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PEEP is defined as the pressure above atmospheric maintained in the airway at the end of expiration. It must be utilised for all intubated and ventilated patients except in rare situations as indicated below. An initial setting of 5 cm H2O should be utilised which is then titrated in concert with the required inspired oxygen setting.
Start with a PEEP of 5 cm H2O up to 40% oxygen, and 15 cm H2O up to 60% oxygen. Thereafter the level of PEEP should be customised to the patient's condition following discussion with the Intensive Care Specialist. Whenever there are significant changes in oxygenation or if the PaO+2+ is less than 60mmHg (FiO2 60%) the Intensive Care specialist must be called.
PEEP is a therapeutic technique which increases functional residual capacity (FRC), prevents alveolar derecruitment, decreases ongoing lung injury, and improves lung compliance. It is specifically helpful in patients with cardiogenic and non cardiogenic pulmonary oedema. PEEP also minimises aspiration risk by reducing the leakage of oropharyngeal secretions past the endotracheal tube cuff.
In ARDS, alveolar recruitment and derecruitment occur throughout inspiration and expiration respectively. PEEP keeps freshly recruited alveolar units open during expiration, by preventing the alveolar pressure falling below its threshold closing pressure (TCP). However the pressure required to open a collapsed unit, known as the threshold opening pressure (TOP), is usually much greater than the TCP. In some cases, patients require a recruitment manoeuvre using controlled inflation pressures of 45-50 cmH2O to open de-recruited regions. Recruitment manoeuvres should only be used, if you are familiar with the technique and after discussion with the intensive care specialist.
From Sundaresan A. Applications of Model-Based Lung Mechanics in the Intensive Care Unit; PhD thesis in Mechanical engineering, University of Canterbury, New Zealand 2010
The above pressure volume diagram of lung with ARDS, shows the increasing PEEP with a recruitment manoeuvre may improve FRC. Increasing FRC improves oxygenation for any given level of inspired oxygen.
The level of PEEP needs to be discussed with the Intensive Care Specialist in all patients with a bronchopleural fistulae, other significant injuries to lung leading to air leak, or severe 'gas trapping' e.g. asthma.
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