Question 20

College Answer

Structured answers separating effects of positive pressure on right and left ventricle, on preload and on afterload were expected. Overall there was a lack of depth and many candidates referred to pathological states such as the failing heart. Simply stating that positive pressure ventilation reduced right ventricular venous return and/or left ventricular afterload, without some additional explanation was not sufficient to achieve a pass level.

Discussion

Mechanisms affecting the right ventricle and the pulmonary circulation are:

• Increased intrathoracic pressure is transmitted to cenral veins and the right atrium, decreasing right ventricular preload
• Increased intrathoracic pressure is transmitted to pulmonary arteries
• Transmitted alveolar pressure increases pulmonary vascular resistance
• Increased pulmonary vascular resistance increases right ventriular afterload
• Thus, increased afterload and decreased preload has the net effect of decreasing the right ventricular stroke volume. 

Mechanisms affecting the left ventricle and the systemic circulation are:

• Decreased preload by virtue of lower pulmonary venous pressure
• Decreased afterload due to a reduction in LV end-systolic transmural pressure and an increased pressure gradient between the intrathoracic aorta and the extrathoracic systemic circuit
• Thus, decreased LV stroke volume

The consequences of this are:

• Decreased cardiac output
• Decreased myocardial oxygen consumption

Most of this information comes from only two textbooks. With "Basic Assessment and Support in Intensive Care" by Gomersall et al (was well as whatever I picked up during the BASIC course) as a foundation, I built using the humongous and canonical "Principles and Practice of Mechanical Ventilation" by Tobins et al – the 1442 page 2nd edition.

Oakes, Dennis L. Physiological Effects of Positive Pressure Ventilation. AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH, 1992. -this is somebody's Masters of Science thesis! They received their degree in 1992, but one expects that the fundamentals of physiology have remained the same since then.

Kumar, Anil, et al. "Continuous positive-pressure ventilation in acute respiratory failure: effects on hemodynamics and lung function." New England Journal of Medicine 283.26 (1970): 1430-1436.

Luecke, Thomas, and Paolo Pelosi. "Clinical review: positive end-expiratory pressure and cardiac output." Critical Care 9.6 (2005): 607.

Cassidy, S. S., et al. "Cardiovascular effects of positive-pressure ventilation in normal subjects." Journal of Applied Physiology 47.2 (1979): 453-461.

Luce, John M. "The cardiovascular effects of mechanical ventilation and positive end-expiratory pressure." Jama 252.6 (1984): 807-811.

Duke, G. J. "Cardiovascular effects of mechanical ventilation." Critical Care and Resuscitation 1.4 (1999): 388.