Application of a high respiratory rate with a low tidal volume during carbon dioxide-pneumoperitoneum in rabbits

ABSTRACT

BACKGROUND: During laparoscopic surgery, high airway pressures are generally followed by a diaphragmatic shift and hyperventilation. We hypothesize that normocapnea can be maintained with the same amount of CO2 output (VCO2) during pneumoperitoneum (PP). METHODS: Six anesthetized rabbits were mechanically ventilated at a respiratory rate of 20/min with FIO2 0.5. At the end of the expiratory limb of the ventilator, the mean partial pressure of CO2 was measured. The internal carotid artery was catheterized. Baseline values for blood pressure, heart rate, arterial blood gas analysis, and ventilatory variables were obtained. CO2 gas was introduced into the peritoneum with an intra-abdominal pressure of 12 mmHg. The measurements at baseline and at PP1 were compared. The respiratory rate was changed from (20/min PP1, to 40/min PP2, 80/min PP3 or 120/min PP4) while calculating VCO2 and comparing ventilatory variables under PP at the same time. RESULTS: The peak inspiratory pressure (PIP) and tidal volume (VT) at PP1 increased, compared with baseline. With the same PaCO2, the VT decreased significantly from (45 +/- 8 ml PP1 to 29 +/- 5 ml PP2, 19 +/- 4 ml PP3 and 15 +/- 2 ml PP4), respectively. The PIP was reduced. However, the dead space to tidal volume ratio (VD/VT) was greater at higher RR during PP. CONCLUSIONS: PP increased the PIP and VT for the removal of overloaded CO2. Less VT at a higher respiratory rate could be used with the same amount of VCO2 during PP. However, the VD/VT was elevated by the induction of PP and by the increase in respiratory rate.