Effect of positive pressure ventilation on impedance plethysmography

It has been stated that positive pressure ventilation (PPV) may cause false positive impedance plethysmography (IPG) results because PPV can decrease venous outflow. We studied 36 patients who were being weaned from mechanical ventilation to determine the effect of PPV on IPG. Patients had IPGs performed during spontaneous ventilation and while receiving mechanical ventilation in the assist/control (A/C) mode without positive end expiratory pressure (PEEP) and with 10 cm H2O PEEP. No patient developed a positive IPG with institution of PPV (mean airway pressure = 6.8 cm H2O), or with PEEP (mean airway pressure = 14.9 cm H2O). The IPG values for venous capacitance and venous outflow did not change during either mode of mechanical ventilation. There also was no tendency for positive pressure ventilation to move IPG results in the direction of a positive test. In summary, PPV does not cause IPG results to be false positive, nor does it move IPG results toward the discriminant line that separates normal from abnormal results.