Evaluation of cardiac function using heart-lung interactions

Abstract

Heart lung interactions can be used clinically to assist in the evaluation of cardiac function. Application of these interactions and understanding of the physiology underlying them has formed a focus of research over a number of years. The changes in preload induced by changes in intrathoracic pressure (ITP) with the respiratory cycle, have been applied to form dynamic tests of fluid responsiveness. Pulse pressure variation (PPV), stroke volume variation (SVV), end expiratory occlusion test, pleth variability index (PVI) and use of echocardiography are all clinical assessments that can be made at the bedside. However, there are limitations and pitfalls to each that restrict their use to specific situations. The haemodynamic response to treatment with continuous positive airway pressure (CPAP) in left ventricular failure is explained by the presence of heart lung interactions, and works predominately through afterload reduction. Similarly, in other disease states such as acute respiratory distress syndrome (ARDS), the effects of a change in ventilation can provide information about both the cardiac and respiratory system. This review aims to summarise how assessment of cardiac function using heart lung interactions can be performed. It introduces the underlying physiology and some of the clinical applications that are further explored in other articles within the series.

Keywords: Cardiac; haemodynamics; respiratory.

Figure 1

Frank-Starling and venous return curves in patients receiving positive pressure ventilation (curve A is a responder, curve B a non-responder). There is an increase in intrathoracic pressure in inspiration. This triggers an increase in RAP. As venous return depends on the gradient between Pmsf and RAP, venous return decreases during inspiration. Note that Pmsf does not change during the respiratory cycle, and the movement across venous return curves implies different Frank-Starling curves, as the changes are isovolaemic. The decrease in venous return in inspiration decreases right ventricular filling and subsequently the output from the right ventricle falls. The effects of this are seen on the left ventricle (and on the arterial pressure) during expiration, due to pulmonary transit time. RAP, the right atrial pressure; Pmsf, mean systemic filling pressure.