Un bout de souffle: ventilatory support for pulmonary oedema

What is the best treatment for acute pulmonary oedema? There is a wealth of large scale randomized studies conducted in chronic stable heart failure which informs the management of the condition and has helped in drawing up guidelines. In contrast, there have been very few trials of treatment for acute pulmonary oedema. Pulmonary oedema is an appalling experience. For many patients, it is the most distressing thing they ever experience. The pathophysiological model that best appears to describe the syndrome is the straightforward haemodynamic model: left ventricular filling pressure rises, causing a rise in pulmonary venous and pulmonary capillary pressure. The pressure exceeds the colloid osmotic pressure in the vessels and oedema fluid accumulates in the air spaces; the rate of accumulation exceeds the rate of lymphatic drainage, and the patient starts to drown. Traditional management has focussed on the need to reduce ventricular filling pressures, principally with nitrates, and on inducing a diuresis with loop diuretics. Oxygen at high flow rates is given. In passing, there is a certain lack of logic to this approach: in acute pulmonary oedema, there has been little time for the patient to develop fluid overload, and the problem is mainly haemodynamic. Circulatory support with positive inotropic drugs is sometimesneeded. Intubation and positive pressure ventilation is the last resort and offers the distressed and tiring patient almost immediate relief of symptoms, whilst buying time for pharmacological treatment to work. Data are slowly accumulating on the possible role of non-invasive respiratory support but the techniques and equipment are fertile areas for confusion. Continuous positive airway pressure (CPAP) should be distinguished from non-invasive positive pressure ventilation, although both can be applied with similar looking nasal or full-face masks. Patients on CPAP are breathing spontaneously and determine the depth and frequency of their breaths. The pressure is communicated to the airway at a constant level throughout the respiratory cycle and is positive in relation to atmospheric pressure at the end of expiration. In contrast, positive pressure ventilation blows air and entrained gases into the lung. Gas flow is primarily determined by the ventilator and the pressure