Clinical Implications of H-L Interaction in the ARDS Patient

While the protective ventilation regimen applied in ARDS makes standard dynamic indices of fluid responsiveness based on H-L interaction unreliable, the pathophysiology of the haemodynamic effects of H-L interaction represents a strong rationale for managing mechanical ventilation in the light of RV physiology and for addressing weaning failure from a different pathophysiological approach.

Therefore, the main clinical implication with regard to ARDS management consists of preventing or improving ACP by adhering to lung-protective strategies during mechanical ventilation [5, 9, 11-13] and in addressing the cardiac issue at the time of difficult weaning.

The management of mechanical ventilation to prevent effects on the RV requires an echocardiographic approach. Echocardiography is currently considered the gold standard in this setting [5].

The comprehensive description of the ultrasound assessment of the heart under mechanical ventilation goes beyond the aim of this chapter. However we suggest that any ARDS patient under PPV should be evaluated by echocardiography on a daily basis, and reassessed any time the clinical situation shows changes in the haemodynamic profile or the clinician suspects that a RV dysfunction is developing. The echo approach should aim at evaluating the RV anatomy and function, addressing the RV/LV ratio, and detecting and assessing the tricuspid valve.

The RV assessment allows one to balance lung recruitment and lung overdistention [5]. A strategy based on the use of echocardiography in combination with a protective ventilatory approach allows one to pursue the following: “what is good for the lung is good for the right ventricle” [5].

Positive pressure ventilation can also have beneficial effects on haemodynamics in ARDS patients if appropriately managed. The use of PEEP to treat hypoxaemia may reduce pulmonary vascular resistance through lung recruitment and aeration of hypoxic alveoli, which contribute to blunt hypoxic pulmonary vasoconstriction. Applying the least amount of PEEP necessary to achieve an adequate PaO₂/FiO₂

ratio should lead to the least detrimental haemodynamic effects. This application can be achieved by titrating PEEP under echocardiography monitoring of the RV response [12, 13].