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Mechanical Ventilation Effects — FRCA Primary MCQ

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ModeratePhysiology - RespiratoryMechanical Ventilation EffectsFRCA Primary

In a normovolaemic adult with normal cardiac function, which statement best describes the immediate effect of the inspiratory phase of controlled positive-pressure ventilation on systemic venous return, compared with spontaneous inspiration?

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Correct answer: A — It decreases as right atrial pressure rises and the venous-return pressure gradient narrows

During positive-pressure inspiration, airway and pleural pressures rise. Transmission of pleural pressure to the right atrium increases right atrial pressure, thereby narrowing the pressure gradient between mean systemic filling pressure and right atrial pressure that drives systemic venous return. Venous return and right ventricular preload therefore tend to decrease; the effect is particularly important in hypovolaemia. In spontaneous inspiration, pleural and right atrial pressures fall, tending to widen this gradient and augment venous return, so A is reversed. Mean systemic filling pressure does not predictably rise enough to produce B. Pleural pressure is transmitted to the right atrium, excluding D. Circulating volume influences mean systemic filling pressure, but venous return also depends on right atrial pressure and resistance to venous return, excluding E.

Reference: Berger D, Takala J. Determinants of systemic venous return and the impact of positive pressure ventilation. Annals of Translational Medicine. 2018;6(18):350. https://pubmed.ncbi.nlm.nih.gov/30370277/