Renal Function and Cardiovascular Responses during Positive Airway Pressure

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Abstract
Cardiovascular, renal and hormonal responses to increased airway pressure during continuous positive-pressure ventilation (CPPV) and continuous positive airway pressure (CPAP) were determined. Nine healthy, hydrated laboratory swine had appropriate catheters placed to allow for measurement of intrapleural, aortic, inferior vena caval and left ventricular end-diastolic pressures, cardiac output and urinary flow. Samples of arterial blood were analyzed for O2 and CO2 tensions, pH, plasma vasopressin, osmolality and creatinine and Na concentrations. Urine was analyzed for osmolality and creatinine and Na concentrations and volume was recorded. Intrapleural pressure was subtracted from left ventricular end-diastolic pressure to calculate transmural pressure, a reflection of left ventricular filling pressure. Glomerular filtration rate and urinary free-water and osmolal clearances were calculated. Expiratory left ventricular filling pressure was decreased equally by CPAP and CPPV. Inspiratory left ventricular filling pressure and cardiac output were decreased by CPPV only. Urinary flow and glomerular filtration rate were decreased equally by CPAP and CPPV. Na excretion was decreased and plasma vasopressin increased by CPPV but not by CPAP. Urinary free water and osmolal clearances were not changed by either ventilatory pattern. Many of the renal-function variables were affected similarly by CPPV and CPAP, but the changes were not influenced solely by cardiac output or vasopressin, because only CPPV depressed cardiac output and increased vasopressin levels.