Negative effect of insufflation on cardiac output and pulmonary blood volume

In 14 anaesthetized young pigs the changes in pulmonary blood flow and pulmonary blood volume (Qp) during mechanical ventilation were quantified. Ventilation was performed at 10 cycles per min and tidal volume (VT) was adjusted to an arterial Pco2 of about 40 mmHg (5.3 kPa). In 4 animals, 7 ventilatory cycles with an inspiratory pause (IP) of 7.2 s but different tidal volumes were inserted at intervals of 5 min to determine the decrease in Qp (ΔQp) from the differences between right ventricular (Qs,rv) and left ventricular (Qs,lv) stroke volume, and to relate ΔQp to VF We measured pressure in the aorta (Pao, central veins (Pcv), right and left ventricles (Prv,Plv) pericardium (Pit), and trachea (PT). Blood flow was measured electromagnetically (EM) in the pulmonary artery (Q′pa) and aorta (Q′ao). Stroke volumes were derived from the EM‐flow curves. In the other 10 experiments, Qs,lv was derived from the aortic pulse contour. Beat‐to‐beat analyses of Qs,rv and Qs,lv and blood pressures during the normal ventilatory cycles and those with an IP revealed the following: 1) The end‐expiratory RV output and LV output were constant and were defined as baseline values. 2) The accumulated decrease in Qs,rv during insumation caused a mean deficit in cardiac output of 10.3 ± 3.2% (s.d.), n= 135; the same was found for Qs,lv, indicating the pulse contour as a useful method to estimate the variations in cardiac output during a ventilatory cycle. 3) Part of the pulmonary blood volume shifted into the systemic circulation during insumation and recovered during expiration with a maximum of about 1.5 ml ▾ kg‐1 body weight for a VT above 20 ml ▾ kg‐1. The decreases in cardiac output and pulmonary blood volume were attributed to an accumulation of blood in the compliant systemic venous system due to the increased intrathoracic pressure. The decreased aortic pressure at early expiration contributed to the decrease in cardiac output. A squeezing effect on the pulmonary circulation during insumation contributed to the decrease in pulmonary blood volume.