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

 

List:

·       Intro

·       Benefits

·       Harm

·       Clinical use

 

Intro:

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

 

Benefits of PEEP:

↑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

 

Harm from PEEP:

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:

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