Heart-lung interactions: Implications for non-invasive evaluation of changes in blood volume

Loss in blood volume due to bleeding or critical illness may be difficult to detect based on clinical signs such as blood pressure and heart rate alone. Monitoring of cardiac output is recommended for haemodynamic evaluation, but methods are often invasive and skill-dependent. However, peripheral circulatory and ventilatory changes following heart-lung interactions may reflect central hypovolemia. Heart-lung interactions may be recognizable as respiratory induced changes in pulse pressure and the photoplethysmographic waveform amplitude; referred to as “dynamic variables”. Dynamic variables have been found to reliably predict volume changes during mechanical ventilation. Mechanical ventilation induces regular cyclic changes in intrathoracic pressure, and is assumed to be a prerequisite for the use of dynamic variables. Change in exhaled carbon dioxide is another dynamic measure of heart-lung interactions which has been shown to reflect large changes in cardiac output, mostly in experimental studies. The aims of this thesis were to explore less invasive methods based on heart-lung interactions in new experimental and clinical settings. In two different experimental studies on healthy volunteers we found that dynamic variables revealed blood loss induced by lower body negative pressure also during non-invasive positive pressure ventilation and positive expiratory pressure; two frequently used clinical interventions. In a clinical study on patients after open heart surgery we found that changes in exhaled CO2 accurately reflected moderate reductions in cardiac output induced by right ventricular pacing. These findings indicate that dynamic variables may be applicable also in patients without mechanical ventilation and obtainable by less invasive monitoring than previously assumed. This may enable earlier identification of hypovolemia due to for instance bleeding also in patients without invasive monitoring