Effect of venous hypercarbia and hyperventilation on myocardial contractility in canine haemorrhagic shock

To study the effect of venous hypercarbia on myocardial contractility, haemorrhagic shock was produced in six healthy mongrel dogs by ex-sanguination of 15 ml of blood/kg body weight every 20 minutes till a loss of 45 ml/kg was achieved. After recording haemodynamic and respiratory parameters, the dogs were hyperventilated by positive pressure ventilation for 30 minutes and haemodynamic and blood gas parameters reassessed. During haemorrhagic shock, mean cardiac output decreased from 4.23 l min to 0.98 l min (p < 0.01), stroke index from 2.25 to 0.35 ml/kg (p < 0.05) and left ventricular stroke work index from 3.72 to 0.19 g. m/kg. The mean mixed venous pCO2 increased from 35 mmHg to 56.7 mmHg (p < 0.05). During hypoventilation, mixed venous pCO2 decreased to 40 mmHg (p < 0.05) and without any volume replacement, mean cardiac output increased 2.5 l min (P < 0.05), stroke index to 1.13 ml/kg (p < 0.05) and left ventricular stroke work index, and index of myocardial contractility, increased to 0.78 g.m/kg (p < 0.05). Thus, although hypovolaemia is the major cause of low cardiac output in haemorrhagic shock, this study shows that venous hypercarbia (which probably indicates tissue respiratory acidosis) further worsens circulatory failure by decreasing myocardial contractility. Hyperventilation improves cardiac functions and increases output by relieving tissue hypercarbia in spite of persistent hypovolaemia.