Pathophysiological effects of non-invasive ventilation

EFFECTS ON THE RESPIRATORY SYSTEM

Extrinsically applied positive end expiratory pressure (ePEEP) increases alveolar size and recruitment.^{2, 3} This expands the area available for gas exchange, reduces intrapulmonary shunt, improves lung compliance, and decreases the work of breathing.^3–6 It acts to negate the effects of intrinsic PEEP (iPEEP), which is the cause of dynamic airway compression and gas trapping.^{7, 8} Ventilation is improved with beneficial effects on the alveolar-arterial gradient, hypercarbia and, to a lesser extent, hypoxia.^{9, 10}

Pressure support (alone or as part of bilevel positive airway pressure, BiPAP) further augments alveolar ventilation and allows some respiratory muscle rest during the inspiratory phase.¹¹

EFFECTS ON THE CARDIOVASCULAR SYSTEM

PEEP reduces venous return to the right side of the heart.¹² Left ventricular preload, transmural pressure, and relative afterload are all decreased without altering myocardial contractility.^12–14 Thus, the ejection fraction improves without an increase in myocardial oxygen consumption.^{12, 15} It appears that those with worst ventricular dysfunction show the most significant improvement in stroke volume index with continuous positive airway pressure (CPAP).¹⁶

Overall, CPAP leads to a decrease in arterial pressure, heart rate, and rate-pressure product within 10 minutes, without exacerbation of hypotension.¹⁷ …