The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

Anatomic Dead Space Can Not be Predicted by Body Weight

Joseph Orr, Ph.D.1 and Lara Brewer, M.S.1 and Dinesh Haryadi, Ph.D. 2 1. Anesthesiology, University of Utah, Salt Lake City, Utah, United States. 2. Respironics Inc, Wallingford, Connecticut.

Introduction:
Anatomic, or tracheal, dead space is the part of the tidal volume that does not participate in gas exchange. As newer protocols, such as the one from the ARDS network, call for the use of smaller tidal volumes, the percent of each delivered breath that is wasted due to the anatomic dead volume increases. Respiratory and medical textbooks state that anatomic dead space can be estimated from the patient's body weight [1,2,3]. Specifically, these references suggest dead space can be predicted by one ml per pound, or 2.2 ml per kilogram, of body weight. Anatomic dead space can be measured using Fowler's method in which dead space equals the exhaled volume up to the point when CO2 rises above a threshold [4]. Using a respiratory profile monitor, that incorporates an on-airway flow and CO2 monitoring (NICO2, Respironics, Wallingford CT), anatomic dead space can be automatically and directly measured.

Methods:
We retrospectively analyzed data collected in 48 (47 male, 1 female) patients in the operating room and ICU. The average age was 66.7 +/1 10.9 years. Patient weight ranged from 140.8 to 301.4 Lbs. These patients were intubated and a combination CO2/flow sensor was connected directly to the endotracheal tube. Average anatomic dead space was measured during the first 10 minutes of monitoring. These dead space values were compared to that obtained using the patient weight and height.

Results:
The correlation between the anatomic dead space and body weight was r2 = 0.07 and the correlation between patient height and dead space was r2=0.01. The average airway dead space was 225 ± 24 ml. The average weight was 201 ± 31 Lbs. The ratio of average dead space (ml) to average weight (Lbs) was 1.12.

Discussion:
The poor correlation between patient weight and dead space appears to contradict the suggestion that dead space can be estimated from body weight. It appears that the average anatomic dead space corresponds to the average body weight in pounds for the overall population; however, based on our data, there is no basis for estimating an individual patient's anatomic dead space volume from his/her body weight or height. To assess the impact of an incorrectly estimated dead space, assume a tidal volume set at 6 ml/kg for this set of study patients. The average tidal volume would be 548 ml; applying the average dead space of 225 ml gives an alveolar tidal volume of 323 ml with a possible range of 275 to 371ml (mean ± 2SD), which may be much different from the volume predicted by body weight (347 ml). Therefore, the error in calculating alveolar minute ventilation when dead space is estimated from body weight for these patients would be on the order of -20% to +8%. Given these data, direct measurement appears to be the only reliable method for assessing an individual patient's anatomic dead space to unsure adequate alveolar ventilation.
References:
1. JB West, Respiratory Physiology - the Essentials, Williams and Wilkins 2nd Ed., 1979, Page 19.

2. MP Hlastala, AJ Berger, Physiology of Respiration, Oxford University Press, 1996, Pages 73-4.

3. WF Ganong, Review of Medical Physiology, 19th Ed. 1999, Appleton & Lange, Stamford, CT.

4. WS Fowler, Lung Function Studies. II The Respiratory Dead Space, Am J Physiol. 154, 405, 1948.


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