Interaction of hypoxia and hypercapnia on respiratory drive in patients with COPD

The objective of this study was to compare the response of respiratory drive to progressive hypoxia under eucapnic and hypercapnic conditions in patients with severe COPD. Twenty-five patients with severe COPD and 13 nonsmoking young men were studied. The pressure in the occluded airway measured 0.1 second after the onset of inspiration was used as an index of respiratory drive. The occlusion pressure was measured at levels of SaO2 between 97 and 85 percent while eucapnic. The PETCO2 was then increased 10 mm Hg and the study repeated. The response of respiratory drive to hypoxia as measured by the slope of the regression line relating occlusion pressure to SaO2 was weak and variable in eucapnic hypoxia, and some subjects had no demonstrable response. When mild respiratory acidosis was created by increasing the PETCO2, the response to hypoxia was much greater and occurred in all subjects studied. Respiratory acidosis resulting from acute elevation of the PaCO2 greatly potentiates the increase in respiratory drive in response to hypoxia in normal subjects and in patients with severe COPD. Increase in occlusion pressure may occur with slight degrees of hypoxia when acute hypercapnia is present. These observations suggest that patients with acute respiratory failure complicating COPD, treated with controlled oxygen administration with only partial correction of hypoxia and continued respiratory acidosis, will have high respiratory drive.

Ventilatory and metabolic effects of glucose infusions

It has been demonstrated that total parenteral nutrition (TPN) results in increased O2 consumption (VO2), CO2 production (VCO2) and minute ventilation (VE). TPN consists of a mixture of glucose and amino acids. The individual role of each of these nutrients in mediating these changes has not been well established. To examine the effects of the individual nutrients, continuous infusions of glucose in hypo- and hypercaloric amounts were given to four normal volunteer subjects and four acutely ill patients for a six-day period, with three days on each dietary intake. After each three-day period, gas exchange, VO2, VCO2, and ventilatory variables (VE), tidal volume (VT), frequency (f), mean inspiratory flow (VT/TI), inspiratory time (TI) and expiratory time (TE) were measured. With the high carbohydrate diet, CO2 production increased 18 percent (p greater than .05) and 7 percent (p greater than .05) in the normal subjects and the patients, respectively. VO2 did not change, while the RQ rose. VE rose in parallel with VCO2, with no significant change in ventilatory sensitivity to CO2. In light of previous observations, these results suggest that during administration of TPN, the protein component plays a major role in the observed ventilatory changes: a) by bringing about a rise in VO2, which acts to magnify the effect of an increased RQ on VCO2, and b) by increasing ventilatory sensitivity to CO2.