Peak pressures or plateau pressures in acute asthma

Abstract

Sir: We read with interest the article “Isoflurane therapy for severe refractory status asthmaticus in children” wherein the authors describe the use of inhaled isoflurane in a series of children with lifethreatening asthma [1]. However, it was surprising to note that in the algorithm the authors used the peak inspiratory pressure rather than plateau pressure for titration of tidal volumes and as a surrogate for lung hyperinflation. Peak inspiratory pressure does not reflect peak alveolar pressure or the degree of overdistension of alveolar structures, as the high pressures are encountered largely by robust proximal airways. On the other hand, plateau pressures (measured after an inspiratory hold of 0.5–1.5 s) are most reflective of the peak alveolar pressure (also used in calculation of transpulmonary pressure and static lung compliance). Plateau pressures of more than 30–35 cmH2O risk barotrauma and hemodynamic compromise without tangible benefit to gas exchange or oxygen delivery [2]. Ina classical case of acute asthma requiring mechanical ventilation, the peak inspiratory pressures and plateau pressures are in the range of 75–100 cmH2O and 25–35 cmH2O respectively [3]. Therefore high peak pressures are generally accepted, and in fact recent data have failed to show a relationship between peak pressures and complications of mechanical ventilation [4]. Also, a strategy aimed at reducing inspiratory time with fast inspiratory flow rates and the square inspiratory flow waveform is usually associated with high peak inspiratory pressures. The importance of recognizing high peak inspiratory pressures lies in setting the alarms to a level higher than the peak pressure, otherwise the ventilator would cycle with every breath and would not deliver the set tidal volume, leading to hypoventilation and worsening gas exchange [5]. In addition, the difference between the peak and plateau pressure represents the contribution from airway resistance to the value of the peak pressure [6]. In conclusion, the measurement of peak pressures has limited clinical relevance; because of their important resistive component, the peak pressures do not reflect the alveolar distension pressure in most of the lung.