Effects of Ventilation with Large Tidal Volumes or Positive End-Expiratory Pressure on Cardiorespiratory Function in Anesthetized Obese Patients

The cardiorespiratory effects of ventilation with large tidal volumes (LTV) or positive end-expiratory pressure (PEEP) were investigated in 10 extremely obese patients during anesthesia for a jejuno-ileal by-pass operation.
Elevation of the tidal volume by 35% and insertion of a dead space (LTV-group: five patients) or applying a PEEP of 1.0 kPa (PEEP-group: five patients) resulted in significant rises in arterial oxygen tensions (Pa_o2) of 87.4% and 72.4%, respectively. The alveolar-arterial oxygen tension differences (P(a-a)_o2) decreased by 29.2% in the LTV-group and 25.6% in the PEEP-group when compared to control values. No significant differences were found between the groups in Pa_o2 or P(A-a)o₂.
PEEP-ventilation caused a maximal increase in compliance of 42.6%, but there was an increase of only 20.8% in the LTV-group.
Stroke index (the impedance cardiography method) decreased by 19.7% in the PEEP-group, whereas no changes occurred in the LTV-group. The decrease in stroke index was probably due to a reduction in venous return, as reflected in the systolic time intervals. In the PEEP-group a prolongation of the pre-ejection period (PEP) was observed, causing an increase in the PEP/LVET-ratio of 41.2%. A 13.8% increase in PEP/LVET-ratio was found in the LTV-group.
In spite of the increased arterial oxygen contents, no improvements in arterial oxygen delivery were found in either of the groups.