Cardiopulmonary interactions and volume status assessment

Abstract

Assessment of the hemodynamics and volume status is an important daily task for physicians caring for critically ill patients. There is growing consensus in the critical care community that the "traditional" methods-e.g., central venous pressure or pulmonary artery occlusion pressure-used to assess volume status and fluid responsiveness are not well supported by evidence and can be misleading. Our purpose is to provide here an overview of the knowledge needed by ICU physicians to take advantage of mechanical cardiopulmonary interactions to assess volume responsiveness. Although not perfect, such dynamic assessment of fluid responsiveness can be helpful particularly in the passively ventilated patients. We discuss the impact of phasic changes in lung volume and intrathoracic pressure on the pulmonary and systemic circulation and on the heart function. We review how respirophasic changes on the venous side (great veins geometry) and arterial side (e.g., stroke volume/systolic blood pressure and surrogate signals) can be used to detect fluid responsiveness or hemodynamic alterations commonly encountered in the ICU. We review the physiological limitations of this approach.

Arterial Pressure Variation in Elective Noncardiac Surgery: Identifying Reference Distributions and Modifying Factors

“…Recent studies note similar findings, placing a stronger emphasis on the relationship between driving pressure and dynamic waveform indices compared to tidal volumes. 16,49 In an era of lung-protective ventilation favoring lower tidal volumes and plateau pressures, our findings suggest a similar trend in SPV and PPV values which anesthesiologists may need to consider; indeed, the utility of an arterial pressure variation indexed to tidal volume has been demonstrated. 48 Despite this association, a study also demonstrates that the ability of dynamic waveform indices to predict fluid-responsiveness remains intact even at lung-protective ventilation levels and continues to greatly exceed predictive capabilities of static measures.…”

“…Compared to SVV, SPV and PPV have demonstrated similar or superior ability to predict fluid responsiveness without the need for specialized SVV measurement equipment, 13,14 and eclipse other static measures. 5,15 Limitations of SPV and PPV include requiring a regular heart rhythm 16,17 and standardized ventilator settings, 17,18 as well as sensitivity to confounding factors including positioning, 19 intrathoracic pressure, 16 intraabominal pressure, 16,20 and heart failure. 21 Despite limitations, SPV and PPV have endured as commonly used measures of fluid-responsiveness, owing to literature support, clinical safety, lack of other cost-effective, reliable measures, and automatic near-continuous calculation by newer physiologic monitors.…”

Context-sensitive fluid therapy in critical illness

“…Cardiac preload is determined by venous return, which is equal to the difference between mean systemic filling pressure and right atrial pressure divided by the resistance to venous flow return [ 6 ]. Veins of the systemic circulation contain approximately 70 % of the blood volume in the whole body.…”

“…In contrast, the stressed volume represents blood volume above the opening and thus contributes directly to mean systemic filling pressure and venous return. Changes in vascular tone alter the ratio of unstressed to stressed volume [ 5 , 6 ]. Specifically, venoconstriction shifts blood from unstressed volume to stressed volume, whereas venodilation shifts blood from stressed volume to unstressed volume.…”

“…Surgical insults and severe infections affect all of these features, the magnitudes of which depend on clinical context such as the phase of illness. For example, fluid administration increases mean systemic filling pressure through intravascular volume expansion, thus increasing venous return and cardiac output [ 5 , 6 ]. However, the degree of intravascular volume expansion is context-sensitive as demonstrated by larger plasma volume expansion following fluid infusion in the hypovolemic state compared to the normovolemic state [ 7 ].…”