Positive end expiratory pressure (1)

Positive end expiratory pressure (PEEP) is a modality used in positive pressure ventilation to prevent the alveoli returning to atmospheric pressure during expiration. It is routinely set between 3–5cmH2O; however, in severe respiratory failure, it will often need to exceed 10cmH2O to be above the lower inflexion point of the pressure–volume curve. This has been suggested as beneficial in patients with severe ARDS. It rarely needs to exceed 20cmH2O to avoid cardiorespiratory complications and alveolar over-distension (see below). It neither prevents nor attenuates ARDS, reduce capillary leak, or lung water.

Respiratory effects

PEEP improves oxygenation by recruiting collapsed alveoli, re-distributing lung water, decreasing A–V mismatch, and increasing functional residual capacity (FRC).

Haemodynamics

PEEP usually lowers both left and right ventricular preload, and increases RV afterload. Though PEEP may increase cardiac output in left heart failure and fluid overload states by preload reduction, in most other cases, cardiac output falls, even at relatively low PEEP levels. PEEP may also compromise a poorly functioning right ventricle. Improved PaO2 resulting from decreased venous admixture may sometimes arise solely from reductions in cardiac output.

Physiological PEEP

A small degree of PEEP (2–3cmH2O) is usually provided physiologically by a closed larynx. It is lost when the patient is intubated or tracheostomised and breathing spontaneously on a T-piece with no CPAP valve (see CPAP).

Intrinsic PEEP (auto-PEEP, air trapping, PEEPi)

Increased level of PEEP due to insufficient time for expiration, leading to ‘air trapping’, CO2 retention, increased airway pressures, and increased FRC. Seen in pathological conditions of increased airflow resistance (e.g. asthma, emphysema) and when insufficient expiratory time is set on the ventilator. Used clinically in inverse ratio ventilation to increase oxygenation and decrease peak airway pressures. However, high levels of PEEPi can slow weaning by an increased work of breathing; use of extrinsic PEEP may overcome this. PEEPi can be measured by temporarily occluding the expiratory outlet of ventilator at end-expiration for a few seconds to allow equilibration of pressure between upper and lower airway, and then reading the ventilator pressure gauge (or print-out).

‘Best’ PEEP

Initially described as the level of PEEP producing the lowest shunt value. Now generally considered to be the lowest level of PEEP that achieves SaO2 ≥90% allowing, wherever possible, lowering of FIO2 (ideally ≤0.6), though not at the expense of peak airway pressures >35–40cmH2O or significant reductions in DO2.