Knowledge of heart–lung interactions is key to manage haemodynamics in mechanically ventilated patients (see also Chapter 5). It allows intensivists to understand the meaning of blood and pulse pressure respiratory variations (PPV). Unlike spontaneous breathing, positive pressure ventilation increases blood pressure and pulse pressure during inspiration following by a decrease during expiration. This is called reverse pulsus paradoxus and includes a ‘d-down’ and a ‘d-up’ effect. No variation means no effect of mechanical ventilation on the heart and especially on the right heart. In case of significant PPV, tidal volume usually reduces right ventricular stroke volume by way of reducing preload where systemic venous return is decreased (fluid expansion is useful to restore haemodynamics, when impaired) or increasing afterload (obstruction of pulmonary capillaries due to alveolar inflation and, in this case, fluid expansion is useless or even sometimes deleterious). Clinical examination as well as evaluation of respiratory variations of superior vena cava by echo, helps to distinguish between these two situations. By studying the beat-by-beat changes in echo parameters induced by positive pressure ventilation heartbeat by heartbeat, echocardiography is perfectly suited to study heart–lung interactions and then to propose an appropriate optimization in case of haemodynamic impairment.
Is There an Optimal Positive End-expiratory Pressure to Measure the Internal Jugular Vein Collapsibility Index? A Pilot Study in Mechanically Ventilated Patients
