Question 5(p.2)

College Answer

Points required included a definition of PEEP, both intrinsic and extrinsic.
The important physiological consequences that need to be discussed are respiratory including
increased FRC, increased compliance and decreased work of breathing.
Cardiovascular consequences include decreased venous return and subsequently decreased
cardiac output and an increased pulmonary vascular resistance.
Renal consequences include decreased renal blood flow and increased ADH
Effects on intra-abdominal pressure, hepatic blood flow and the beneficial effects in cardiac
failure earned marks.
Syllabus B1k.2a
Reference: Nunn 6th edition p. 431.

Discussion

The question did not specifically ask for a definition, let alone a definition of intrinsic PEEP, and so the trainees would have been quite surprised to find this among the essential points expected of a passing answer. In spite of these hidden gems, 40% seem to have passed.

Definitions:

• PEEP (positive end-expiratory pressure) is  "a residual pressure above atmospheric maintained at the airway opening at the end of expiration” (Joint Committee on Pulmonary Nomenclature of the American Thoracic Society and the American College of Chest Physicians, 1975) 
• Intrinsic PEEP has no official definition, but one example of a good description is from Brander & Slutsky (2012): "when expiratory flow has not emptied alveoli to their resting FRC values by the end of exhalation... [intrinsic PEEP is] the residual positive pressure within the lungs referenced to atmospheric pressure or to PEEP"  

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

American College of Chest Physicians. "Society of Thoracic Surgeons. Pulmonary terms and symbols: a report of the ACCP-STS Joint Committee on Pulmonary Nomenclature." Chest 67 (1975): 583-93.

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.