Question 8

College Answer

This question required a breadth of response that was largely unrepresented in the answers provided by candidates. Better answers divided responses into cardiovascular effects and respiratory effects however many did not provide sufficient detail or provided only detail on some of the effects and missed the breadth of the question. Cardiovascular effects could be grouped into effects on the; right heart, left heart, ventricular interdependence, cardiac output, other circulations, baroreceptor and hormonal reflexes. Respiratory effects could be grouped into lung volumes, dead space, V/Q matching, compliance, airway resistance and the effect of excessive pressures on the lung. Despite this division in the question many candidates abandoned a structure which made their answers and explanations lack clarity. 
The examiners commented that most candidates who wrote an answer in paragraph format were likely to provide long vague answers rather than those more factual and succinct answers presented in a format with headings as above and pertinent associated dot point descriptions underneath these headings. 

Discussion

Respiratory effects of positive pressure ventilation:

• PEEP increases functional residual capacity (FRC)
• By increasing FRC, PEEP:
  • Increases alveolar recruitment, which gives rise to:
    • Improved V/Q matching
    • Increased total gas exchange surface
  • Increases lung compliance
  • Decreases the work of breathing (done against compliance)
• Positive pressure may also redistribute lung water out of the lung interstitium
• Excessive positive pressure leads to
  • Overdistension and lung injury
  • Worsening V/Q matching
  • "Biotrauma", i.e. cytokine leak and extrapulmonary organ dysfunction

Cardiovascular effects of positive pressure ventilation:

• Effects on the right ventricle and the pulmonary circulation:
  • Increased intrathoracic pressure is transmitted to central 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 ventricular afterload
  • Thus, increased afterload and decreased preload has the net effect of decreasing the right ventricular stroke volume. 
• Effects on the left ventricle and the systemic circulation:
  • 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
• Effects on overall cardiovascular function:
  • Decreased cardiac output
  • Decreased myocardial oxygen consumption

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.

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

Morgan, Beverly C., et al. "Hemodynamic effects of intermittent positive pressure respiration." Anesthesiology 27.5 (1966): 584-590.