2016B01 Describe the respiratory effects of adding positive end-expiratory pressure (PEEP) to intermittent
positive pressure ventilation (IPPV).

Definition

·   Positive end-expiratory pressure

Overall purpose

·   Counteract the fall in FRC that occurs with anaesthesia

·   Prevent collapse of small airways and alveoli

o Especially dependent regions

o Especially at extubation

o Note recruitment manoeuvres are required to re-open collapsed areas

Typical range

·   5-15cmH₂O

↑Oxygenation

·   ↑Lung volume -> ↑Alveolar surface area -> ↑gas exchange

·   Prevent collapse -> ↓shunt->↑PaO₂ -> ↑DO₂

·   Note theoretical ‘sweet spot’ for DO₂

(might not be so simple - see Nunn’s Applied Respiratory Physiology, Fig 30.3)

↑Lung compliance

·   ↑Static compliance: due to ↑lung volume -> ↑alveolar radius

·   ↑Dynamic compliance: due to prevention of small airway and alveolar collapse

    ->↓Airway pressure, ↓damage

(modified from West’s Respiratory Physiology)

↓Airway resistance

·   Laminar flow: R = (8 x length x viscosity) / (π x radius⁴)

·   Turbulent flow: (P₁-P₂) ∝ (length x density) / (radius⁵)

·   PEEP -> ↑Lung volume -> ↑airway radius -> ↓resistance

·   Radius is the major factor, since power 4 or 5

    ->↓Airway pressure, ↓damage

↓Pulmonary vascular resistance

·   Low volume: compress extra-alveolar vessels

·   High volume: compress alveolar vessels

    -> PVR minimized at natural FRC

Prevent damage

·   ↓Alveolar collapse, ↓atelectrauma

    -> ↓Shear stress

    -> ↓Inflammation

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

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