2016B Question 01
Describe the respiratory effects of adding positive end-expiratory pressure (PEEP) to intermittent positive pressure ventilation (IPPV).
Examiner Report
57% of candidates achieved a pass in this question.
This question was repeated from the last exam where the pass rate was 1%. Candidates that had read and studied the examination report showed a more detailed understanding of the relevant points and scored better. A pass could be achieved by acknowledging that anaesthesia causes a reduction in the FRC, that PEEP is applied to mitigate this effect, by outlining the effects on compliance and airway resistance that occurs, and by showing an understanding that this improves FRC in relation to closing capacity thereby recruiting dependent airways, improving V/Q matching and hence oxygenation. Unfortunately many of these marks were missed by candidates who spend time on definitions of PEEP, IPPV and describing the modes (pressure and volume) of ventilation that can be used, which no marks were awarded for.
Answers that included comments on the adverse effects of PEEP generally scored better. Many candidates correctly drew the pulmonary compliance curve, and accurately described the increase in compliance (right shift) that occurs with the application of PEEP, however few went on to describe the reduction in compliance that occurs with 'over-PEEP’ and the damage this many cause. Few answers had the x-axis of the compliance curve correctly labeled with positive pressure numbers reflecting a lack of integration of book knowledge into the answer.
A reduction in work of breathing (due to improved compliance and reduced airway resistance) does occur with optimal FRC, however in a mechanically ventilated patient this is manifested by a reduction in ventilator pressures and effort by the machine rather than energy expenditure from the patient. PEEP does not prolong apnoea time at anaesthesia induction (RSI) by providing a increased oxygen reservoir, because it has not been applied until after intubation and commencement of ventilation. Candidates who correctly acknowledge that PEEP provides benefits at extubation, and its use is associated with less atelectrauma, shear stress and inflammatory response scored extra marks.
Cardiovascular effects of PEEP only gained marks when describing V/Q changes that may occur and the effect of PEEP on PaO2.
Model Answer
Structure:
- Introduction
- Benefits
- Harm
- Clinical use
Introduction
| Factor | Description |
|---|---|
| Overall purpose | - Counteract the fall in FRC that occurs with anaesthesia - Prevent collapse of small airways and alveoli - Especially dependent regions - Especially at extubation - Note recruitment manoeuvres are required to re-open collapsed areas |
| Typical range | - 5-15cmH2O |
Benefits of PEEP
| Effect | Mechanism |
|---|---|
| ↑ Oxygenation | - ↑ Lung volume → ↑ Alveolar surface area → ↑ Gas exchange - Prevent collapse → ↓ Shunt→↑ PaO2 → ↑ DO2 |
| ↑ Lung compliance | - ↑ Static compliance: Due to ↑ lung volume → ↑ Alveolar Radius - ↑ Dynamic compliance: Due to prevention of small airway and alveolar collapse → ↓ Airway pressure, ↓ damage
|
| ↓ Airway resistance | - Laminar flow: - Turbulent flow: - PEEP → ↑ Lung volume → ↑ Airway radius → ↓ Resistance - Radius is the major factor, since power 4 or 5 → ↓ Airway pressure, ↓ damage
|
| ↓ PVR | - Low volume: Compress extra-alveolar vessels - High volume: Compress alveolar vessels → PVR minimized at natural FRC
|
| Prevent damage | - ↓ Alveolar collapse, ↓ atelectrauma → ↓ Shear stress → ↓ Inflammation |
Harm from PEEP
| Effect | Mechanism |
|---|---|
| Alveolar damage | - Over-PEEP → Barotrauma, volutrauma, pneumothorax, pneumomediastinum |
| Breath stacking | - ↓ Expiration rate → Stacking → Trauma - Higher risk if asthma, COPD |
| Dead space | - ↑ Alveolar pressure → ↑ West zone 1 |
| Impede cardiac output | - ↑ Intrathoracic pressure → Obstruction to venous return - Higher risk if hypovolaemia |
Clinical Use
| Scenario | Effect |
|---|---|
| General anaesthesia | - Prevent ↓ FRC, with all the above benefits |
Acute cardiogenic pulmonary oedema (as CPAP) |
- Obstruction of venous return → Normalization of preload - ↓ Transmural pressure- > ↓ Afterload - Move alveolar water into interstitium - Increases lymphatic drainage - (No effect on total lung water) |
| Type 2 respiratory failure (as BiPAP) | - Splints airways during expiration → Shift equal pressure point proximally → ↓ Airway collapse → ↓ Obstruction |