In vivo and simulation results of mechanical ventilation effects on energetic variables in cardiosurgical patients

Controlled ventilation of the lungs by means of respirators is the most often used technique of respiratory support. Since its introduction in fifties into intensive care practice, both methods and means of artificial ventilation have changed enormously. In contrary to a well established, conventional mechanical ventilation, with a relatively large tidal volume (adjusted as 10 ml per unit of body weight), so called "lung protective" ventilation was proposed about a decade ago. This last strategy is based on the assumption that large tidal volume may cause a damage to the pathologically changed alveoli, so tidal volume is adjusted as 5 ml per unit of body weight. As the result of diminished alveolar ventilation in this case (ventilatory frequency is equal in both methods, for adults is 10-16 cycles min) usually hypercapnia appears, often called "permissive" hypercapnia. The consensus on the optimal tidal volume during ventilatory support has not been reached up to now, in spite of some attempts to establish it. It has been shown in various studies that one of the main detrimental side-effect of positive pressure ventilation may be the drop of lung perfusion, venous return, arterial pressure and cardiac output. In this chapter the effects of different artificial ventilation methods on energetic ventricular variables - left and right stroke work are presented. Changes of these variables caused by conventional, lung protective and high frequency ventilation are shown. This last ventilatory method may be treated as a special technique that in some cases may assure an appropriate gas exchange with low intrathoracic pressure. Collecting hemodynamic variables from clinical measurements for calculation of energetic ventricular ones and using a computer simulator of cardiovascular system we examined numerical simulation as a tool to foresee energetic effects of various ventilatory support.