EFFECT OF POSITIVE AND NEGATIVE STEP CHANGES IN INTRATHORACIC PRESSURE ON LEFT-VENTRICULAR FUNCTION IN CONSCIOUS MAN

1. Breathing affects left ventricular stroke volume (LVSV) in normal s ubjects. 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 lun g volume, beat-by-beat LVSV (pulsed Doppler ultrasound) and systemic b lood pressure (Finapres) were measured during obstructed inspiratory a nd expiratory efforts causing step changes in IP of +/- 15 cmH2O for 1 0 s, in seven subjects. Changes in mouth pressure (MP) during airway o cclusion were used to indicate changes in IP. Group-averaged data for each second were compared to that in the second before the change in M P using Dunnet's multiple range test. 3. Step reductions in MP resulte d in immediate and significant falls in LVSV (P < 0.05) and systolic b lood pressure (P < 0.01) and increased heart rate, although this was n ot significant. These responses were transient, lasting only 3 s despi te 10 s of reduced MP. 4. Step increases in MP caused biphasic cardiov ascular responses. LVSV increased immediately, then fell significantly below control after 8 s (P < 0.01). Heart rate increased significantl y between 5 and 9 s after the onset of the increase in MP (P < 0.05), suggesting activation of the baroreflexes by the accompanying progress ive fall in systolic blood pressure. 5. The asymmetry in time course a nd magnitude between the responses to positive and negative pressure m ay 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 press ure. 6. With negative pressure, systolic blood pressure fell by more t han IP; this argues against an important role for LV after-load in dep ressing LVSV during negative pressure. These results therefore provide indirect support for the concept of ventricular interdependence, in w hich enhanced right ventricular filling impairs left ventricular filli ng.