The Science Journal of the American Association for Respiratory Care

2003 OPEN FORUM Abstracts

Comparison of two methods to measure physiologic dead-space-to-tidal volume ratios (VD/VT).

RH Kallet MS RRT FAARC, University of California, San Francisco Department of Anesthesia, at San Francisco General Hospital.

Background:  Bedside measurement of VD/VT is commonly done with a metabolic monitor. The VD/VT measurement can be falsely elevated in patients requiring high inflation pressure ventilation because a substantial portion of the VT is compressed in the circuit. This gas mixes with CO2-laden gas from the lungs and lowers the mean expired carbon dioxide tension (PeCO2). Typically, the effect of compression volume contamination is estimated by formula, which is an accurate method of determining the actual VD/VT.1 An alternative method is to use volumetric capnometry that simultaneously measures VT and PeCO2 at the patient Y-adapter. Thus, the effects of compression volume contamination are negated. I inquired whether these two methods yield similar results. 

Methods
: Twenty-six VD/VT measurements were made in 4 ARDS patients and one trauma patient with normal lung function. Patients were ventilated primarily in a pressure-regulated mode at a VT of 7.44 ± 1.08 mL/kg. Mean PeCO2 was calculated from fractional expired CO2 concentration measured with a metabolic monitor (Deltatrac, Sensormedics, Yorba Linda, CA.) and corrected for both H2O vapor pressure and compression volume contamination. Volumetric capnography was done with the NICO pulmonary mechanics monitor (Respironics, Carlsbad, CA).* Both monitors were calibrated prior to each measurement. As both monitors use minute-to-minute measurement averaging, the internal clocks were synchronized. After 10 min of stable readings in well-sedated patients, an arterial blood sample was withdrawn over one minute from an indwelling catheter. VD/VT was calculated using the Enghoff modification of the Bohr Equation. Data was compared by 2-sided paired t-test; correlation was measured by Pearson product-moment coefficient (R) and bias/precision was assessed by Bland-Altman test. 

RESULTS:
 Uncorrected VD/VT and PeCO2 obtained with a metabolic monitor were 0.64 ± 0.11 and 14.8 ± 3.8 mm Hg respectively. Estimated VT delivered to the patient was not different from that directly measured by the NICO (473 ± 125 mL vs. 479 ± 92 mL respectively; p = 0.88)

  NICO Metabolic Monitor (corrected) p
VD/VT 0.60 ± 0.13 0.58 ± 0.13 0.45
PeCO2 (mm Hg) 16.2 ± 4.1 17.1 ± 4.4 0.45


VD/VT and PeCO2 measured with the NICO correlated strongly with metabolic monitor measurements corrected for compression volume (R = 0.94 and 0.91 respectively; p < 0.0001). VD/VT measured with the NICO had a bias of -0.02 and a precision of 0.04 compared to the corrected metabolic monitor measurements. Comparisons of PeCO2 between the NICO and the metabolic monitor had a bias of 0.96 mm Hg and a precision of 1.8 mm Hg. 

CONCLUSION:
 VD/VT measured by volumetric capnometry, which avoids the problem compression volume contamination, yields results close to those based upon estimates compression volume and appears to be an equally valid method for measuring VD/VT.

1. Forbat AF, Her C. Correction for gas compression in mechanical ventilators. Anesth Analg 1980; 59 (7): 488-493.

*The NICO monitor and related supplies used in this evaluation were donated by Respironics.

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