The Work of Breathing: Potential for Clinical Application and the Results of Studies Performed on 100 Normal Males

T. V. N. BALLANTNE, LT., MC, USNR,* H. J. PRoCrOR, LCDR., MC, USNR,** N. D. BROUSSARD, LCDR., MC, USN,*** B. D. LrrT, M.A.t From the Division of Surgical Research, Station Hospital, Naval Support Activity, Da Nang, Republic of Vietnam and U. S. Naval Medical Research Institute, Bethesda, Maryland 20014

THERE is increasing interest in the re- more useful index of pulmonary function. sponse of the lungs in patients sustaining The time required and the complexity of hemorrhagic shock and massive, nontho- the equipment used by previous investi- racic, trauma.9' 11, 12, 16, 24 gators 2, 13, 18, 20, 21 led us to evaluate a spe- The routine postoperative assessment of cial purpose, on-line, analogue computer pulmonary function in such patients is fre- to calculate the work of breathing. This quently left to "clinical judgment" which is evaluation was a preliminary step under- often inadequate. Respiratory rate is the taken prior to a study of pulmonary func- only quantitative measure of pulmonary tion in massively injured patients.28 function which is widely recorded. Most Normal subjects were selected initially currently used measures of pulmonary for evaluation of the equipment, for deter- function are based on calculations made mining the practicality of its use in a clini- with one or two observations during a res- cal setting, and for comparing the values piratory cycle. To compute the work of obtained with those previously obtained breathing, continuous observations of flow using other technics.4 10, 20, 25 and pressure from an entire respiratory cycle are required. Hence the work of Materials and Methods breathing might be a less variable and The subjects for study were 100 healthy, ambulatory men between the ages of 18 Submitted for publication June 30, 1969. * Division of Surgical Research, Box 40, NSA and 26. All subjects were studied with Da Nang, FPO San Francisco 96695. their chests elevated 30 degrees from the ** Division of Experimental Surgery, U. S. horizontal. They were allowed to adjust to Naval Medical Research Institute, Bethesda, the respiratory apparatus for 15 minutes Maryland 20014. before measurements were recorded, but *** Department of Surgery, Portsmouth Naval Hospital, Portsmouth, Virginia 23708. were otherwise untrained. f Naval Medical Data Service Center, National Transpulmonary pressure was measured Naval Medical Center, Bethesda, Maryland 20014. by a Stratham No. PM-131-TC differential From the Bureau of Medicine and Surgery, pressure transducer, one limb of which was Navy Department, Research Task No. M4305.05- connected by a length of PE 200 tubing 3007. The opinions or assertions contained herein to a 15 cm. long by 2 cm. circumference are the private ones of the authors and are not to latex balloon * positioned in the mid- be construed as official or reflecting the views of the Navy Department or the Naval service at * Instrumentation Associates, 17 W. 60th St., large. New York, N. Y. 10023. 590 Volume 171 WORK OF BREATHING Number 4 THE 591

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FIG. 1. Simultaneous E tracing of air flow and transpulmonary pressure prior to any work done on the incoming signals by the computer.

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esophagus to approximate intrapleural pres- By noting the change in transpulmonary sure.14, 17, 22 Prior to pressure measurements pressure associated with the change in vol- the latex balloon was inflated gently and ume at beginning and end inspiration, dy- allowed to collapse passively before con- namic compliance was calculated. necting to the transducer to eliminate elas- C = AV/AP tic tension in the wall of the balloon. The other limb of the transducer was connected The computer integrated the product of to the side of a rubber mouthpiece t by flow multiplied by transpulmonary pres- a blunt 15 gauge needle to measure oral sure over a respiratory cycle, pressure. rc Airflow was measured using a No. 1 W =JP(t)*V(t) dt Fleisch pneumotachograph connected to a Sanborn No. 270 transducer. where V is flow, P is transpulmonary pres- A nose clip was applied to the subjects' sure, and the integration is carried out for nose during the recording of measurements a period c, corresponding to one respira- to insure that all respired air passed This was recorded, and is re- through the pneumotachograph. tory cycle. as of a breath The analogue voltages produced by the ferred to here the work transducers were then recorded on a San- (Fig. 3). born two channel recorder (7712) giving The result recorded for each patient was a display of airflow and transpulmonary the mean of five respiratory cycles. Prior pressure (Fig. 1). A special purpose ana- to each series of measurements the com- logue computer * similar to that described puter was calibrated with known airflow by Peters,27 interfaced with the recorder, and known pressures. After correction of integrated air flow over the course of each gas flow for body temperature and water respiratory cycle to give a tracing of tidal vapor pressure the accuracy of the calibra- This was simultaneously volume. displayed tion procedure was within 5%o. Neglecting with the transpulmonary pressure (Fig. 2). inertial forces, the major forces to be over- f Warren E. Collins, Inc., 220 Wood Road, come during inspiration are tissue elasticity Braintree, Mass. Cat. No. P-357. and flow resistive forces.5 19,25,26 Since I SHP Electronics, Inc., No. 1 Mt. Bolus Rd., Chapel Hill, North Carolina. work used to overcome elastic forces dur- BALLANTINE, PROCTOR, BROUSSARD AND LITT Annals of Surgery 592 April 1970 tion (r2 values) were below 10%o in all cases. The resulting predictive equations, X = aMb, offered no improvement over an estimate made solely on a 1:1 ratio using weight, i.e., without transforming the val- ues to logarithms. Discussion Previous data of the work of breath- ing5"13 20, 25 have either been obtained from small populations of patients or have been reported from studies of trained, upright subjects. To use such data to evaluate an ill patient who is untrained and frequently unable to sit up does not seem reasonable. FIG. 2. Simultaneous tracing of tidal volume Since the data in this report resulted from and transpulmonary pressure. Sample lines depict- ing method of obtaining pressure change for a study of numerous untrained subjects, given volume have been drawn. measured in the semi-supine position, we feel their application to a clinical situation ing inspiration is available as stored en- is more meaningful. ergy for expiration (normally passive), the Our mean values for tidal volume and computer was able to furnish an estimate respiratory rate compare closely with those of work used to overcome flow resistive generally considered to be normal.10 The forces by noting the difference between the observed mean value for compliance, 0.114 work required for inspiration and that re- liters/cm. H2O, is less than that reported covered during expiration (Fig. 3). It by Chiang8 (p = 0.02) for supine indi- should be kept in mind that a small per- viduals. This decrease possibly is caused centage of the energy lost is due to heat by the different technic required to posi- loss, inertia of the system, and continued tion the esophageal balloon utilized by contraction of inspiratory muscles during Chiang. No significant difference was noted expiration.2 For a more detailed discussion of this subject the reader is referred to the work of Otis,26 Butler,5 and Cain.6 Results A f v \ ; S K The mean values (x), standard devi- ations (6), and number of samples (n) are WORK listed in Table 1. Data for height, weight and surface area were lost in some sub- jects. Work/liter, both total and resistive, was obtained by dividing the work/breath 0i by the tidal volume. U To facilitate the application of our data VOLUME to other populations the data were reduced to a height and weight independent form FIG. 3. Simultaneous tracing of tidal volume through the use of allometry as described and the work of the breath. Stylus re-zeroes at end of each breath. This distance represents work by Stahl.29 The results of these analyses are not recovered during expiration and is referred not shown, as the coefficients of determina- to as resistive work. Volume 171 THE WORK OF BREATHING Number 4 593 between our observed value for compli- TABLE 1. Mean and One Standard Deviation for ance and the values listed by Attinger3 Measurements Made on 100 Young, Adult Males (p > 0.18). Absence of sample number and standard deviations precludes com- Parameter X 6 n parison with the data of Comroe et al.,10 Age 20.8 2.8 83 although his mean of 0.200 liters/cm. H20 Height (cm.) 178.8 6.2 84 is higher than our mean value. Body weight (Kg.) 74.2 8.8 83 After conversion to similar units, vari- Body surface area (m2) 1.9 0.1 83 Respiratory rate (b.p.m.) 15.8 4.9 100 ations in our mean computed values for the Tidal volume (cc.) 559.6 221.8 100 work of breathing do not significantly differ Minute volume (1.) 8.9 2.8 100 from the values reported by Fritts et al.13 Compliance (1/cm. H20) 0.114 0.047 100 Total work/breath (Kg.) 0.028 0.018 100 (p = 0.2), and are nearly identical to the Resistive work/breath (Kg.) 0.011 0.009 100 mean values quoted by Collins et al.,10 Mc- Total work/I. (Kg.) 0.048 0.022 100 Ilroy et al.,20 and Brownlee and Allbritten.4 Resistive work/I. (Kg.) 0.019 0.012 100 Total work/minute (Kg.) 0.442 0.274 100 Although our mean value is lower than Resistive work/min. (Kg.) 0.174 0.137 100 that reported by Otis et al.25 (p = 0.02), % resistive work 40.3 15.6 100 the discrepancy in sample number (100 vs. 3) and differences in populations makes interpretation of this difference difficult. equipment enables serial or continuous Although we used untrained subjects, measurement which avails sufficient res- our standard deviations were in the same piratory effort to undergo analysis. order of magnitude as reported by oth- The amount of effort expended by the ers.4' 20, 25 We feel the low r2 values deter- patient to achieve adequate alveolar ven- mined from the regression analysis are tilation is an issue of clinical interest. Cau- caused by the small variation (±3.5%) in tion should be used in equating pulmonary mean subject height, and in the small vari- effort, when computed by the method re- ation (±+11.9%) in the mean subject ported with metabolic effort on the part weight, and are not a reflection of the of the subject. Depending upon the level standard deviation. of respiration with respect to the relaxation The concept of using the work of breath- volume of the lung, pulmonary effort may ing as an index of pulmonary function is or may not accurately reflect metabolic not new 4,13, 19, 20 25 but as has been pointed effort, since during the initial effort of nor- out, the time required for calculations and mal inspiration, energy is transferred from the complexity of the equipment has here- the compressed chest wall to the lung, and tofore limited its clinical application. By at the end of expiration is transferred back using this equipment in a clinical environ- to the chest wall,5' 7, 26 without metabolic ment, we were able to record data easily. expenditure by the patient. This measurement integratively assesses Summary pulmonary function throughout the res- piratory cycle, as it is affected by altera- Believing a need existed for better, tions in surfactant, tissue elasticity, inertial easier, and more meaningful measurement forces, friction between tissue planes, and of pulmonary function in a clinical en- airway resistance." 6 The contributions of vironment, the authors have explored the rate,21' 23 depth, level of respiration,5 and feasibility of measuring the respiratory ef- pulmonary changes 15, 20 are incorporated fort of untrained subjects by use of a spe- into the measurement. The facility of the cial bedside analogue computer. 594 BALLANTINE, PROCTOR, BROUSSARD AND LITT Annals of Surgery April 1970 It was found that the respiratory effort Normal Subjects and in Dyspneic Patients with Either Chronic Pulmonary Emphysema of untrained patients could be measured or Obesity. J. Clin. Invest., 38:1339, 1959. easily and that standard deviations com- 14. Fry, D. L., Stead, W. W., Ebert, R. V., Lubin, R. I. and Wells, H. S.: The Measurement of pared favorably with measurements of Intraesophageal Pressure and Its Relation- trained subjects obtained in a laboratory ship to Intrathoracic Pressure. J. Lab. and of respiratory effort Clin. Med., 40:664, 1952. setting. The measure 15. Garzon, A. A., Seltzer, B. and Karlson, K. E.: integratively assesses the interaction of Physiopathology of Crushed Chest Injuries. pressure and flow throughout the respira- Ann. Surg., 168:128, 1968. 16. Henry, J. N., McArdle, A. H., Scott, H. J. tory cycle and provides a valuable index and Gurd, F. N.: A Study of the Acute to determine a patient's pulmonary status. and Chronic Respiratory Pathophysiology of Hemorrhagic Shock. J. Thorac. Cardiovasc. In this report normal values for 100 Surg., 54:666, 1967. young male subjects are presented. 17. Knowles, J. H., Hong, S. K. and Rahn, H.: Possible Errors Using Esophageal Balloon in Determination of Pressure-Volume Charac- References teristics of the Lung and Thoracic Cage. J. Appl. Physiol., 14:525, 1959. 1. Agostoni, E. and Fenn, W. O.: Velocity of 18. Lewis, J. F., Shimizu, T., Scofield, A. L. and Muscle Shortening as a Limiting Factor in Rossi, P. S.: Analysis of Respiration by an Respiratory Air Flow. J. Appl. Physiol., 15: On-line Digital Computer System: Clinical 349, 1960. Thoracoabdominal Surgery. 2. Agostoni, E.: A Graphical Analysis of Tho- Data Following racoabdominal Mechanics during the Breath- Ann. Surg., 164:547, 1966. ing Cycle. J. Appl. Physiol., 16:1055, 1961. 19. Margaria, R., Milie-Emili, G., Petit, J. M. 3. Attinger, E. O., Monroe, R. G. and Segal, and Cavagna, G.: Mechanical Work of M. S.: The Mechanics of Breathing in Dif- Breathing During Muscular Exercise. J. ferent Body Positions. J. Clin. Invest., 35: Appl. Physiol., 15:354, 1960. 904, 1956. 20. McIlroy, M. 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