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Research ArticleOriginal Contributions

Anatomic Dead Space Cannot Be Predicted by Body Weight

Lara M Brewer, Joseph A Orr and Nathan L Pace
Respiratory Care July 2008, 53 (7) 885-891;
Lara M Brewer
Department of Anesthesiology, University of Utah Health Sciences Center, Salt Lake City, Utah.
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  • For correspondence: [email protected]
Joseph A Orr
Department of Anesthesiology, University of Utah Health Sciences Center, Salt Lake City, Utah.
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Nathan L Pace
Department of Anesthesiology, University of Utah Health Sciences Center, Salt Lake City, Utah.
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Abstract

BACKGROUND: Anatomic dead space (also called airway or tracheal dead space) is the part of the tidal volume that does not participate in gas exchange. Some contemporary ventilation protocols, such as the Acute Respiratory Distress Syndrome Network protocol, call for smaller tidal volumes than were traditionally delivered. With smaller tidal volumes, the percentage of each delivered breath that is wasted in the anatomic dead space is greater than it is with larger tidal volumes. Many respiratory and medical textbooks state that anatomic dead space can be estimated from the patient's weight by assuming there is approximately 1 mL of dead space for every pound of body weight. With a volumetric capnography monitor that measures on-airway flow and CO2, the anatomic dead space can be automatically and directly measured with the Fowler method, in which dead space equals the exhaled volume up to the point when CO2 rises above a threshold.

METHODS: We analyzed data from 58 patients (43 male, 15 female) to assess the accuracy of 5 anatomic dead space estimation methods. Anatomic dead space was measured during the first 10 min of monitoring and compared to the estimates.

RESULTS: The coefficient of determination (r2) between the anatomic dead space estimate based on body weight and the measured anatomic dead space was r2 = 0.0002. The mean ± SD error between the body weight estimate and the measured dead space was 60 ± 54 mL.

CONCLUSIONS: It appears that the anatomic dead space estimate methods were sufficient when used (as originally intended) together with other assumptions to identify a starting point in a ventilation algorithm, but the poor agreement between an individual patient's measured and estimated anatomic dead space contradicts the assumption that dead space can be predicted from actual or ideal weight alone.

  • respiratory dead space
  • anatomic dead space
  • tidal volume
  • gas exchange
  • body weight
  • respiratory distress syndrome
  • mechanical ventilation
  • ventilator
  • capnography
  • lung volume
  • pulmonary ventilation
  • respiratory function tests
  • ventilation-perfusion ratio

Footnotes

  • Correspondence e-mail: lbrewer{at}abl.med.utah.edu.
  • Ms Brewer presented a version of this paper at the 52nd International Respiratory Congress of the American Association for Respiratory Care, held December 11–14, 2006, in Las Vegas, Nevada.

  • Copyright © 2008 by Daedalus Enterprises Inc.
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Respiratory Care: 53 (7)
Respiratory Care
Vol. 53, Issue 7
1 Jul 2008
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Anatomic Dead Space Cannot Be Predicted by Body Weight
Lara M Brewer, Joseph A Orr, Nathan L Pace
Respiratory Care Jul 2008, 53 (7) 885-891;

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Anatomic Dead Space Cannot Be Predicted by Body Weight
Lara M Brewer, Joseph A Orr, Nathan L Pace
Respiratory Care Jul 2008, 53 (7) 885-891;
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Keywords

  • respiratory dead space
  • anatomic dead space
  • tidal volume
  • gas exchange
  • body weight
  • respiratory distress syndrome
  • mechanical ventilation
  • ventilator
  • capnography
  • lung volume
  • pulmonary ventilation
  • respiratory function tests
  • ventilation-perfusion ratio

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