Ventilating horses: moving away from old paradigms.

All veterinarians who administer general anaesthesia to horses are regularly confronted with severely impaired gas exchange if not hypoxaemia in their patients. This is of particular concern given the exceptionally high perianaesthetic mortality in horses (Johnston et al. 2002). It has now been generally accepted that anaesthesia-induced atelectasis in dependent lung regions causing intrapulmonary shunting of blood is a major cause of this impaired arterial oxygenation (Nyman et al. 1990). In humans atelectasis and hypoxaemia are the typical features of Acute Lung Injury (ALI) or the Acute Respiratory Distress Syndrome (ARDS) with mortalities between 30% and 50% (Bernard et al. 1994). In the 1990s both, Lachmann and Amato proposed novel therapeutic concepts aimed at protecting the lungs from the additional injury brought about by the mechanical ventilator. They suggested that atelectatic lungs should be actively ‘opened’ by a dedicated intervention using high inspiratory pressures and then ‘kept open’ by applying sufficient levels of positive end expiratory pressure (PEEP) (Lachmann 1992; Amato et al. 1995). These concepts were not only physiologically interesting but also life-saving as Amato showed in his now famous study (Amato et al. 1998). The encouraging results of this pioneer work have given birth to different types of so called ‘recruitment manoeuvre’ (RM) for ARDS patients. Airway opening pressures as high as 60 or even 85 cmH2O have been reported for ARDS patients in whom the lungs became extremely stiff due to massive disturbances of surfactant function (Schreiter et al. 2004; Borges et al. 2006). Surprisingly few pulmonary complications following the use of RM and high airway pressures are reported in humans. It has long been recognized that atelectasis and impaired gas exchange also occur in about 90% of all human patients undergoing general anaesthesia (Moller et al. 1991). Modifying Lachman¢s ‘Open Lung Concept’, Tusman et al. (1999) were the first to apply these ideas also in anaesthetized patients. To date the beneficial physiological effects on oxygenation, CO2 elimination, lung mechanics and pulmonary vascular resistance of their cycling ‘Alveolar Recruitment Strategy’ have been documented for small infants, for morbidly obese and geriatric patients but also for one lung ventilation or during laparoscopic surgery. Since then many different recruitment maneuvers have appeared in the human literature. These recruitment manoeuvres can be divided into two basic categories: 1) sustained inflations such as the typical 40/40 manoeuvre where a continuous positive airway pressure (CPAP) of 40 cmH2O is applied for 40 seconds or 2) cycling recruitment manoeuvres where the normal breathing pattern is maintained but at elevated PEEP and Peak Inspiratory Pressures (PIP) (i.e. 10 breaths at a PEEP of 20 cmH2O and a PIP of 40 cmH2O are applied at a respiratory rate of 8 breaths minute). In principle both fulfill Lachmann¢s imperative to ‘open up the lung’ but the impact of the former on haemodynamics is usually more pronounced than in the latter case (Tusman & Belda 2010; Tusman & Böhm 2010). Now, the question for the veterinary anaesthetist arises: can we apply such treatment principles also in our large equine patients? Shall we really do it and if so, how? In this issue of Veterinary Anaesthesia and Analgesia, Hopster et al. (2011) investigated, in horses, the efficacy of a ventilatory strategy which included a specific RM and compared gas exchange in this group (n = 12) with that of a control group (n = 12). The horses were suffering from colic and undergoing exploratory laparotomy in dorsal recumbency, a position known to increase the risk of hypoxaemia. These horses were treated with perioperative fluids and if needed with inotropic drugs because such horses are often hypovolaemic and hence cardiovascularly unstable. The RM the authors designed is a mix of the types described