Reflex effects on circulation and respiration from contracting skeletal muscle.

Isometric exercise induced by ventral root stimulation in the anesthetized cat caused heat rate (HR) to increase from 199 +/- 6 beats/min to 206 +/- 6 (P less than 0.001); mean arterial pressure (MAP) from 99 +/- 5.3 mmHg to 112 +/- 5.8 (P less than 0.001); and left ventricular dp/dt at developed pressure of 25 mmHg from 3,960 +/- 204 mmHg/s to 4,380 +/- 220 (P less than 0.001), while end-diastolic pressure remained unchanged. Respiratory volume (RV) increased from 755 +/- 71 ml/min to 1,160 +/- 123 (P less than 0.001). These changes were abolished by cutting the dorsal roots receiving afferents from the exercising muscle. Beta-adrenergic blockade with propranolol (1 mg/kg) abolished the changes in heart rate and contractile state; however, changes in MAP and RV still occurred. Induced isometric exercise causes a small rise in HR with a more marked increase in MAP, the contractile state of the left ventricle and RV. In this preparation the cardiovascular and respiratory changes reflexly originate from contracting skeletal muscle as proven by section of the corresponding dorsal roots, and the cardiovascular changes are mediated in part by activation of beta-adrenergic receptors in the heart.

Noninvasive determination of cardiac output by a modified acetylene rebreathing procedure utilizing mass spectrometer measurements.

A modified acetylene rebreathing technique for measuring cardiac output has been developed utilizing a mass spectrometer for measuring gas concentrations. Helium serves as an indicator of mixing and system volume. The disappearance rate of acetylene is proportional to pulmonary capillary blood flow, which is taken as an index of cardiac output. The method also provides an estimate of pulmonary tissue volume. The technique was assessed by comparing 22 simultaneous acetylene and dye-dilution cardiac outputs measured at rest and during exercise in six healthy subjects. There was no significant difference between the mean acetylene and dye-dilution cardiac output values (11.5 l/min vs. 11.24 l/min, respectively). The correlation coefficient was 0.94 and the S.E.M. difference was 0.53 l. The effect of the procedure on cardiac output was evaluated in a separate study of six subjects. There was a significant positive correlation between respiratory rate and cardiac output; but the magnitude of this effect was small, indicating that accurate measurements may be made at spontaneous respiratory rates.