Effects of PEEP on extra vascular lung water and central blood volume in the dog

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Abstract
Twenty-four mongrel dogs were anesthetized and ventilated mechanically in the supine position. Extravascular lung water (EVLW) and central blood volume (CBV) were measured with a double indicator (dye/cold) dilution technique. Both indicators were detected intravascularly in the aortic root with a fibreoptic thermistor catheter. Seven dogs ventilated with a positive end-expiratory pressure (PEEP) of 1.0 kPa (10 cm H2O) for a short period of time (<20 min) displayed no significant change in EVLW as measured with the indicator dilution technique (=EVLWi), while reductions were seen in both CBV (15%, P<0.01) and cardiac output (CO-thermodilution technique) (10%, P<0.05). Another seven dogs ventilated with a PEEP of 1.0 kPa for 8 h showed a gradual increase in EVLWi. After 8 h, a mean increase of 34% (P<0.01) was recorded, and the increase was also verified by post-mortem gravimetric determination of EVLW (=EVLWg), displaying an increase of 61% (P<0.01). In five dogs ventilated with zero end-expiratory pressure (ZEEP) for 8 h, no changes in EVLWi, CO, and CBV were observed, and EVLWg was mean 4.39 g/kg body weight (BW). Five additional dogs were sacrificed after 15 min of anesthesia without catheterization and EVLWg was found to be 4.24 g/kg BW. It is concluded that EVLWi does not change measurably during ZEEP or short periods of PEEP. However, long periods (8 h) of PEEP result in elevated EVLWi values. Gravimetry supports these conclusions. Although PEEP recruits lung volume after lung damage and in this way enhances gas exchange, it appears that prolonged ventilation with PEEP also causes a significant increase of extravascular lung water in otherwise healthy lung tissue.