Expanding the usefulness of hemodynamic waveform analysis in the critically Ill

Excerpt

Right ventricular (RV) function is usually not important clinically in defining cardiovascular status until it is. The role RV function plays in maintaining normal cardiovascular homeostasis defines that as long as it maintains a low right atrial pressure (Pra) in response to circulatory stresses like exercise, acute respiratory failure, or sepsis, its function can be ignored [1]. Under normal conditions pulmonary arterial systolic pressure remain low, usually below 16 mmHg, such that the ejecting RV can transfer its blood easily into the pulmonary arterial bed. Ventilation, by altering both intrathoracic pressure (ITP) and lung volume, affects this process in multiple ways that include varying right atrial pressure independent of RV filling pressures and pulmonary vascular resistance. Spontaneous inspiration by decreasing ITP artifactually decreases Pra while simultaneously increasing the rate of RV filling [2]. This is good because if RV filling pressure does rise with increased filling, Pra does not, thus fulfilling the requirements of an adaptive RV in sustaining a maximal venous return. Positive pressure breathing, regrettably does the exact opposite, it increases ITP artifactually increasing Pra and impeding venous return. Thus, increased Pra values are commonly seen in patients on mechanical ventilation even if they have normal RV function. Still, many disease states commonly seen in the acute care setting are associated with increased pulmonary vascular resistance. Operationally, this is compensated for reflexively by fluid retention and fluid resuscitation by the clinician. Thus, in the setting of acute illness Pra is commonly elevated but its relationship to RV function is unknown. Presently, echocardiographic analysis of right ventricular function and ventricular interdependence are used to assess RV function at the bedside, but these methods are user dependent, not universally available or available all the time, cannot be easily used to continually followed RV function as time or treatments occur, and define only a specific state not the response to stress or cardiovascular reserve, which are what are really needed for the bedside assessment of RV function to be clinically useful. …