Effect of intermittent positive pressure ventilation on cardiac systolic time intervals

The measurement of systolic time intervals (STI) has been widely used as a non-invasive method of assessing the inotropic state of the heart, and normal values are available for healthy individuals breathing spontaneously. The present study was performed in order to evaluate how intermittent positive pressure ventilation (IPPV) affects STI. Ten subjects were investigated before and during halothane anaesthesia for routine surgery. Oesophageal pressure, respiratory minute volume and frequency, arterial blood-gas tensions, cardiac output and heart rate were also measured simultaneously. As expected, the institution of IPPV was associated with a reduction in cardiac output and an increase in oesophageal pressure. Paco2 was reduced. These changes were associated with a considerable lengthening of electro-mechanical systole. This was due to a lengthened pre-ejection period (PEP), whereas the left ventricular ejection time (LVET) was slightly shortened. These changes were even more marked during artifical hyperventilation. The changes in STI are attributed mainly to the reduction of venous return to the heart, subsidiary factors being intrathoracic pressure, myocardial inotropy and vascular resistance.