
of’
Wiscowin,
qf
Wisconsin.
near sea level (250 m) and during
under
decreased
College
The pH, Pco,, and PO2 of arterial
were measured an indwelling
Uniuersit)
Medicd
U.S.A.
fluid was sampled from
D. D. BUSS,
Medicine,
Medicine.
Wiscor~.sir~53226,
adult female ponies
and 4300 m (N=7)
of cerebrospinal
days)
of Prerentil:e
and Depurtment
und Department
Anmterdnnl
PONIES
J. A. ORR, G. E. BISGARD, H. V. FORSTER, J. A. DEMPSEY and J. A. WILL Department
Company,
sojourns
and pH and
(1 hr) and chronic
of acute magna
In both
blood
of the awake
arterial
catheter.
groups
= -0.6
mm Hg at 3400 m; -3.9
(APa,,,
= -7.2
pony
of animals,
(l-45 and Pa,,>
mm Hg at 4300 m),
mm Hg at 3400 m; - 12.3 mm Hg at
after 6 days of chronic
hypoxia
(APa,oZ=
+4.1
mm Hg at
3400 m; +4.7 mm Hg at 4300 m). Although Pa,, decreased markedly during acute hypoxia, subsequent changes m Pa,,, at high altitude did not alter Pa,, from that observed during acute hypoxia (Pa,,=52
mm
hypoxia
(ApH=
Hg at 3400 m; 41 mm +0.013
l-2 days at high altitude remained
alkaline
Hg at 4300
unit at 3400 m; (ApH=
to control
f0.031
unit at 3400 m: +0.064
values throughout
hypoxia, CSF pH was imperfectly regulated than (4300 m) arterial blood. Furthermore, arterial
[HCO,]
local ‘CSF-specific’
during
chronic
hypoxia
mechanisms Acid-base Control
sojourn.
status fluid
Under
and regulated the similarity
may indicate
as previously
Cerebrospinal
m). The
pH
of CSF
+Q.O33 unit at 4300 m) and
increased
became
more
during
acute
alkaline
,after
unit at 4300 m). At 4300 m, CSF pH these conditions
of chronic
hypocapnic
in a magnitude equal to (3400 m) or less of relative changes in CSF [HCO;] and
a passive
regulation
of CSF [HCO;]
rather
than
proposed. High altitude
acclimatization
Hypoxia
of breathing
Conflicting views exist concerning the regulation of cerebrospinal fluid (CSF) pH in man during chronic hypocapnic hypoxia. In 1963, Severinghaus et al. presented data from 4 subjects sojourning at 3880 m that indicated near complete compensation of lumbar spinal fluid (LSF) after 2-8 days at this altitude. The return of CSF pH to near normoxic levels in spite of a sustained arterial alkalosis was postulated to be due to mechanisms specific for local regulation of brain extracellular fluid (ECF) [HCO;] and pH. Possible mechanisms included active transport of H+ from capillary blood to brain ECF (Severinghaus et al., 1963) or increased Accepted
for
publication
2 June 1975. 23
24
J. A. ORR
production
et (I/.
of lactic acid by brain tissue (Sorensen,
of these data, Severinghaus
et al. (1963) proposed
H+ played a key role in the stimulation chronic hypoxia.
1971). Furthermore, that the relative
of ventilation
during
on the basis
increase
in CSF
acclimatization
to
More recently, Dempsey et al. (I 974) and Forster et al. (1975) studied 7 subjects during IO-day to 34week sojourns at 3100 and 4300 m altitude. During the sojourn LSF pH remained significantly alkaline to normoxic measurements and was compensated to a similar extent as was arterial blood pH. Consequently no local mechanism for CSF pH regulation need be postulated as changes in CSF [HCO;] seemed to be passively following decreases in plasma [HCO; 1. Furthermore the data were inconsistent with the concept of a positive role for CSF H+ as a mediator of ventilatory acclimatization. The primary problem of interest pertains to the regulation of [HCO;] and [H ‘1 in brain ECF, and in this regard studies in humans are limited by the required use of the lumbar sampling site. It has been shown that lumbar and cisternal CSF aciddbase status are not identical in the steady state (Plum and Price, 1973). It is
commonly presumed that long-term those
in cisternal
and brain
changes in lumbar CSF acid-base status parallel extracellular fluid (Plum and Price, 1973); but this
assumption has not been adequately tested. A further limitation in the human studies described above is the relative absence of data obtained during the time course of the acclimatization process. We felt that most of these limitations would be avoided. and therefore the question of CSF [H+] regulation in chronic hypocapnic hypoxia could be better answered. if studies were carried out on experimental animals. The pony is well suited for these studies: this animal is highly trainable. can be studied in the unanesthetized state, has a large CSF volume permitting repeated sampling of CSF during the acclimatization process, and the pony is a non-ruminant so that possible changes in CO, elimination via the rumen are not a complicating factor. In addition, the data collected could provide unique information on an animal species not previously utilized in high-altitude respiratory studies. For these reasons, two studies in consecutive years were designed to study the regulation of acid-base status in blood and CSF during acclimatization to high altitude. Each study consisted of data collected 2) during acute hypoxia and 3) during sojourns 4300 m altitude.
1) near sea level during normoxia, of up to 45 days at either 3400 or
Materials and methods ANIMALS
STUDIED
Two groups of 6 and 7 female ponies were selected for these studies. The average weight of these ponies was 200 kg (range: 150-260 kg) and did not vary significantly throughout the course of the studies. The animals were purchased in Wisconsin and were free of any clinical signs of disease. The ponies were trained to stand in a
ACID-BASE
wooden
stanchion
STATUS
restrained
experiments
were the animals
COLLECTION
OF DATA
OF THE PONY
only by a halter anesthetized
DURING
HYPOXIA
and at no time during
25 any of these
or tranquilized.
A standardized protocol was used to study these animals in each experiment. The basic protocol consisted of 1) initially collecting several arterial blood samples, 2) simultaneous sampling of blood and cisternal CSF and 3) measurement of resting ventilation. At the beginning of each experiment the pony was led into the stanchion and a teflon catheter was placed percutaneously in the descending aorta (Will and Bisgard, 1972). After the placement of the catheter, several blood samples were collected anaerobically and the acid-base status of each sample was measured. If Paco2 and Pao2 did not fluctuate more than 1.0 mm Hg on these consecutive samples, the pony was judged to be in a steady state and preparations were made for CSF collection. After local anesthesia (27: lidocaine) of the skin, CSF was sampled from the cisterna magna of the pony. A 20-ga, 85-mm, thin-walled needle was advanced into the cistern and 10 ml of CSF was withdrawn anaerobically. Although necessary only in 2 instances, samples contaminated with air bubbles or blood were discarded. The fluid was then immediately analyzed for Pco, and pH. During the collection of CSF, blood was simultaneously withdrawn from the aorta for similar acid-base determinations. After collections of arterial blood and CSF, \j~ was measured in a manner similar to that previously described for calves (Bisgard et al., 1974). A plastic mask was fitted over the animal’s nose and mouth and taped securely in place. A low resistance three-way valve fitted to the mask allowed the volume of expired air to be measured on a Parkinson Cowan gasmeter (Instrumentation Associates, New York, N.Y.). Temperature of expired air was read directly after each measurement and ‘irk was corrected to BTPS. The equipment and procedure used for the measurement of ‘?E did not disturb the animal’s breathing as evidenced by the close agreement of Pacoz and Pao, of blood samples collected with and without the breathing mask and valve system attached. Resting VE was expressed as ‘?E per square meter body surface area (BSA). BSA(m’)= 10.5.weight(g)“/lOOOO (Spector, 1956). SOJOURN
TO 3400
m
ALTITUDE
Control studies were completed in Madison, WI (altitude 250 m, PB = 740 mm Hg) while the animals breathed ambient air. Control measurements were made on 3 separate occasions, but because the acid-base measurements of blood and CSF did not differ between studies (P >0.05), the data were pooled and normoxic measurements represent a mean of 3 determinations. Following the control study, simulation of 3400 m was carried out for 1 hour during which the ponies breathed 14.7_tO.l% 0, from a large neoprene balloon. Arterial blood was sampled every 15 min in order to follow the time course of
J. A. ORR
26 acid-base
changes
in blood.
A duration
et d.
of 1 hr of hypoxia
was selected
because
PacoZ remained relatively stable between 30 and 60 min of hypoxia. Arterial blood and CSF were simultaneously collected during the last several minutes of acute hypoxia. The acute hypoxic exposure was repeated on the six ponies studied, but since no differences
between
studies were found (P > 0.05) the data presented
in the results
represent a mean of two determinations on each pony. After the completion of the sea-level studies the ponies were transported by truck to Climax, Colorado ( PB = 510 mm Hg). Studies were completed after 2, 5, 10 and 45 days at 3400 m altitude. The protocol was similar to the control studies, except \jE was not measured on day 2 at 3400 m. In addition to the above measurements the ventilatory response to acute hypercapnia was determined for these six ponies at 250 m and after 5. 10 and 45 days at 3400 m. The ponies were given 40x, 0, to breathe and the concentration of CO, was increased stepwise while ii~ was continuously recorded. Levels of CO, added to the inspired gas were approximately 0, 2, 4, 5 and 6’5,. Each level of COZ concentration was breathed for 3 min after which \ir;: was measured and a blood sample withdrawn for P+o, determinations. At 3400 m the ponies were given 60:‘:, O2 to breathe to approximate breathing 40’;; 0, at 250 m. A further explanation of this method is offered by Bisgard et ul. ( 1973) in a recent report on calves. SOJOURN
TO
4300 m
ALTITUDE
Studies near sea level were similar to the control and acute hypoxia (FIEF= 12.0f 0.17; 0,) studies previously described. However, unlike the previous year, only 1 control and 1 acute hypoxia study was completed on consecutive days in the 7 ponies included in these studies. In addition to the 60-min acute hypoxic exposure, 4 ponies were selected for an additional experiment. These 4 ponies breathed 12% O2 for 6 hr and at the end of this time. CSF and arterial blood were collected simultaneously and the acid-base status of each was determined. Exposure to chronic hypoxia was done at Mt. Evans, Colo. The seven ponies were trucked to 4300 m and studied after at this altitude. Minute ventilation was only measured on day Unfortunately, insufficient time and equipment space prevented responses in this group of ponies. However, blood proteinized and frozen for later lactate analysis. ANALYSIS
and
CSF
(Pe=450 mm Hg). 1, 4. 6 and 9 days 2 and 8 at 4300 m. the testing of CO, samples
were
de-
TECHNIQUES
with conArterial Po,, Pco, and pH and CSF PcoZ and pH were measured ventional Radiometer electrodes (The London Co., Cleveland, Ohio) maintained at 38 “C. In addition to conventional calibration procedures using humidified gases, blood from several ponies was tonometered at a known PcoJ and P,, to check and correct the electrode measurements made on arterial blood. Artificial CSF with a known [HCO;] was prepared and tonometered at known Pco,‘s to check and The accuracy and reproducibility correct CSF PcoZ and pH electrode calibrations.
ACID-BASE STATUS OF THE PONY DURING HYPOXIA
27
of these methods have been published previously (Dempsey el al., 1974). In addition, measurement of total CO, (CT& in the Van Slyke apparatus was completed on several CSF samples and close agreement was found between calculations of CSF [HCO;] based on CT,,, USelectrode measurements of Pco, and pH similar to our previous reports (Dempsey et al., 1974). In two instances the rectal temperature of the pony was 39 “C rather than 38 ‘C. In these cases acid-base measurements were corrected to 38 ‘C using temperature coefficients published for humans (Severinghaus, 1966; Mitchell et nl., 1965). Bicarbonate concentrations in blood and CSF were calculated from measured PcoL and pH using solubility coefficients and dissociation constants previously reported for human blood (Severinghaus, 1965) and CSF (Mitchell et al., 1965). To compare the compensation of pH in blood and CSF, a method offered by Siesjo ( 1971) was used to calculate the per cent of pH compensation for the respiratory alkalosis occurring at high altitude. Lactate concentration in CSF and blood was determined by a modified calorimetric technique (Barker and Summerson, 1941). Statistical analysis of the data was done by use of an analysis of variance or Student’s t test (Steel and Torrie, 1960). Differences of P -=c 0.05were considered significant. Results STuDtEs AT 3400 m ALTITUDE The mean Pa,-o, and Pa,, data of the six ponies studied at 250 m and after sojourn at 3400 m are shown in fig. 1. Arterial Po, fell from 99.3 mm Hg during normoxia to 52.1 mm Hg during acute hypoxia and did not differ by more than 2.5 mm Hg from this value during any of the measurements at 3400 m altitude. The response of Pat,,, to acute and chronic hypoxia was triphasic in nature. Control Paco2 (39.3 mm Hg) decreased 0.6 mm Hg during acute hypoxia and fell an additional 6.6 mm Hg the first 2-5 days at 3400 m. After 10 days, Pacoz increased 4-5 mm Hg and remained at this level on the 45th day of sojourn. At 250 m, mean pHa was 7.403. During acute hypoxia, pHa increased 0.03 units and remained unchanged through the first 10 days at 3400 m. After 45 days mean pHa had returned to near the control value (7.398). Arterial [HCO;] increased from 24.9 meq/l during normoxia to 26.3 meq/l during acute hypoxia, and after 2 and 5 days at 3400 m, was reduced to 21.9 meq/l. On day 10 and 45 at 3400 m, [HCO;]a had returned to control levels. Changes in CSF Pco2 p aralleled previously described changes in Pacoz (fig. 1). Cerebrospinal fluid Pea, decreased from a mean normoxic measurement of 47.9 mm Hg to 46.0 mm Hg during acute hypoxia. The Pcoz of CSF then decreased 7.0-8.6 mm Hg after 2 and 5 days at 3400 m, and increased 4-5 mm Hg after 10 and 45 days. Mean cerebrospinal fluid pH of these ponies was 7.324 at 250 m. During acute hypoxia CSF pH increased 0.013 units. On the 2nd day at 3400 m, CSF pH was 7.339 and by 10 days CSF was not different from measurements at 250 m.
7.44
28
,,~---c-___r______~___--------~.
.\\(
~ ‘\
fPHa
‘\
I
I
/-+-
1
I”
E
3
2
$
22
E
I 18
mo2
99.2
52.1
NOIMOX,A 1 hr
52.0
54.6 ’
2da
5da 3400
Fig. 1. Meanskstandard 3400-m
altitude.
error
of the mean (SEM)
In all cases N=6
except
during
m
of arterial the j-day
blood
and CSF data
(N=4)
and
lo-day
during
sojourn
at
(N= 5) measurements.
PaC~2(mmw AT INTERCEPT 9
n
10.0
35
.
A
0%
-P OAO
6.0
T
6.0
0 f
AA
30
A
.
2.0
l
Y!!!!lT 0
4.0
n
t
0
+
.
A
.
A
n
-
25
Sea Level
5D +
3400m
Fig. 2. Individual
IOD
45D
b
Sea Level
Altitude
and mean (kSEM)
data obtained
to characterize
-
50 u
.
IOD
450
3400 m Altitude ventilatory response
of ponies
to
CO,. The Aii~/AParo~ values, that is, the slope of the CO, response curve, were obtained using the least_squares method to determine the line of best fit. The Pa,,, at the intercept is the point on the Pa,,, axis that the line of best tit intersects when it is extended downward.
ACID-BASE
mo,
Fig.
Bf8
STATUS OF THE PONY DURING
41.2, 43.7 “1 lhr Ida N*n*OXII
44.8 1 2da
41.7 1 4du 4300
45.4 I 6Q
HYPOXIA
43.4 8do
29
42.6 I 9do
m
3. MeansiSEM of acid-base data of arterial blood and CSF during sojourn at 4300-m altitude. In all cases N=7 except during the I-hr (N=6), 2-day (N=6) and 4-day (N=S) measurements.
were similar to [HCO;]a changes except during acute Changes in CSF[HCO;] hypoxia. While [HCO;]a rose 1.4 meq/l during acute hypoxia, CSF[HCO;] remained unchanged from normoxic concentrations (23.8 meq/l). The mean X&/m2 (BTPS), respiratory rate, and VT (BTPS) are listed in table 1. Changes in $‘E did not parallel previously described changes in Paco2. Minute ventilation increased 4.0 I/min/m2 during acute hypoxia, but ii~, respiratory rate and VT did not change s~gni~cantly from control during exposure to chronic hypoxia. Changes in the ventilatory response of these ponies to CO2 is shown in fig. 2. On the 5th day at 3400 m the slope of the COz response was increased as evidenced by a 1350/, greater increase in \j~ for each mm Hg increase in Pa,,,. This increased sensitivity to COz was transitory; however, as on the 10th day at 3400 m Aii~lAPa,-,, was only 48% greater than at 250 m. By 45 days at 3400 m the slope of the COz response curve was not different from that measured at 250 m. Changes in the intercept of the COz response paralleled changes in the slope of the response. STUDIES AT
4300 m
The acid-base status and Paoz of blood during sojourn at 4300 m are presented
Normoxia Acute hypoxia (1 hr) 2 days 5 days 8 days 10 days 45 days
-
13.511.6 12.7+1.8 12.5kO.8
4
5 6
10.6+2.7 14.5f3.7
6 6
-
N OEjrll~ (1 mPs~min~~‘~m~2)
34Wm
1923.2 21F2.7
21 +2.4 2924.7 _ 221 1.0
-.______-
~. _.~_. f (breaths~min)
1.9420.25 1.94+0.16 1.96f0.24
1.74t0.10 1.84+0.10
.-
VT (l/breath)
~_
______
7
7 6 5
-._
_I_~
6.9rtO.S -
7.51_ 1.2 13.2k2.1 6.8&0.8
_
~:\I iiE/rnZ (1 BTPS~min-‘~m~m2)
4300 m
_
2.57;tO.28 9,1.7
-
2 T--
. --.-______-
1.72-&0.23 2.09+0.26 2.39kO.35 -
?
15+2.3 22i 5.4 to+ 1.6
RTPS.
? 0 $
.__~
to
VT (l/breath) __
f (br~dths~min)
_--.
Resting ventilatnry data measured at Madison, Wisconsin and after indicated exposures to 3400. and 4300-m altitude. Volumes were corrected Values are means&standard error of the mean (SEM). ~-_---. ~ __ .~___ --
TABLE 1
ACID-BASE
STATUS OF THE PONY DURING
in fig. 3. Arterial Par fell from 85.8 mm Hg during during acute hypoxia and as at 3400 m was statistically throughout
the sojourn
at 4300 m. The response
31
HYPOXIA
normoxia to 41.3 mm Hg unchanged from this value
of PacoL to hypoxia
was again
triphasic with respect to time. The first phase was a reduction of PacOz of 3.9 mm Hg from the control normoxic measurement (39.6 mm Hg). The 2nd phase was a further reduction in PacOz after 14 days at 4300 m at which time Pacoz was 6.9 to 8.3 mm Hg lower than the Pacoz measured during acute hypoxia. After 6 days at 4300 m PacOz increased 4.7 mm Hg and remained at this level through day 9. The mean normoxic pHa of the 7 ponies included in this study was 7.409. During acute hypoxia pHa increased 0.07 units above the normoxic mean pH. After 1 day at 4300 m mean pHa was 7.452 or only 0.04 units above the mean normoxic pHa. Arterial pH remained at this level until day 8 when it was no longer different from the mean pHa at 250 m. Arterial [HCO;] remained unchanged during acute hypoxia from the control measurements at 250 m (24.5 meq/l). After 1 day at 4300 m [HCO;]a fell 5.0 meq/l and remained 2.7 to 3.4 meq/l below normoxic measurements for the entire 9-day sojourn. Changes in CSF Pco2 again paralleled changes in PaCOL (fig. 2). The Pco, of CSF decreased from a mean normoxic measurement 47.8 mm Hg to 41.7 mm Hg during acute hypoxia. The PcoL of CSF then decreased 9.3 to 12.0 mm Hg from day 1 through day 4 at 4300 m and increased 34 mm Hg after 6 and 9 days. CSF pH remained alkaline as compared to control measurements during the entire sojourn at 4300 m. The mean pH of CSF increased from 7.331 at 250 m to 7.364 during acute hypoxia and increased again to 7.395 after 1 day at 4300 m. On day 4,6 and 9 CSF pH was not different from the pH measured during acute hypoxia. Changes in CSF [HCO;] were again similar to changes in [HCO;]a except during acute hypoxia, during which CSF[HCO;] decreased 1.4 meq/l while [HCO;]a remained unchanged. Lactic acid concentrations in blood and CSF during sojourn at 4300 m are shown in table 2. The lactic acid concentration of blood did not change during altitude sojourn
while
CSF
lactic
acid concentration
TABLE Mean lactic acid concentrations
( f SEM) in blood (reported
increased
0.52 meq/l
during
acute
2
and CSF during
normoxia
and sojourn
at 4300 m
as meqil) ~~_______
N
CSF
Arterial
blood
Normoxia
7
2.71kO.21
2.03 f 0.28
1 hr (4300 m)
6
3.23 + 0.22
2.43 k 0.49
1 day (4300 m) 4 days (4300 m)
7 5
3.70 & 0.06 3.40 f 0.09
I .99 * 0.28
6 days (4300 m)
7
2.95 + 0.09
1.33kO.07
9 days (4300 m)
7
2.91 +O.lO
1.46i_O.l5
1.65+0.15
_.~
32
J. A. ORR el al.
hypoxia and remained elevated for the first four days at 4300 m. As at 3400 m, VE did not increase during the I-9-day sojourn at 4300 m (table 1). Minute ventilation did increase during acute hypoxia, but returned to control measurements during chronic hypoxia. Respiratory rate was increased during acute hypoxia while VT did not change throughout altitude sojourn. As previously mentioned, in order to determine if 1 hr was sufficient time to attain a steady state, 4 ponies were given 12% O2 to breathe for 6 hr. The acid-base status of blood and CSF during normoxia and after I and 6 hr of hypoxia is shown in table 3. After 6 hr of hypoxia P+,,, decreased 0.7 mm Hg and [HCd;]a increased 1.1 meq/l relative to values obtained after 1 hr of hypoxia. Cerebrospinal fluid PC-,, decreased 2.6 mm Hg and [HCO,] decreased 1.0 meq/l relative to measurements after 1 hr of hypoxia. CSF pH was not appreciably altered by the longer duration of hypoxia.
TABLE 3 Acid-base status of CSF and arterial blood during normoxia and after 1 and 6 hr of 12% Flo*. Values are means+ SEM (N = 4). -._ --._ PH
Pcoi (mm W
__~_
Wosl
Lactate
(mew? --
(me@) -_-
Normoxia
CSF arterial blood
7.334+0.002 7.424kO.010
47.6 f I .O 38.7 + 1.0
24.7 + 0.5 24.5kO.3
2.89kO.36 2.21 io.50
1 hr (12% FI,,)
CSF arterial blood
7.375 i 0.004 7.470*0.003
40.9i: 1.3 33.5+0.8
22.4kO.7 23.7iO.7
3.36i 0.30 2.36rtO.53
6 hr
CSF arterial blood
7.370 + 0.007 7.495 + 0.015
38.3 k 0.2 32.8 _t 0.9
21.4+0.2 24.8 k 1.4
3.84f0.16 2.14kO.42 .-~
(12%Fb,)
cohmimo~
OF
3400-m
AND
4300-m DATA
The similarity of the changes in the variables studied in blood and CSF during sojourn to these two altitudes is evident. At both 3400 and 4300 m, Paoz decreased during acute hypoxia and remained essentially unchanged during the altitude sojourns. Likewise, the response of Pace, to hypoxia was similar at both altitudes, differing only in the magnitude of the response. Arterial Pcoz decreased slightly during acute hypoxia, decreased further after several days, and began to rise after 6 days at high altitude. Changes in CSF pH and pHa were also similar at the 2 altitudes studied. Arterial pH was most alkaline during acute hypoxia and became progressively compensated with time at high altitude. Cerebrospinal fluid pH increased during acute hypoxia, became more alkaline as the hypocapnia progressed over the first several days, and then returned to levels measured during acute hypoxia.
33
ACID-BASE STATUS OF THE PONY DURING HYPOXIA
,
x
8
[HCo;l~:q, 1
t
’
I
’
1
-4.0
-6.0 1
Fig. 4. Relative changes in [HCO;] from normoxia during sojourn to 3400-m (closed circles) and 4300-m (open circles) altitudes. Values were calculated from mean [HCO;] in arterial blood and CSF.
0
10do l I
2do 04do
0 hr
bda
l 45da
93 l&
0
9 da
V
I
0
I
20 %
4’0 PM,
I
60
1 80
Id0
compenroG3n
Fig. 5. Compensation of pH in arterial blood and CSF during sojourn to 3400-m (closed circles) and 4300-m(open circles)altitude. Mean pH compensation was calculated from mean pH and [HCO;] values.
J. A. ORR
34
et Cd.
Discussion Present
findings
is accompanied
have shown that in healthy, by a compensation
awake ponies,
sojourn
of CSF pH, which is incomplete
to high altitude and for the most
part, parallel to that in arterial blood. The pertinent comparisons between blood and CSF are summarized in figs. 4 and 5. After 1 hr of hypoxia, with mild hypocapnia, the reduction in [HCO,] and hence. degree of pH compensation, in CSF exceeded that in blood. However, as hypocapnia progressed over the ensuing l-5 days of hypoxia, the decrease in [HCO;] and pH compensation in the CSF was equal to (at 3400 m) or less than (at 4300 m) that in arterial blood. Finally, beyond 5 days of sojourn. the Pcoz of the pony rose gradually in blood and CSF. and the pH and [HCO;] of each fluid also returned toward sea-level control values. the rate of return being somewhat greater in blood than CSF at 4300 m. The equivalence of pH compensation observed between blood and CSF suggests that the regulation of CSF [HCO;] during chronic exposure to a relatively wide range of hypoxemic hypocapnia is governed by passive exchange processes. Furthermore, recent evidence obtained during short-term hypocapnia in the anesthetized dog (Pelligrino et ul., 1974) suggests that the close correspondence presently observed in pH compensation between CSF and blood reflects a critical dependence of CSF [HCO;] on corresponding changes in plasma [HCO;]. Obviously the findings on these ponies during chronic hypoxic hypocapnia do not support local ‘CSFspecific’ mechanisms involved in the precise regulation of CSF [HCO;] or pH. However, a clear contribution from local mechanisms to CSF [HCO,] regulation is evident in the acute stages of the hypoxic exposure. The [HCO;] of blood does not change during acute hypoxia, while CSF [HCO;] decreases slightly after l-6 hr at 4300 m. This may be attributed in part to a corresponding increase in CSF lactic acid (table 2) secondary to a selective cerebral hypoxia. Although we are unable to determine if this mechanism continued to effect a reduction in CSF [HCO;] beyond 6 hr of hypoxia, the close correspondence in [HCO;] changes in CSF and blood suggest that increases in cerebral lactic acid had a negligible effect on CSF [HCO;] re g u 1a t’ton during chronic hypoxia. We are confident that the above discussion of CSF acid-base status accurately reflects changes in brain ECF Hf throughout sojourn at high altitude. Fencl er al. (1966) indicated that a steady state between CSF and brain ECF is reached in goats after several days of sustained metabolic alkalosis or acidosis. One could assume the same to be true during chronic hypocapnic hypoxia. As mentioned previously, the data collected from 4 ponies after 6 hours of hypoxia are similar to the results obtained after 1 hour of hypoxia (table 3). This may indicate that a steady state is reached between CSF H ’ and brain ECF H + after only 1 hr of hypoxia. If CSF H ’ is an accurate measure of cerebral ECF H + then it is obvious that the ECF H + of these ponies was imperfectly regulated during chronic hypocapnic hypoxia. As outlined in the introduction, these data on the pony are relevant to two basic problems previously investigated in the human sojourner to high altitude. First, as previously discussed, these data support previous data from our laboratory on man
ACID-BASE
STATUS OF THE PONY DURING HYPOXIA
35
sojourning at 3100 and 4300 m altitude in that pH compensation in CSF is incomplete and does not exceed that in blood during chronic hypocapnic hypoxia. Secondly, these data have limited implications concerning the regulation of respiration during altitude sojourn in man. Before discussing the regulation of respiration in the pony during high-altitude sojourn, the question arises as to whether or not the pony actually exhibits ventilatory acclimatization to high altitude. Two points of evidence support the contention that this animal species shows an increased ventilatory drive during chronic hypoxia. First, similar to man (Forster et al., 1971), these animals exhibited a positive interaction between acute hypercapnia and chronic hypoxia. That is, the slope as well as the extrapolated intercept of the CO, response curves were increased after several days at 3400 m compared to CO* responses tested at 250 m. Secondly, Pacoz decreases in the pony during progression from acute (1 hr) to chronic (1-5 days) hypoxia similar to responses in men taken to similar altitudes (Dempsey et al., 1974; Forster et al., 1975). However while Pa,,, of man remains lowered throughout sojourn, the Pace, of the pony increases after several weeks’ exposure to hypoxia. The transient nature of this response is similar to reports on goats (Lahiri et al., 1971) and Hereford steers (Grover et al., 1963). Although increases in the acute response to hypercapnia during chronic hypoxia and the decrease in Paco2 during the first week at high altitude relative to acute hypoxia support ventilatory acclimatization in the pony, changes in VE do not. In man, decreases in Pacoz during ventilatory acclimatization are associated with . increases m VE. In the pony, Paco2 is lower during the first few days at high altitude than at 250 m, but VE is not statistically elevated above control measurements. In other words, changes in PacOz are poorly correlated with changes in QE. The reason for failure of \j~ to increase in the pony while Pacoz decreases is not known. Although a decrease in tissue CO, production in the pony during altitude sojourn may explain this phenomenon, it seems unlikely since \jco2 does not change in man at high altitude under a constant workload (Lenfant and Sullivan, 1971). Consequently if we assume that \jco2 did not change in these ponies, a decrease in Pace, at a constant \j~ may indicate an increase in VA with a corresponding decrease in VD. Until these data become available on the pony, no conclusions can be made regarding ‘?A in the pony at high altitude. If, however, PacOz changes do reflect increases in VA, the changes in the measured chemical stimuli cannot explain this increase in VA. For example, Pacoz was significantly lower after several days at either elevation compared to the PacOz after 1 hr exposure to the same PIN,. Yet in examining the known chemical stimuli to breathing in either instance, one sees that CSF pH rose, Pao, remained constant and arterial pH either remained constant or decreased slightly (0.03 units at 4300 m). Thus the proposed increase in ventilatory drive at high altitude in the pony cannot be explained by relative decreases in CSF pH and probably not by peripheral chemical stimuli such as Pao2 or pHa. In summary, the pony appears to acclimatize to high altitude in a different way
36
J. A. ORR et a/.
from man. The pony does not increase VE at high altitude but may increase There may be other mechanisms operating with respect to gas exchange in the pony since Paoz remained essentially constant during chronic hypoxia (> 6 days) while Pa,,, was increasing. VA.
Acknowledgements
This study was supported by grants from PHS HL (No. 13154, 15473 and 15469902) U.S. Army Research and Development Command (No. 17-74-C-4020), the American Lung Association, and the College of Agricultural and Life Sciences, University of Wisconsin, Madison. The authors wish to extend thanks to all who provided technical assistance including Mr. Gordon Johnson, Ms. Monique Wanner, Ms. Jean Vaughn, Mr. Michael Madalon, Ms. Chris Alexander, Ms. Rita Leczynski, Mr. John Pope, Ms. Gail Jamieson, Mr. William Bayer and to Ms. Karen Griffin. We are also indebted to Dr. Mario Iona and the University of Denver, Department of Physics who provided the laboratory facilities at Mt. Evans, Colorado and the Climax Molybdenum Mine for their assistance at Climax, Colorado. References Barker.
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