Dynamic Parameters Pulse Pressure Variation

During positive-pressure mechanical ventilation, intrathoracic pressure increases with lung insufflation, which reduces venous return to the right heart. If the right ventricle is preload-dependent, this will inevitably reduce the right ventricular outflow and ultimately result in a reduced left ventricular preload. Hence, positive- pressure mechanical ventilation induces cyclic changes in intrathoracic pressure that transiently affect ventricular preload, resulting in greater cyclic changes in stroke volume (SV) when the ventricle operates on the steep (responder) rather than on the flat (nonresponder) portion of the Frank-Starling curve. These cyclic changes in SV can be evaluated by changes in pulse pressure over the respiratory cycle because the pulse pressure is proportional to SV [53].

A number of studies have demonstrated that pulse pressure variation (PPV) is a reliable indicator of fluid responsiveness in critically ill patients including those with ARDS, provided that the conditions of its validity are fulfilled. Pulse pressure variation (APP) has been proposed to predict fluid responsiveness in patients with ARDS [54, 55]. However, the respiratory variation in pulse pressure as a marker of fluid responsiveness is not valid under some conditions that commonly affect patients with ARDS. First, in the setting of spontaneous breathing activity, stroke volume variations are related more to the respiratory irregularity rather than to preload dependence [56]. Second, in the setting of cardiac arrhythmias, the variation of stroke volume is related more to arrhythmia itself than to heart-lung interactions

[57]. A third important limitation occurs in the setting of low tidal volume ventilation and/or with low lung compliance which are common in patients with ARDS. In this setting the low tidal volumes and lung compliance can reduce the transmission of changes in alveolar pressure to the intrathoracic structures, thereby diminishing the magnitude of the ventilation-induced changes of intravascular pressure [55]. This may result in false-negative predictions of fluid responsiveness by PPV. To improve the performance of APP for assessment of fluid responsiveness in patients managed with low tidal volume, a recent study reported correction of APP by pleural pressure changes. APpl values facilitate discrimination between responders and nonresponders. In their analysis, a APP/APpl greater than 2 predicted fluid responsiveness with a sensitivity of 92.3% and a specificity of 93.2% [58]. While this work shows promise, further validation is needed.