Breathing affects left ventricular stroke volume (LVSV) in normal subjects. The observed relationship may result from interaction between the effects of changing lung volume and intrathoracic pressure (IP). 2. To investigate the effect of IP on LVSV with minimal changes in lung volume, beat-by-beat LVSV (pulsed Doppler ultrasound) and systemic blood pressure (Finapres) were measured during obstructed inspiratory and expiratory efforts causing step changes in IP of +/- 15 cmH2O for 10 s, in seven subjects. Changes in mouth pressure (MP) during airway occlusion were used to indicate changes in IP. Group-averaged data for each second were compared to that in the second before the change in MP using Dunnet's multiple range test. 3. Step reductions in MP resulted in immediate and significant falls in LVSV (P < 0.05) and systolic blood pressure (P < 0.01) and increased heart rate, although this was not significant. These responses were transient, lasting only 3 s despite 10 s of reduced MP. 4. Step increases in MP caused biphasic cardiovascular responses. LVSV increased immediately, then fell significantly below control after 8 s (P < 0.01). Heart rate increased significantly between 5 and 9 s after the onset of the increase in MP (P < 0.05), suggesting activation of the baroreflexes by the accompanying progressive fall in systolic blood pressure. 5. The asymmetry in time course and magnitude between the responses to positive and negative pressure may reflect asymmetrical effects of MP on systemic venous return, right ventricular output, pulmonary venous return and left ventricular (LV) after-load, with the baroreceptors limiting changes in arterial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)
