The use of positive end-expiratory pressure (PEEP) and alveolar recruitment manoeuvre (ARM) in equine anaesthesia – a review

Abstract

Summary: Impairment of gas exchange and arterial oxygenation during general anaesthesia and recumbency of the horse are a common and well-known problem. Often these alterations are poorly responsive to intermittent positive pressure ventilation (IPPV) and increasing levels of supplemental oxygen. Over the last decades the understanding of the underlying pathophysiology developed more and more, recognizing that a decrease in functional residual capacity (FRC), lung collapse and atelectasis, and ultimately ventilation/perfusion mismatch lead to poor oxygenation. Accordingly, new ventilation strategies arose over the course of time including alveolar recruitment manoeuvres (ARMs) and positive end expiratory pressure (PEEP). In 1992 Burkhard Lachmann presented his idea of an 'open lung' concept using high peak inspiratory pressures to reopen atelectatic lung regions followed by sufficiently high PEEP to keep the recruited lung areas open. In equine anaesthesia, early attempts with continuous positive pressure in spontaneously breathing horses yielded rather disappointing results. In combination with IPPV the use of PEEP resulted in unreliable improvements in the partial pressure of oxygen in arterial blood (PaO 2 ). Once the pre-treatment with an ARM prior to PEEP was introduced, improvement in PaO 2 could be seen to be more reliable. Despite these promising results, there is a risk of adverse effects with the use of PEEP and ARM in horses due to the high pressures that are necessary to achieve an effect. A reduction in cardiac output proportional to the increase in intrathoracic pressure has been observed and may compromise peripheral blood flow as well as gastrointestinal perfusion once a certain pressure threshold is reached. Similar, an increase in right to left shunt due to forcing blood into already collapsed lung areas can compromise the desired effect. Finally, early signs of ventilator induced lung injury have been detected in horses and it remains to be seen how future ventilation strategies will succeed in finding the optimal balance for each individual horse. This review presents the development and use of ARMs and PEEP in horses and its potential effect and side effects.

Successful management of hypoxaemia in a mule during general anaesthesia

“…It is generally considered that mechanical ventilation should better be started early after induction of general anaesthesia to avoid large areas of lung atelectasis 12 , which may make recruitment more difficult. 41 This probably partially explains the failure of standard mechanical ventilation to improve the situation and delays the success of recruitment manoeuvres in the present case. A previous study suggests that mechanical ventilation with PEEP initiated from the beginning of anaesthesia is more effective to decrease development of atelectasis and hypoxaemia.…”

“…Half an hour of standard mechanical ventilation did not improve the situation; rather, the values worsened revealing the extent of V/Q mismatch and pulmonary shunt. It is generally considered that mechanical ventilation should better be started early after induction of general anaesthesia to avoid large areas of lung atelectasis 12 , which may make recruitment more difficult 41 . This probably partially explains the failure of standard mechanical ventilation to improve the situation and delays the success of recruitment manoeuvres in the present case.…”

Assessment of Magnetic Resonance Imaging Artefacts Caused by Equine Anaesthesia Equipment: A Cadaver Study

“…Previous studies have demonstrated that the degree of alveolar collapse can be reduced by application of an alveolar recruitment manoeuvre (ARM) and continuous PEEP [16,27,28]. An alternative approach consists of an ARM provided by stepwise incremental and decremental peak inspiratory pressure (PIP) and PEEP, which has been found to be amongst the most effective methods to redistribute ventilation and improve oxygenation [29,30]. Additional strategies involve the use of continuous CPAP, which also proved effective in redistributing ventilation to the dependent lung regions, thereby decreasing ventilation/perfusion mismatch [23].…”

Persistent hypoxaemia and pulmonary oedema in a horse anaesthetised for emergency exploratory laparotomy

“…PEEP increases functional residual capacity (FRC) over the critical closing volume, hence helping prevent alveolar collapse at the end of expiration 23,27 . While application of PEEP without an ARM did not prove effective in improving gas exchange in spontaneously breathing anaesthetised horses, 28 it was demonstrated to change distribution of ventilation and increase PaO 2 and FRC when mechanical ventilation was used 29–31 .…”

“…PEEP increases functional residual capacity (FRC) over the critical closing volume, hence helping prevent alveolar collapse at the end of expiration. 23,27 While application of PEEP without an ARM did not prove effective in improving gas exchange in spontaneously breathing anaesthetised horses, 28 it was demonstrated to change distribution of ventilation and increase PaO 2 and FRC when mechanical ventilation was used. [29][30][31] Nonetheless, high levels of PEEP are associated with high intrathoracic pressures, which may decrease the ventilation/perfusion ratio by forcing blood into already atelectatic lung areas, leading to a higher degree of right-to-left shunt and decreased oxygenation.…”