Carbon dioxide re-breathing with close fitting face respirator masks
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Guidelines for the use of personal protective equipment when caring for patients who are affected by highly infectious pathogens such as SARS, avian influenza and tuberculosis include the use of high efficiency respirators that filter at least 95% of particles with a median diameter of 0.1 micron. These must therefore be close fitting to prevent air leakage [1, 2]. A healthy intensivist (SF), wearing such a respirator (Tecnol Fluidshield PFR95, Kimberly Clark Corporation, Roswell, GA) to perform a percutaneous tracheostomy on a patient with multidrug resistant pulmonary tuberculosis, experienced dyspnoea, tachycardia and tremor after 30 min. End-tidal carbon dioxide measured at the mouth by hand-held capnometry was 6.3 kPa (normal value 5.3 kPa). We postulated that the symptoms were due to hypercapnia.
We measured the end-tidal carbon dioxide levels in four anaesthetists wearing the same design of mask, before and after performing tracheal intubation on another patient with pulmonary tuberculosis. Measurements were taken by sidestream capnometer (Poet LT, Criticare, Waukesha, WI) using a 15-mm T-piece held between the lips. The mean baseline end-tidal carbon dioxide level was 5.18 kPa. Post-procedure (20 min later) the mean end-tidal carbon dioxide level was 5.95 kPa (p = 0.007). No subjects reported symptoms of hypercapnia. The rise in end-tidal carbon dioxide is due to rebreathing of expired alveolar gas that is ‘trapped’ in the respirator, with the degree of rebreathing being proportional to the volume of the respirator (‘dead space’). It is likely that all tight-fitting, high efficiency respirators will behave similarly, with only the size of dead space varying between designs.
The respiratory response to hypercapnia is an increase in minute ventilation, giving rise to the sensation of dyspnoea. Moderate (6.18 kPa) to high (7.5 kPa) levels of end-tidal carbon dioxide have also been shown to impair significantly cognitive and psychomotor performance and it is likely that this effect of carbon dioxide is dose related with no threshold [3]. Clearly, our findings are of uncertain practical significance and further trials would be required employing cognitive and psychomotor measurements and arterial blood gas analysis.
In the event of an influenza pandemic, large numbers of healthcare workers may need to wear these respirators for prolonged periods and problems with hypercapnia might reduce the tolerability of these devices. Whether psychomotor performance is affected also remains to be seen.
