Bradycardia during severe but reversible hypovolemic shock in man

Severe bleeding and hypovolemic shock causing hypotension are most often associated with tachycardia. In response to passive head-up tilt, five healthy men exhibited an increase in heart rate (HR) from 62 to 79 beats X min-1 and a gradual increase in the plasma concentration of aldosterone and protein. The increase in HR was followed by a decrease of 29 beats X min-1 (range 11-46) at the time when blood pressure decreased 38 mmHg (6-73). When tilted back to 0 degree, blood pressure immediately reversed while HR remained unchanged. Hypotension was associated with large but variable increases in plasma vasopressin (86 +/- 28 pg X ml-1) accompanied by peripheral vasoconstriction. In two cases where patients with internal bleeding presented with a moderate HR of 96 beats X min-1, the ensuing fall in blood pressure was associated with a decrease in HR to 68 and 76 beats X min-1, respectively. Administration of albumin solution and blood normalized cardiovascular function. Two other patients showing initial HR of 130 and 100 beats X min-1, respectively, also developed relative bradycardia in conjunction with a decrease in blood pressure. Administration of ephedrine and atropine increased HR temporarily from 56 to 90 and from 36 to 110 beats X min-1, respectively. The latter two patients died in extreme bradycardia and autopsies revealed severe internal bleeding. It is concluded that although hypovolemic shock is most often associated with an increase in HR, the increase is modest and a paradoxical bradycardia develops in severe but potentially reversible hypotensive hypovolemic shock.