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The effect of continuous increases in intrathoracic pressure by positive end-expiratory pressure (PEEP) on common carotid artery blood flow (CCABF) and arterial blood gas tensions and pH during standard closed-chest cardiopulmonary resuscitation was studied in dogs. CCABF was measured at various levels of PEEP by means of the bubble flowmeter in hypervolemic, normovolemic, and hypovolemic dogs. PEEP was increased and then decreased in 5-cm H2O increments with the highest level being 20 cm H2O. Arterial blood samples were obtained approximately 15 minutes before ventricular fibrillation was induced with a transthoracic electric shock during zero end-expiratory pressure during increases in PEEP to 20 cm H2O, and again during decreases to zero end-expiratory pressure. CCABF increased stepwise with increasing levels of PEEP in hypervolemic dogs, whereas it decreased stepwise with increasing levels of PEEP in hypovolemic dogs. In normovolemic dogs, CCABF increased only when the PEEP was 5 cm H2O. Severe metabolic acidosis developed in both hypovolemic and normovolemic dogs at the end of 18 minutes of cardiopulmonary resuscitation. It is speculated that PEEP can either increase CCABF by increasing intrathoracic pressure during chest compression or decrease the CCABF by decreasing venous return during chest compressions, depending on the circulating blood volume. Data suggest that even low levels of PEEP are contraindicated in hypovolemia. Although changes in cerebral perfusion caused by PEEP were not investigated, it is suggested that some degree of PEEP may be beneficial in the hypervolemic state during cardiopulmonary resuscitation.