How far can we go with permissive hypercapnia? A case presentation and some biased comments with emphasis on maintaining normal haemoglobin level.

The respiratory management strategy of small tidal volume with permissive hypercapnia has been adopted to avoid further aggravation of lung injury due to high airway pressure with some impressive success (1). No consensus, however, has been established in terms of the rate of increase in PaCO2 and its upper limit. Recently, our colleague in the intensive care unit experienced a severe case of ARDS successfully treated with the above strategy despite of the fact that during the course of treatment, the highest PaCO2 reached 177 mmHg and the lowest pH, 7.03 (2). The fact that PaCO2 may reach a very high level in the clinical setting and the well-known role of haemoglobin (Hb) in buffering CO2 led us to study effects of different Hb levels on pH and haemodynamic changes in response to acute CO2 loading in the blood. We will summarize the case report first with permission of authors (the case report was published in Japanese) (2) and then discuss the studies conducted in our animal laboratory.