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THE RESPIRATION OF THE HORSE UNDER DIFFERENT ANAESTHETIC MEDICATIONS
112. THE RESPIRATION OF THE HORSE UNDER DIFFERENT ANAESTHETIC MEDICATIONS
U.
Schatzmann
Klinik fur Nutztiere und Pferde, Universitat Bern, Langgass-Strasse 124, CH-3012 Bern, Switzerland.
It is well recognised that general anaesthesia in the horse results in high alveolar-arterial oxygen tension differences with dangerously low arterial tensions unless the inhaled air is enriched with oxygen. Previous investigations give evidence that hypoxia is due mainly to the change from the upright to the lateral position, and that the depressive effect of the anaesthetic agent on the respiratory centre is not the main cause of the phenomenon. It was therefore interesting to evaluate and compare the effect of different anaesthetics on respiration, and to stUdy the influence of various parameters (time, bOdyweight, blood pressure etc.) on the alveolar-arterial tension difference. The experiments were carried out in healthy adult horses during routine surgical anaesthesia after sedative premedication (propiopromazine* 10 mg/kg and I-polamidone** 2.5 mg/100 kg) and induction with guaifenesine (80.8 mg/kg). The horses were intubated and the facial artery was cannulated. The following anaesthetics were used: 1.
Chloral hydrate
15 g/lOO kg + 1 repeat dose
2.
Thiopentone
to effect for superficial surgical anaesthesia (with repeat doses)
3.
Halothane/room air
to effect for superficial surgical anaesthesia
*
**
Combelen R POlamivet R
Bayer Hoechst
113.
4.
Halothane/oxygen
to effect for superficial surgical anaesthesia
5.
Halothane/02/N20 (1:1)
to effect for superficial surgical anaesthesia
Arterial oxyaen and carbon dioxide tensions (Pa02, PaC02) Hypoxaemia was observed in all horses breathing room air with dangerously low levels in the animals anaesthetised with halothane without enrichment with oxygen (Table 1, Figure 1, Figure 2). horse in this group died during the experiment.
One
No difference was
calculated between the group under chloral hydrate and the group under thiopentone. The corresponding carbon dioxide tensions are listed (Table 2, Figure 1, Figure 2).
Very low levels, indicating hyperventilation,
were measured under halothane/room air.
The fact that ,no carbon dioxide
retention was observed whenever hypoxia was present (Table 2), or that hypercapnia is always observed together with high arterial oxygen pressures, may indicate that hypoxia is responsible for increased ventilation under room air inhalation. Alveolar-arterial tension differences (CP(A-a)02,
~P(a-A)C02)
Alveolar-arterial tension differences were measured and calculated in the horses under halothane/room air (4 animals) and under chloral hydrate (24 animals).
End-tidal (= alveolar) gases were collected
through a catheter placed in the trachea. In the group under chloral hydrate the pressure differences were correlated with the bodyweight, the arterial carbon dioxide tension systOlic, mean and diastolic blood pressure and the age of the horse using the least square method. Oxygen and carbon dioxide tensions, and alveolar-arterial tension differences are listed in Table 3 and Figure 3.
114. It can be demonstrated that the increased alveolar-arterial difference in oxygen tension occurs as soon as lateral recumbency is achieved (corresponding values of the standing horse 16.0~SD9,~
P(a-A)C02
4.6~1.8
rom Hg).
~
P(A-a)02
The oxygen differences are
higher in the horses under halothane/room air compared with those under chloral hydrate; the overall ventilation was increased in the halothane/room air group (statistically significant lower PaC02 pressures).
The tension difference and therefore the oxygenation
appears not to be influenced by increased ventilation with room air. These findings support our previous suspicion that the disturbances in oxygenation generally observed under inhalation anaesthesia are not greatly influenced by increasing the respiratory volume (with artificial ventilation) • The previous findings that the tension differences remain relatively unaltered during the period of lateral recumbency could be confirmed.
In hypoxia and hypercapnia, however, a statistically
significant increase could be calculated (Table 3). In Table 4 correlation coefficients between oxygen tension differences and various parameters are summarised.
With the exception
of the bodyweight neither alterations in blood pressure nor PaC02 nor the age of the horse appears to be correlated.
It must therefore be
assumed that alveolar-arterial tension differences are not influenced or caused by the decreased systemic blood pressure under general anaesthesia. and~
The lack of a significant correlation between PaC02
P(A-a)02 is another indicator that the difference is not
influenced by increasing or decreasing the ventilation.
115. Conclusions All anaesthetic procedures with the horse in lateral position without enrichment of inspiratory air with oxygen lead to arterial hypoxaemia of a significant degree. In the adult horse hypoxaemia is mainly due to the lateral position.
Oxygenation is further decreased by the anaesthetic.
Hypercapnia is only observed when high arterial oxygen pressures are measured. Hyperventilation under room air ventilation indicates dangerously low oxygen tensions.
Death may occur.
Hypoxaemia (and increased alveolar-arterial oxygen pressure differences) are generally not influenced by increasing the ventilation (neither under spontaneous nor artificial respiration). Alveolar-arterial oxygen pressure differences are statistically not correlated with systemic blood pressure or PaC02'
116.
- ...
T.-efT'OtE (n • • H~)
-
5
ZO
10
2S
30
315
.,g
...
Mean oxygen and carbon dioxide tensions
FIGURE 1
during thiopentone anaesthesia (witn standard deviations) mrnHg 70
Halothane/room air
60
!SO
40
30
40
30
20
-
10
20
30
40
50
"*'
Alterations in oxygen and carbon dioxide tensions under influence of halothane
117.
70
eo !IO
30
zo 10
FIGURE 3
Alveolar-arterial oxygen and carbon dioxide tension
differences in 4 adult
TABLE 1
horses in lateral positi.on under halothane/room air.
Oxygen tensions (mrn Hg) during different anaesthetic procedures in adult horses (with .standard deviations) Number
10'
20'
30'
40'
50'
58(9)
62(11)
62(11)
64(3)
62(3)
60'
.:"'naesthetic
of horses
Chloral hydrate
n = 24
71 (0)
60(7)
5B(8)
Thiopentone
n = 6
82(16)
59(13)
56(5)
Halothane/ room a i r e
n
4
62
46
44
38·
42
Ha1othane!02
n =
6
351 (69)
339 (95)
324 (136)
288 (130)
Halothane! °2!N20 (1.1)
n •
6
141 (64)
142 (64)
129 (48)
122 (40)
*
Lateral
no standard deviations, limited number of horses
40
41
118. Mean carbon dioxide tensions with standard deviations under dif:erent anaesthetic procedures in the horse
Pa c02
(nun Hg)
Chloral hydrate
39 SO :': 2,6
Thiopentone
41 SO
Halothane/room air
33 SO :': 3,9
Halotha.le/axygen
61 SO :': 13.9
Halothane/nitrous oxide/oxygen (1:1)
63 SO :': 5,6
z 1,1
Mean oxygen and carbon dioxide tensions in arterial blood and
correspondi~g
alveolar-arterial tension differences during lateral recumbency under chloral hydrate (with standard deviatlons).
20'
25'
3D'
40'
Lateral
5'
10 '
15'
Pa02
71.!.lO*
63:':8
60:':7
58:':8
58:,:8-
58:':9
58:':9
58.!.lO
62.:'.11
Pa C02
34.!.4*·
35:':8
38:':4
39:':4
40:':3
40~3·*
41:':4
411:.4
41:':4
34:':13
34:':13
38.:'.14
38:':14
41:':15
39:':18
42:':17
36:':11
7:':4
7:':4
8:':5
8:':5
11:':4
7:':4
10:':5
10:':3
6 P ( A- a ) 02 30:':15 L::J'(a-A)C02
7:':3
35'
the difference is statistically significant
Correlations with alveOlar-arterial
tension
differences Correlation coe:ficient
Bodyweight (l
0.54 (almost significant
PaC02 (nun Hg)/6P(A_a)02
0,15 not significant
Systolic blOOd pressure (mm Mean blood pressure (mm
Hg)/~P(A-a)02
Hg)/~P(A-a)02
0,10 not significant 0.40 not significant
Systolic blOod pressure
/6 P (a - A) C0 2
0.15 not significant
Age of the horse
/6 P(A-a)02
0.27 not significant
p
-c 0,05)
U.
Schatzmann
Klinik fur Nutztiere und Pferde, Universitat Bern, Langgass-Strasse 124, CH-3012 Bern, Switzerland.
It is well recognised that general anaesthesia in the horse results in high alveolar-arterial oxygen tension differences with dangerously low arterial tensions unless the inhaled air is enriched with oxygen. Previous investigations give evidence that hypoxia is due mainly to the change from the upright to the lateral position, and that the depressive effect of the anaesthetic agent on the respiratory centre is not the main cause of the phenomenon. It was therefore interesting to evaluate and compare the effect of different anaesthetics on respiration, and to stUdy the influence of various parameters (time, bOdyweight, blood pressure etc.) on the alveolar-arterial tension difference. The experiments were carried out in healthy adult horses during routine surgical anaesthesia after sedative premedication (propiopromazine* 10 mg/kg and I-polamidone** 2.5 mg/100 kg) and induction with guaifenesine (80.8 mg/kg). The horses were intubated and the facial artery was cannulated. The following anaesthetics were used: 1.
Chloral hydrate
15 g/lOO kg + 1 repeat dose
2.
Thiopentone
to effect for superficial surgical anaesthesia (with repeat doses)
3.
Halothane/room air
to effect for superficial surgical anaesthesia
*
**
Combelen R POlamivet R
Bayer Hoechst
113.
4.
Halothane/oxygen
to effect for superficial surgical anaesthesia
5.
Halothane/02/N20 (1:1)
to effect for superficial surgical anaesthesia
Arterial oxyaen and carbon dioxide tensions (Pa02, PaC02) Hypoxaemia was observed in all horses breathing room air with dangerously low levels in the animals anaesthetised with halothane without enrichment with oxygen (Table 1, Figure 1, Figure 2). horse in this group died during the experiment.
One
No difference was
calculated between the group under chloral hydrate and the group under thiopentone. The corresponding carbon dioxide tensions are listed (Table 2, Figure 1, Figure 2).
Very low levels, indicating hyperventilation,
were measured under halothane/room air.
The fact that ,no carbon dioxide
retention was observed whenever hypoxia was present (Table 2), or that hypercapnia is always observed together with high arterial oxygen pressures, may indicate that hypoxia is responsible for increased ventilation under room air inhalation. Alveolar-arterial tension differences (CP(A-a)02,
~P(a-A)C02)
Alveolar-arterial tension differences were measured and calculated in the horses under halothane/room air (4 animals) and under chloral hydrate (24 animals).
End-tidal (= alveolar) gases were collected
through a catheter placed in the trachea. In the group under chloral hydrate the pressure differences were correlated with the bodyweight, the arterial carbon dioxide tension systOlic, mean and diastolic blood pressure and the age of the horse using the least square method. Oxygen and carbon dioxide tensions, and alveolar-arterial tension differences are listed in Table 3 and Figure 3.
114. It can be demonstrated that the increased alveolar-arterial difference in oxygen tension occurs as soon as lateral recumbency is achieved (corresponding values of the standing horse 16.0~SD9,~
P(a-A)C02
4.6~1.8
rom Hg).
~
P(A-a)02
The oxygen differences are
higher in the horses under halothane/room air compared with those under chloral hydrate; the overall ventilation was increased in the halothane/room air group (statistically significant lower PaC02 pressures).
The tension difference and therefore the oxygenation
appears not to be influenced by increased ventilation with room air. These findings support our previous suspicion that the disturbances in oxygenation generally observed under inhalation anaesthesia are not greatly influenced by increasing the respiratory volume (with artificial ventilation) • The previous findings that the tension differences remain relatively unaltered during the period of lateral recumbency could be confirmed.
In hypoxia and hypercapnia, however, a statistically
significant increase could be calculated (Table 3). In Table 4 correlation coefficients between oxygen tension differences and various parameters are summarised.
With the exception
of the bodyweight neither alterations in blood pressure nor PaC02 nor the age of the horse appears to be correlated.
It must therefore be
assumed that alveolar-arterial tension differences are not influenced or caused by the decreased systemic blood pressure under general anaesthesia. and~
The lack of a significant correlation between PaC02
P(A-a)02 is another indicator that the difference is not
influenced by increasing or decreasing the ventilation.
115. Conclusions All anaesthetic procedures with the horse in lateral position without enrichment of inspiratory air with oxygen lead to arterial hypoxaemia of a significant degree. In the adult horse hypoxaemia is mainly due to the lateral position.
Oxygenation is further decreased by the anaesthetic.
Hypercapnia is only observed when high arterial oxygen pressures are measured. Hyperventilation under room air ventilation indicates dangerously low oxygen tensions.
Death may occur.
Hypoxaemia (and increased alveolar-arterial oxygen pressure differences) are generally not influenced by increasing the ventilation (neither under spontaneous nor artificial respiration). Alveolar-arterial oxygen pressure differences are statistically not correlated with systemic blood pressure or PaC02'
116.
- ...
T.-efT'OtE (n • • H~)
-
5
ZO
10
2S
30
315
.,g
...
Mean oxygen and carbon dioxide tensions
FIGURE 1
during thiopentone anaesthesia (witn standard deviations) mrnHg 70
Halothane/room air
60
!SO
40
30
40
30
20
-
10
20
30
40
50
"*'
Alterations in oxygen and carbon dioxide tensions under influence of halothane
117.
70
eo !IO
30
zo 10
FIGURE 3
Alveolar-arterial oxygen and carbon dioxide tension
differences in 4 adult
TABLE 1
horses in lateral positi.on under halothane/room air.
Oxygen tensions (mrn Hg) during different anaesthetic procedures in adult horses (with .standard deviations) Number
10'
20'
30'
40'
50'
58(9)
62(11)
62(11)
64(3)
62(3)
60'
.:"'naesthetic
of horses
Chloral hydrate
n = 24
71 (0)
60(7)
5B(8)
Thiopentone
n = 6
82(16)
59(13)
56(5)
Halothane/ room a i r e
n
4
62
46
44
38·
42
Ha1othane!02
n =
6
351 (69)
339 (95)
324 (136)
288 (130)
Halothane! °2!N20 (1.1)
n •
6
141 (64)
142 (64)
129 (48)
122 (40)
*
Lateral
no standard deviations, limited number of horses
40
41
118. Mean carbon dioxide tensions with standard deviations under dif:erent anaesthetic procedures in the horse
Pa c02
(nun Hg)
Chloral hydrate
39 SO :': 2,6
Thiopentone
41 SO
Halothane/room air
33 SO :': 3,9
Halotha.le/axygen
61 SO :': 13.9
Halothane/nitrous oxide/oxygen (1:1)
63 SO :': 5,6
z 1,1
Mean oxygen and carbon dioxide tensions in arterial blood and
correspondi~g
alveolar-arterial tension differences during lateral recumbency under chloral hydrate (with standard deviatlons).
20'
25'
3D'
40'
Lateral
5'
10 '
15'
Pa02
71.!.lO*
63:':8
60:':7
58:':8
58:,:8-
58:':9
58:':9
58.!.lO
62.:'.11
Pa C02
34.!.4*·
35:':8
38:':4
39:':4
40:':3
40~3·*
41:':4
411:.4
41:':4
34:':13
34:':13
38.:'.14
38:':14
41:':15
39:':18
42:':17
36:':11
7:':4
7:':4
8:':5
8:':5
11:':4
7:':4
10:':5
10:':3
6 P ( A- a ) 02 30:':15 L::J'(a-A)C02
7:':3
35'
the difference is statistically significant
Correlations with alveOlar-arterial
tension
differences Correlation coe:ficient
Bodyweight (l
0.54 (almost significant
PaC02 (nun Hg)/6P(A_a)02
0,15 not significant
Systolic blOOd pressure (mm Mean blood pressure (mm
Hg)/~P(A-a)02
Hg)/~P(A-a)02
0,10 not significant 0.40 not significant
Systolic blOod pressure
/6 P (a - A) C0 2
0.15 not significant
Age of the horse
/6 P(A-a)02
0.27 not significant
p
-c 0,05)