Question 2

College Answer

This topic has been asked previously. It was expected candidates could detail the impact of PEEP on a variety of respiratory parameters such as lung volume, dead space, arterial pO2 and intrapleural pressure. The cardiovascular consequences are well described including the effect on cardiac output, blood pressure and oxygen delivery. 
The physiological impact of lower levels PEEP in a young healthy person is different to that often seen in the critically ill and this was not appreciated by most candidates. 

Discussion

The college did ask about the effects on a "healthy mechanically ventilated adult", which basically means you can't spend half a page discussing how PEEP is beneficial in the setting of left heart failure, or how it reduces inspiratory effort in a patient with bronchospasm and gas trapping.

In summary:

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

Other organ system effects of positive pressure ventilation:

• Raised intracranial pressure, if the PEEP is very high
• Water retention due to increased ADH release and aldosterone secretion
• Sodium retention due to decreased ANP release and aldosterone secretion
• Decreased renal perfusion and GFR (due to decreased cardiac output and increased renal venous pressure)
• Decreased hepatic perfusion and thus decreased metabolic clearance of drugs
• Decreased splanchnic perfusion, resulting id decreased intestinal motility and poor gastric emptying
• Decreased gastric perfusion, increasing the risk of stress ulceration
• Neutrophil retention in the pulmonary capillaries
• Impaired lymphatic drainage from the lungs

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.