Dispersal of respiratory droplets with open vs closed oxygen delivery masks: implications for the transmission of severe acute respiratory syndrome.

Nosocomial transmission of droplet-borne respiratory infections such as severe acute respiratory syndrome (SARS) may be influenced by the choice of oxygen face mask. A subject inhaled saline mist and exhaled through three oxygen masks to illustrate the pattern of dispersal of pulmonary gas. In two commonly used masks, exhaled gas formed a plume emanating from the side vents, while a third mask with a valved manifold, which was modified by adding a respiratory filter, retained the droplets. Maintaining respiratory isolation during the administration of oxygen may reduce the risk of the nosocomial transmission of respiratory infections such as SARS.

The effects of carbon monoxide on respiratory chemoreflexes in humans.

As protection against low-oxygen and high-carbon-dioxide environments, the respiratory chemoreceptors reflexly increase breathing. Since CO is also frequently present in such environments, it is important to know whether CO affects the respiratory chemoreflexes responsiveness. Although the peripheral chemoreceptors fail to detect hypoxia produced by CO poisoning, whether CO affects the respiratory chemoreflex responsiveness to carbon dioxide is unknown. The responsiveness of 10 healthy male volunteers were assessed before and after inhalation of approximately 1200 ppm CO in air using two iso-oxic rebreathing tests; hypoxic, to emphasize the peripheral chemoreflex, and hyperoxic, to emphasize the central chemoreflex. Although mean (SEM) COHb values of 10.2 (0.2)% were achieved, no statistically significant effects of CO were observed. The average differences between pre- and post-CO values for ventilation response threshold and sensitivity were -0.5 (0.9) mmHg and 0.8 (0.3) L/min/mmHg, respectively, for hyperoxia, and 0.7 (1.1) mmHg and 1.2 (0.8) L/min/mmHg, respectively, for hypoxia. The 95% confidence intervals for the effect of CO were small. We conclude that environments with low levels of CO do not have a clinically significant effect acutely on either the central or the peripheral chemoreflex responsiveness to carbon dioxide.

Direct effect of Pa(CO2) on respiratory sinus arrhythmia in conscious humans.

Respiratory sinus arrhythmia (RSA) may improve the efficiency of pulmonary gas exchange by matching the pulmonary blood flow to lung volume during each respiratory cycle. If so, an increased demand for pulmonary gas exchange may enhance RSA magnitude. We therefore tested the hypothesis that CO2 directly affects RSA in conscious humans even when changes in tidal volume (V(T)) and breathing frequency (F(B)), which indirectly affect RSA, are prevented. In seven healthy subjects, we adjusted end-tidal PCO2 (PET(CO2)) to 30, 40, or 50 mmHg in random order at constant V(T) and F(B). The mean amplitude of the high-frequency component of R-R interval variation was used as a quantitative assessment of RSA magnitude. RSA magnitude increased progressively with PET(CO2) (P < 0.001). Mean R-R interval did not differ at PET(CO2) of 40 and 50 mmHg but was less at 30 mmHg (P < 0.05). Because V(T) and F(B) were constant, these results support our hypothesis that increased CO2 directly increases RSA magnitude, probably via a direct effect on medullary mechanisms generating RSA.