tidal volume ratio during exercise in burned children

BumsVol. 21, No. 5, pp. 337-339,1995 Copyright 0 1995 Elsevier Science Ltd for ISBI Printed in Great Britain. All rights reserved 0305-4179/95

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0305-4179(94)00017-4

Increased physiological ratio during exercise

dead space/tidal in burned children

R. P. Mlcak, M. H. Desai, E. Robinson,

R. L. McCauley,

J. Richardson

volume

and D. N. Herndon

Shriners Burns Institute, Galveston, Texas, USA

Exercise testing enables the simultaneous evaluation of fhe cardiovascular and respiratory systems ability fo perform gas exchange. The physiological responses to exercise have not been previously reported in the posfburn child. This investigation was designed to evaluate residual cardiopulmona y impairment in patients convalescing from severe bums. Spiromefy, lung volumes and exercise sfress testing were completed on 40 children with a mean time postburn injury of 2.6f 1.9 years and mean burn size of 443122 per cent TBSA. Respiratory variables studied during exercise included expired volume, tidal volume and respiratory rate, and physiological dead space/tidal volume (VD/ VT) ratios. Stress testing revealed an increased VD/ VT ratio consistent wifh uneven ventilation-perfusion relationships. The data indicafe that pafienfs who survive thermal injuy may not regain normal cardiopulmonary homeosfasis.

Burns, Vol. 21, No. 5,337-339,1995

Introduction Exercise testing enablesthe simultaneousevaluation of the cardiovascular and respiratory systems’ability to perform gas exchange.‘,2. Metabolic and physiological changes occurring asa result of thermal injury may alter the normal gas exchange precess.Gas exchange may be imparied for several reasons:(I) altered lung mechanics,(2) impairment of the alveolar capillary membrane,(‘3)chest wall deformities, or by (4) respiratory muscleweakness3.The exercise endurance levels after thermal injury may be affected by the impaired gasexchange process.There are no studiesof the exercise gas exchange process in children following thermal injury. This investigation was designedaspart of a longitudinal assessmentto evaluate the exercise gas exchange process in children convalescing from thermal injury.

Methods Forty children convalescing from thermal injury were given resting pulmonary function studies (PFTs) and incremental cardiopulmonary stresstests. Three groups of patients with TBSA thermal burns between 15 and 20 per cent, 35 and 50 per cent and > 70 per cent, treated with excisional therapy, were studied. Patients were evaluated

at prescheduledoutpatient clinic visits. At the time of the study, none of the patients had signsof sepsis,pneumonia, pulmonary oedema or any other known acute lung problems. Resting PFT variables included Forced Vital Capacities (FVC), l-second Forced Expiratory Volume (FEV,), forced expiratory flow rate between 25 and 75 per cent of the FVC (FEF,,-,,), FEV,/FVC ratio expressedas a percentage (FEV,/FVC per cent), Peak Expiratory Flow (PEF), Maximum Voluntary Ventilation (MW), Vital Capacity (VC), Total Lung Capacity (TLC), ResidualVolume (RV), Functional Residual Capacity (FRC) and Diffusion Studies (DLCO). All pulmonary function measurements were made with the Horizon MMC 4400 utilizing a digital volume transducer (Sensormedics,Anaheim, CA, USA). Lung volumes and flows were corrected to body temperature and pressure,saturated with water. No spirograms were recorded until the patient had practised at least two forced maximal exhalations and inhalations. At least three satisfactory spirogramswere recorded for eachpatient. All patients were sitting upright at the time of the study. Spirometry was done consistently by one individual and the testing procedure in accordance with the American Thoracic Society standard?. Normal predictive values were obtained from the work of Knudsor?. Cardiopulmonary stress testing variables included: expired volume (VE), tidal volume (VT), respiratory rate (RR), tidal volume/dead space ratio (VD/VT), work rate (WR), 0, consumption (VO,), respiratory quotient (R), heart rate (HR), oxygen pulse (0, pulse),expired volume/ expired volume/ 02 consumption ratio (VE/VO,), maximum voluntary ventilation ratio (VD/MW). Cardiopulmonary variables were collected and analysed with the Horizon MMC 4400 utilizing the breath to breath technique.The Bruceprotocol wasutilized for the incremental stresstest in which the speedand grade increasesevery 3 min6. The subjectswere askedto exercise on the Quinton treadmill for as long as possible. The electrocardiogram was continuously monitored and recorded by the Horizon ECG system (Sensormedics,Anaheim, CA, USA). All gas analysers were calibrated before the test according to manufacturers’ specifications. Baselinehistory and physical examinations along with resting EKGs were performed prior to the exercise test. Informed consent was obtained for all subjects.

Burns: Vol. 21, No. 5, 1995

338

Results

spirometric measurementsconsistent with an obstruc Gve diseasepattern. Flow volume loops were considered to be consistent with an obstructive diseasepattern when both a concave configuration of the FVC curve suggesting diminishing flow rates and decreased FE% and FEF,. i were present. Additionally, an obstructive &ease pattern was confirmed by an increasedRV and ‘I’i.C (Tnbi~ Ii!j. .‘\ combined obstructive/restrictive disease pattern I\‘;lS found in 7 per cent of the patients with inhalation ~~!urv and in 4 per cent of the patients without. This vu’a~ confirmed by a decreasedVC, increasedRV and r~mal ‘TLC. A purely restrictive diseasepattern was preseni ii-i _’1 per cent of the patients with inhalation injury. Paticrlts with restrictive diseasehad a convex configuration of the flo\\, volume loop along with a decreasedVC and TLC Based upon tests of resting spirometry and iung volumes, 64 per cent of the patients with Inhalation In!ury and 27 per cent of the patients withoui !i percentagesoi the predicted indicated increased VE, RR and VD~V7‘ ratios for both groups (?ahlti V!. The resprratory rate was the only variable statistically significank for the two groups. Both groups reached the same endurance ievei ir! the Bruce protocol, as shown by the stage achieved. The patients who had had an inhalation injq; did so with a statistically greater increase in respiratorv rate when compared to the non-inhalation injury group. This indrcates that the patients who had had an inhalation injury were able to exercise as long as the patients who had had no inhalation injury. However, compensationwas made up by an increased respiratory rate. Metabolic and cardiac stressvariables aspercentages of those predicted for both groups indicated some respiratory limitations to exercise (Table VI). This is confirmed by a decreased VOL n,ar, decreasedHR and a decreased0, pulse. had

A total of 40 thermally injured patients were studied, 14 with bronchoscopic evidence of inhalation injury and 26 without. Demographic data (Table I) indicate no significant difference between the two groups. A summary of resting spirometric measurementsin percentagesof those predicted is presented in TableII. Thirtysix per cent of the thermally injured patients with inhalation injury and 23 per cent of the non-inhalation injury patients

Table I. Demographics Inhalation injuries (n = 14)

Characteristics Age W Sex Race* Weight (kg) Height (cm) % TBSA % 3RD”’ PBD tested (yr) “C, Caucasian; ‘Full thickness

Non-inhalation Injuries (n = 26) .--~-

11+3 lOM.4F lOC,2H,2B 40+13 145rt21 51*22 3Ort27 1.9rko.9

12zt5 19M.7F 19C.l H.68 47+23 149f23 40*21 25f21 2.3 f 0.2

H, Hispanic; B, Black. skin injury as percentage

Table II. Restingspirometry measurements: percentageof pre-

dicted Inhalation inpries (n = 14)

Test FVC FEV, FEF,,,, FEV,/FVC% PEF MVV

Non-inhalation injuries (n = 26) -

95&16 95+14 88+21 96zk8 88zt20 79ztz18

92fll 92fl2 go*23 97zk-5 91*20 84kll

Discussion Table III. Resting lung volume

percentageof

measurements:

predicted Inhalation injuries (n = 14)

Test vc TLC RV FRC DLCO *Pi

Non inhalation injuries (n = 26) -____

101*13 92*23 106x+44 87f35 123*22‘

98*12 102*12 122f37 97+20 104+17

0.05.

Exercise testing enablesthe simultaneousevaluation of the cardiovascular and respiratory systems’ ability to perform their major function, gas exchange. Breath by breath measurements during the incremental exercise testing enable the examiner to: (I) titrate the level of the subjects’ exercise limitation, (2) titrate the adequacy of the performance of various components in the external-internal gas exchange coupling, and (3) determine the organ system limiting exercise performance’. Many disorders interfere with the normal metaboliccardiovascular-ventilatory coupling needed for gas

Restingpulmonary function results

Table IV.

Non-inhalation injuries (n = 26)

Inhalation injuries (n = 14) No.

Interpretation WNL Obstructive Combined Restrictive

5 5 1 3

obst./rest.

Total WNL=

% 36 36 7 21 64 per cent abnormal

within

normal

limits.

No. --_ _-.--

__.

-_

19 6

--~-

% .__ 73 23 4

1

27 per cent abnormal

Mlcak

et al.: Physiological

response

to exercise

in burned

children

Table V. Pulmonary stressvariables:percentageof predicted

(maximum) Test

Inhalation injuries (n = 14)

VE VT

124f95 91 f35

RR

140*52* 259 f 54 3.1 f 0.8

W/VT Stage achieved

#on-inhalation &juries (n

= 26)

961t29 101 f50 lOOk48 239+34 3.1 * 0.7

‘P
Table VI. Metabolic andcardiacstressvariables:percentageof

predicted(maximum) Inhalation injuries (n

Test

WR VO, (ml/min) VO, R

(kg)

HR 0, PULSE

= 14)

Non-inhalation injuries (n

13Ok-34 74*22 96+22

135*20 73*23 86k28

112&31 89+10 89+31

110+31 89+5

= 26)

83zk28

These include primary disorders of red blood cell production, the peripheral circulation, the heart, the pulmonary circulation, the lungs, the chest wall, respiratory control and metabolism. Individually, or in combinations, these may alter the gas exchange process. Alveolar ventilation is the theoretical ventilation required to eliminate the CO, produced by metabolismthat would result in a given arterial CO, tension. The physiological dead spaceventilation is the difference between the minute ventilation and the alveolar ventilation. A valuable estimate of the degree of matching of ventilation to perfusion during exercise is the physiological dead space/ tidal volume ratio (VD/VT). The VD/VT is lowest when alveolar ventilation relative to perfusion is uniform. At rest, the physiological deadspacevolume is normally about one-third of the breath. During exercise,it is reduced to about one-fifth of the breath, or even less. In patients with pulmonary disorders in whom ventilation-perfusion relationships are uneven, the VD/VT is increased at rest and fails to decreasenormally during exercise. The VD/VT is a valuable measurement becauseit is typically abnormal in patients with primary pulmonary vascular diseaseor pulmonary vascular diseasesecondary to obstructive or restrictive lung disease.It is sometimes the only gas exchange abnormality evident during exercisetesting. The VD/VT may be only slightly elevated at rest but remainsunchanged, or even increasingrather than

339

decreasing during exercises. Thus, exercise makes the abnormality in ventilation-perfusion mismatching more evident. When this occurs, it is an indication of a ventilatory rather than a cardiac limitation to exercise7.

Conclusions The cardiovascular and respiratory systems’ ability to perform gas exchange in children convalescing from thermal injury has not been previously reported. Metabolic and physiological changes occurring as a result of thermal injury may alter the gas exchange processduring exercise. Late spirometric examination indicates abnormal lung function at rest, predominately obstructive and restrictive diseasepatterns. Cardiopulmonary stress test showed evidence of gas exchange abnormalities. This was confirmed by a decreasedmaximum heart rate, decreased vo 2 max,increasedrespiratory rate and increasedVD/VT ratios. An increased VD/VT ratio during exercise is an indication of a ventilatory rather than a cardiac limitation to exercise.Our resultssuggestthat the cardiovascular and respiratory systems’ ability to perform gas exchange during exercise may not return to normal in children convalescing from thermal injury. Further testing is underway to continue to monitor the gas exchange status following thermal injury in children.

exchange.

References 1 WassermanK, Whipp B. Exercisephysiology in health and disease. Am Rev Resp Dis 1975; 113: 237-245. 2 Wasserman K, HansenJ,SueD, Whipp B.Principles of exercising tesfing and inferpretafions. 1987; 1-6, 37-38. 3 Casey CL, Weber TC. Cardiopulmonary exercise testing physiologic principles and clinical applications. 1988; 16: 272-300. 4 ATS Statement, Snowbird Workshop on Standardizationof Spirometry. Am Rev Resp Dis 1979; 119: 831-837. 5 Knudson RJ, Slatin R, Lebowitz D. The maximal expiratory

flow volume loop curve, normal standards,variability and effectsof age.Am Rev Resp Dis 1976; 113: 587-600. 6 CummingsGR,Everatt D, HastmanL. Brucetreadmilltest in children:normalvaluesin a clinic population.Am ] Cardiol 1978; 41: 69.

7 JonesNL. Pulmonarygasexchangeduringexercisein patients with chronicairway obstruction.Clirz Sci 1966; 31: 39-50. Paper accepted 29 September 1994. Correspondence should be addressed to: Dr R. P. Mlcak, Shriners Burns Institute, 815 Market Street, Galveston, TX 77550, USA.