Cardiovascular response to sudden strenuous exercise.

Cardiac responses to sudden strenuous exercise were studied in seven dogs instrumented to measure cardiac output (Q), stroke volume (SV), left ventricular (LV) pressure, aortic pressure and circumflex coronary blood flow (CBF). The dogs were run on a treadmill at 12.9 km/h, 20% grade for 15s without prior warm-up. During the first 10s of the run, HR and Q increased rapidly. By 2s, myocardial O2 requirements, as estimated from either the tension time index (TTI) or calculated wall tension, had increased significantly above resting values. Simultaneously, mean CBF fell 13% and stroke CBF fell 49% below resting values at the onset of sudden exercise (2s) and was associated with 17% and 21% decreases in positive and negative LV dP/dt, a 9% decrease in (dP/dt)/P and an 11% decrease in SV. After the initial decrease at 2s, these variables then increased rapidly as the run continued except for stroke CBF which never reached resting levels. HR, Q, and LV dP/dt reached near maximal values for this level of exercise by 10s after the start of exercise. In contrast, mean CBF was still increasing when the run was terminated. These data show that sudden strenuous exercise results in a transient decrease in myocardial O2 delivery at a time when myocardial O2 requirements are rapidly increasing. This imbalance between myocardial O2 supply and demand is due to a transient decrease in coronary blood flow followed by a delay in metabolic regulation of CBF.

Left ventricular O2 requirements of pressure and volume loading in the normal canine heart and inaccuracy of pressure-derived indices of O2 demand.

The purpose of this study was to: 1) re-evaluate the left ventricular O2 requirements (MVO2) of pressure and volume-loading; and b) assess the accuracy of pressure-derived indices of left ventricular (LV) O2 demand under pressure-loading and volume-loading conditions. Using a right heart bypass preparation (heart rate 150 beats . min-1), mean arterial pressure (7.3 to 22.9 kPa) and cardiac output (0.8 to 6.0 litre . min-1) were varied independently. Adequate left ventricular O2 supply was demonstrated by normal transmural distribution of blood flow, normal myocardial lactate metabolism, and intact reactive hyperaemic responses. For equivalent increases in cardiac external work (from 2 to 4 joules . min-1) pressure-loading and volume-loading resulted in similar increases in O2 uptake (59% and 49%, respectively, P = 0.21). MVO2 was consistently greater for volume-loading than for pressure-loading at any SPTI, pressure-rate product, and triple product. The prediction of O2 demands by these indices under these loading conditions collectively was unreliable (r = 30 to 0.42). O2 requirements of the left ventricle with both pressure-loading and volume-loading were highly correlated (r = 0.82 to 0.99) with meridional wall stress. We conclude that: 1) volume-loading conditions have a greater O2 requirement than appreciated previously; 2) pressure loads and volume loads require similar O2 uptake in the normal canine heart for similar external work; 3) currently used pressure-derived indices are unreliable predictors of LV O2 demand when loading conditions are varied; and 4) O2 requirements are more uniformly related to meridional wall stress.

Effect of exhaustive exercise on myocardial performance.

Several investigators have reported ultrastructural changes in hearts of animals exercised to exhaustion. The present study was designed to determine whether functional changes occur in the intact heart at exhaustion. Adult mongrel dogs (n = 8) were chronically instrumented to measure cardiac output, coronary blood flow, aortic blood pressure, left ventricular pressure, +dP/dtmax, and -dP/dtmax. After recovery, the dogs were run to exhaustion at a constant work load, eliciting approximately 70% of maximum heart rate. The exhaustive bouts were terminated when the animals either refused or were unable to continue running, at which time their rectal temperatures approaches 42.2 degree C. The mean exhaustion time was 76.7 +/- 11.8 min. All parameters increased from rest to steady state with the exception of stroke volume (23.2 +/- 4.9 vs. 20.5 +/- 1.6 ml), which remained constant. In the transition from steady state to exhaustion, only +dP/dtmax (6,652 +/- 291 vs. 7,689 +/- 479 Torr/s) and -dP/dtmax (4,110 +/- 227 vs. 4,890 +/- 215 Torr/s) increased significantly; all other values exhibited no significant change. Similarly, when maximum cardiovascular parameters were measured before and after exhaustion, no significant changes were found. These data show that cardiac contractile function is not depressed in dogs as a result of exhaustive exercise.

Effects of intensive exercise training on myocardial performance and coronary blood flow.

Five instrumented and eight noninstrumented dogs were progressively trained for 12-18 wk on a motor-driven treadmill. Data were compared with 14 instrumented and 8 noninstrumented control dogs. Gastrocnemius malate dehydrogenase activity was significantly increased in the trained dogs (887 +/- 75 vs. 667 +/- 68 mumol . g-1 . min-1). The trained dogs also showed significant increases in maximum work capacity, cardiac output (7.1 +/- 0.5 vs. 9.1 +/- 0.7 1/min), stroke volume (25.9 +/- 2.0 vs. 32.0 +/- 2.0 ml/beat), and left ventricular (LV) positive dP/dtmax (9,242 +/- 405 vs. 11,125 +/- 550 Torr/s). Negative dP/dtmax was not significantly different. Peak LV systolic pressure increased with exercise, but there was no significant difference between the trained and control dogs. LV end-diastolic pressure did not change with exercise and was the same in both groups. Tension-time index was lower in the trained dogs at rest and submaximum exercise (9.7 km/h, 10%) but was not different at maximum exercise. Diastolic pressure-time index was significantly higher in the trained dogs at rest and during submaximum exercise but was not different at maximum exercise. LV coronary blood flow was significantly reduced at rest (84 +/- 4 vs. 67 +/- 6 mo . min-1 . 100 g-1) and during submaximum exercise (288 +/- 24 vs. 252 +/- 8 ml . min-1 . 100 g-1). During maximum exercise flow was not significantly different (401 +/- 22 vs. 432 +/- 11 ml . min-1 . 100 g-1) between the control and trained groups. The maximum potential for subendocardial flow was unchanged with training despite the development of mild hypertrophy.

Heart rate and ECG responses of fire fighters.

Data were obtained from 35 fire fighters responding to 189 alarms. Fifteen to 30 sec after an alarm heart rate showed a mean increase of 47 beats/min (range 12-117 beats/min). Approximately one minute after the alarm, while on the truck, heart rate still showed a mean increase of 30 beats/min (range 1 to 80 beats/min) above that recorded before alarm. S-T segment changes were observed in the ECG shortly after the alarm sounded. Upon approaching a fire, heart rates as high as 150 beats/min were observed before the men got off the fire truck. During actual fire fighting extremely high heart rates were observed for prolonged periods of time. One fire fighter had a mean heart rate of 188 beats/min for 15 minutes during the initial stages of a structure fire. The heart rate responses observed immediately after the alarm as well as on the truck approaching a fire indicate that the men experience a state of high anxiety. The extremely high heart rates observed for prolonged periods during fire fighting may also indicate a state of high anxiety coupled with the heavy work performed in a hot environment. Repeated exposure to states of high anxiety as well as inhaling pollutants related to the high incidence of ischemic-stress tests previously observed in fire fighters.