Estimation of alveolar pressure variations during panting.

Plethysmographic assessment ofthoracic gas volume (TGV), using Boyle’s law during panting against an occluded airway, necessitates an estimation of alveolar pressure variations (SPA). In the widely used method introduced by DuBois et al in 1956,’ mouth pressure variations ( Pm) are taken as an estimate of SPA. However, these authors noted that in the presence ofcheek movements “the change in mouth pressure is less than the change in alveolar pressure and lags in time because of a pressure fall across the trachea owing to airflow” In a recent issue of Chest, Knudson and Knudson2 studied the error introduced by upper airways using a mechanical model and relating i Pm to pleural pressure variations (APp1). They concluded that “changes in mouth pressure during panting may underestimate changes in pleural, and hence, alveolar pressures.” Although the overall conclusion is true, the point we want to make is that, unless lung compliance is infinite as was the case in that particular model, Ppl is not equivalent to SPA. Pplwas proposed as an alternative to Pm forTGV assessment in obstructive patients.3 The theoretical basis of this method has been fully discussed using electrical analogs and the error introduced was found to be smaller than that resulting from DuBois’ method in severe airway obstruction.34 However, Ppl always overestimates L PA and the error is magnified by increasing lung volume, lung elastance and extrathoracic airways compliance.4 Therefore, the best method to assess TGV may vary according to mechanical properties of the respiratory system. To further illustrate this point, we determined Pplh Pm in six normal subjects panting at 0.8 Hz both at functional residual capacity and near total lung capacity. PplJi Pm was close to unity at functional residual capacity (mean 1.01± SD 0.01) but largely increased near total lung capacity (1.15 ± 0.08). At both lung volumes APm was likely to represent PA owing to normal airway resistance, low frequency of panting and cheek support. However, near total lung capacity Ppl should largely overestimate PA due to the increase in lung volume and lung elastance. In conclusion, i Pm closely reflects APA except in severe airway obstruction. In this case, TCV assessment may be improved by asking patients to support their cheeks and use panting frequencies below 1 Hz.’ if a detectable phase difference between mouth pressure and the volume signal still exists, an error may be present and an alternative method using i Ppl may then be useful. Another possibility is to decrease airway resistance by using a low density gas mixture.’ Paul Begin, M.D. , F.C.C.P ,H#{244}pital Notre Dame, Montreal, Canada; and Ren#{233} Peslin M.D., Iriserm de Physiopathologie Respiratoire, Vandoeuvre, France