H-L Interactions: The Impact of Cardiorespiratory System Characteristics

The description of the anatomy and function of the heart and lungs is basic knowledge that is beyond the scope of this chapter. Therefore, only some key aspects of the cardiorespiratory system relevant to the H-L interaction issue of the ARDS patient will be addressed here.

The human heart shows the interesting combination of physiologically consisting of a double pump system where the two pumps, the left and right ventricles, work in series, with each pump ejecting the blood received from the other pump, and anatomically consisting of two pumps sharing the interventricular septum and the pericardium, thus being anatomically in parallel (Fig. 16.3). One more important aspect to underline in this contest is the anatomic and functional presence of the pulmonary circulation between the two pumps within the chest. This peculiar anatomic and functional condition of two pumps structurally made in parallel and mechanically working in series, with the pulmonary circulation being the circuit connecting the right ventricular ejection to the left ventricle, represents the main reason for the H-L interactions to occur and to have consequences in diseased states,

Fig. 16.3 Pulmonary and systemic circulation (to modify) (Modified with permission from F. Guarracino “Ecocardiografia in Area critica” 2013 Elsevier, Milan)

and it is pivotal to understanding the ventricular interdependence. In fact, any acute and relevant change of the anatomy and/or the function of one pump or of the pulmonary circulation will cause the H-L interactions to be clinically significant.

In the setting of ARDS, it is the relationship between the right ventricle (RV) and the pulmonary circulation that plays a major role, in a setting where the physiological interplay of the pump and vascular elastances is pivotal in determining the right ventriculo-arterial coupling.

It is wise to distinguish H-L interactions under spontaneous ventilation from those observed under positive pressure ventilation.