The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

EFFECTS OF MINUTE VENTILATION AND VENTILATOR SENSITIVITY ON THE OXYGEN COST OF BREATHING IN NORMAL SUBJECTS

Brian Jent CRTT, Grant Liston RRT, Joseph Bailey BS, and William Burke PhD RRT, Indiana University Hospital, Indiana University Respiratory Therapy Program, Indianapolis, IN 46202 and Midwestern State University, Respiratory Therapy Program, Wichita Falls, TX 76308

BACKGROUND Work of breathing (WOB) is an important variable that Respiratory Therapists attempt to use to evaluate a patients ability to support spontaneous breathing. There are two methods used to evaluate a patients WOB. One is to measure the mechanical work of inflation by examining the driving pressure volume curve. In spontaneous breathing subjects this is typically done with the aid of an esophageal balloon, however, this method does not measure total WOB only work done on the lung is quantified. The total WOB of a specific ventilatory task can be measured by comparing the change of total body oxygen consumption (VO_{2}) before and after a load is placed on the ventilatory pump. Ventilator sensitivity, when not set appropriate, can cause a significant increase in the patients WOB. In addition, because of pathology, many patients who are ventilator dependent have high minute volume (MV) requirements. We investigated how minute ventilation affects the oxygen cost associate with increasing efforts needed to initiate inspiration in normal subjects. METHODS We stimulated subjects to breathe by having them turn the peddles of bicycle ergometer, set to zero resistance, at 3 different speeds: 0, 50 and 88 revolutions/min. All subjects were attached to a PB7200 ventilator, set in the CPAP of O mode, via a standard exercise mouth piece. The ventilator's sensitivity was adjusted to three levels: -2, -5 and -10 cm H_{2}O. Expired gas was analyzed with a Sensormedics Deltatrac metabolic monitor. We tested 6 subjects at each speed and sensitivity setting. We used a repeated measures ANOVA model to determine statistical significance. RESULTS MV at the three cycling speeds averaged (ISE) 8.2±0.4 (normal), 11.5±0.4 (moderate) and 17.6±0.4 (high) liters, respectively, for the three cycling speeds. Although there was a significant increase in VO_{2} as MV increased, at each MV level decreasing the sensitivity did not result in an increasing VO_{2}. CONCLUSION In normal subjects there is no oxygen cost associated with the increased effort needed to initiate inspiration. Even high MVs and - 10 cm H_{2}O trigger sensitivity resulted in no increase in VO_{2}.

Reference: OF-96-117

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