Diaphragmatic Energetics during Prolonged Exhaustive Exercise 1- 3
MURLI MANOHAR and ASLAM S. HASSAN
Introduction SUMMARY The present study was carried out to examine diaphragmatic O 2 extraction and lactate Strenuous exertion markedly exaggerand ammonia production during prolonged exhaustive exercise. Experiments were performed on ates the work of breathing. Estimates of nine healthy exerclse-condltloned ponies In which catheters had been Implanted In the phrenic vein respiratory muscle O2 uptake during previously. Blood-gas variables and lactate and ammonia concentrations were determined on simulstrenuous exertion have ranged from 10 taneously obtained arterial and phrenlc-venous blood samples at rest and during 30 min of exertion to 20070 of the whole body O 2 utilization. at 15 mph + 7% grade (heart rate, 200 beats/min; "'90% of maximum). Arterial O2 tension and Conflicting reports on substrate utilizasaturation were maintained near resting value but CO2 tension decreased markedly with exercise, and because of Increased hemoglobin concentration, arterial O2 content rose. Concomitantly, phreniction by the rat diaphragm during exervenous O2 tension, saturation and content decreased markedly (23.6 ± 1 mm Hg, 24.5 ± 2%, 5.2 cise (1-5), and the observation that in ± 0.3 mlldl at 3 min of exertion) and significant fluctuations did not occur as exercise duration people exercising in air, diaphragmatic progresaed to 30 min. Diaphragmatic arteriovenous O2 content difference and O 2 extraction rose fatigue may have occurred after highfrom 4 ± 0.3 to 16 ± 0.5 mlldl and from 30 ± 3 to 75 ± 1% at 3 min of exercise, and significant intensity short-term (6) as well as prodeviations did not occur as exercise duration progressed. Arterial lectate and ammonia levels Inlonged exercise (7) have directed attencreased during exercise, Indicating their release from working 11mb muscles. Phrenlc-venous values tion to the study of diaphragmatic enerof lactate and ammonia did not exceed arterial values. Ponies sweated profusely and were unable geticsin exercise. Recently, diaphragmatic to keep up with the belt speed In the last 4 to 5 min of exercise. Constancy of phrenic arteriovenous perfusion in short-term heavy exercise O2 content difference In exercise Indicated ability to adjust perfusion In diaphragm 80 88 to adehas been studied (8-11),but no attention quately meet Its O2 needs. This and the observation that diaphragm did not have net release of lactate or ammonia suggest Its metabolism In prolonged exhaustive exercise remained aerobic. was paid to prolonged exhaustive AM REV RESPIR DIS 1991; 144:415-418 exercise. Prolonged heavy exerciseindeed poses a dual challenge for the cardiovascular system since it is now required to maintain adequate perfusion to the working dividual muscles is not easily accessed. grade (heart rate of about 200 beats/min) for 30 days. musculature (respiratory muscles includ- However,in the case of costal diaphragm, venous effluent may be cleanly sampled Surgical Preparation of Animals ed) at a time when marked cutaneous by catheterization of the phrenic vein Using general anesthesia induced with intravasodilation is essential to dissipate the thermal burden (12, 13). The challenge (14). Thus, in the present study, our ob- venously administered guifenesin + thiamyis further exacerbated in those species, jectives were to use this approach to lal sodium, and maintained with halothane viz., horse and human, where sweating ascertain changes in diaphragmatic O2 (vaporized in O 2 ) , a left lateral thoracotomy was performed aseptically in the tenth interis the principal means for dissipation of extraction (and blood flow) during costal space. One of the major tributaries prolonged strenuous exercise and to heat (evaporative heat loss). This is bedraining into the main phrenic vein was punccause marked fluid loss from profuse ascertain whether such a stressful condi- tured with a 16-gauge angiocath, and an 18sweating (at a time when cutaneous blood tion would cause diaphragm to engage gauge catheter was then advanced approxivolume is markedly increased) may in net lactate and/or ammonia produc- mately 3 em into the vein. The tip of this cathdiminish central blood volume and ve- tion. Ammonia production in working eter could be palpated in the main phrenic nous return, thereby compromising the skeletal muscles (via deamination of vein. Distance between the tip of this cathestroke volume (12, 13). Whether meta- AMP after the myokinase reaction bolic needs of working musculature un- 2ADP - ATP + AMP) has been reportder such demanding circumstances take ed to be closely related to the onset and (Received in original form April 2, 1990 and in precedence over thermoregulatory needs intensityof anaerobic metabolism (15, 16). revised form November 13, 199(1) of the organism remains a contentious Methods I From the Department of VeterinaryBiosciences, issue (12). Experiments were carried out on nine healthy College of Veterinary Medicine, University of IlTissue O2 consumption is a product at Urbana-Champaign, Urbana, Illinois. of blood flow and arteriovenous O2 con- adult ponies weighing 141 to 280 kg (205 ± linois 2 Supported in part by a grant-in-aid from 12kg; mean ± SE). The animals werehoused tent difference. Thus, at a given level of an air-conditioned building and were fed the Illinois Affiliate of the American Heart tissue O2 uptake, changes in arterio- ainration Association. of alfalfa hay and oats. The animals 3 Correspondence and requests for reprints venous O2 content difference reflect weredewormed, immunized with tetanus toxreciprocallythe alterations in tissue blood oid, and were accustomed to being handled should be addressed to Dr. M. Manohar, Department of Veterinary Biosciences, Room 212, Large supply. This indirect approach to exam- by people. They were trained to run on a Animal Clinic, 1008 West Hazelwood Drive, Coline tissue blood-flow changes poses dif- motor-driven treadmill. Exercise was per- lege of Veterinary Medicine, University of Illinois, ficulty because venous drainage from in- formed daily for 20 min at 15 mph + 70/0 Urbana, IL 61801.
MANOHAR AND HASSAN
ter and the junction of phrenic vein with inferior vena cava was at least 10 em in all instances. This was done to prevent contamination of phrenic venous blood by vena caval blood. This technique did not cause occlusion of the tributary through which the catheter was advanced into the main phrenic vein. The catheter was exteriorized in the thirteenth intercostal space and filled with heparin solution to maintain patency. The thoracotomy was closed in a conventional manner. All ponies recovered uneventfully from this procedure. Catheter location was confirmed at necropsy. About 24 to 48 h before the study, the abdominal aorta was percutaneously catheterized in the sixteenth intercostal space, after local infiltration of 2% lidocaine HCI (17), to obtain arterial blood samples.
Experimental Protocol Experiments were carried out on healthy, afebrile ponies 5 to 9 days after surgery. Simultaneously obtained arterial and phrenic venous blood samples wereanalyzed for bloodgas tensions, pH, O 2 saturation of hemoglobin, hemoglobin concentration, and lactate (Sigma Technical Bulletin 726-UV/826-UV; Sigma Chemical, St. Louis, MO) and ammonia (Sigma Technical Bulletin 170-UV)concentrations. Blood-gas tensions and pH values were corrected to pulmonary artery blood temperature. Bicarbonate concentration was calculated using the Henderson-Hasselbalch equation with pK = 6.1. Blood O 2 content was determined as (hemoglobin concentration [g/dl] x 1.34 x O 2 saturation) + [02 tension in mm Hg x 0.003]. Diaphragmatic O2 extraction was calculated as arteriovenous O 2 content difference divided by arterial O 2 content. Heart rate was also determined. All ponies were first studied at rest (control) while standing quietly in a stanchion. Thereafter, ponies were exercised in an airconditioned laboratory (ambient temperature: 15 to 160 C) at 15 mph + 70/0 grade for 30 min. Pilot trials on separate days had revealed that none of the animals could work at this work load for more than 30 min. In fact, towards the last 4 to 5 min of exercise, they appeared exhausted and were unable to keep up with the belt speed. At this treadmill work intensity,heart rate of ponies approached near 200beats/min, and their respiratory frequency and pleural pressure swings were 110 ± 7 breaths/min and 23 ± 3 em H 2 0 (18). Heart rate of ponies at this work intensity before thoracotomy was not different from values elicited in this study (5 to 9 days postthoracotomy). Simultaneous heparinized arterial and phrenic-venous blood samples were obtained anaerobically at 3-min intervals for determining pH and gas variables and at 5-min intervals for lactate and ammonia assays. For lactate assays, blood was immediately deproteinized with chilled perchloric acid, and supernatant was removed after centrifugation for further analysis (Sigma Technical Bulletin 826-UV). For ammonia assays, blood was promptly centrifuged to separate