Calculation of Physiologic Dead Space: Comparison of Ventilator Volumetric Capnography to Measurements by Metabolic Analyzer and Volumetric CO₂ Monitor

Abstract

BACKGROUND: Calculation of physiologic dead space (dead space divided by tidal volume [V D / V T ]) using the Enghoff modification of the Bohr equation requires measurement of the partial pressure of mean expired CO 2 (P E CO 2 ) by exhaled gas collection and analysis, use of a metabolic analyzer, or use of a volumetric CO 2 monitor. The Dräger XL ventilator is equipped with integrated volumetric CO 2 monitoring and calculates minute CO 2 production (V CO 2 ). We calculated P E CO 2 and V D /V T from ventilator derived volumetric CO 2 measurements of V CO 2 and compared them to metabolic analyzer and volumetric CO 2 monitor measurements. METHODS: A total of 67 measurements in 36 subjects recovering from acute lung injury or ARDS were compared. Thirty-one ventilator derived measurements were compared to measurements using 3 different metabolic analyzers, and 36 ventilator derived measurements were compared to measurements from a volumetric CO 2 monitor. RESULTS: There was a strong agreement between ventilator derived measurements and metabolic analyzer or volumetric CO 2 monitor measurements of P E CO 2 and V D /V T. The correlations, bias, and precision between the ventilator and metabolic analyzer measurements for P E CO 2 were r ‫؍‬ 0.97, r 2 ‫؍‬ 0.93 (P < .001), bias ؊1.04 mm Hg, and precision ؎ 1.47 mm Hg. For V D /V T the correlations were r ‫؍‬ 0.95 and r 2 ‫؍‬ 0.91 (P < .001), and the bias and precision were 0.02 ؎ 0.04. The correlations between the ventilator and the volumetric CO 2 monitor for P E CO 2 were r ‫؍‬ 0.96 and r 2 ‫؍‬ 0.92 (P < .001), and the bias and precision were ؊0.19 ؎ 1.58 mm Hg. The correlations between the ventilator and the volumetric CO 2 monitor for V D /V T were r ‫؍‬ 0.97 and r 2 ‫؍‬ 0.95 (P < .001), and the bias and precision were 0.01 ؎ 0.03. CONCLUSIONS: P E CO 2 , and therefore V D /V T , can be accurately calculated directly from the Dräger XL ventilator volumetric capnography measurements without use of a metabolic analyzer or volumetric CO 2 monitor.

A Comparison of Carbon Dioxide Elimination Measurements Between a Portable Indirect Calorimeter and Volumetric Capnography Monitor: An In Vitro Simulation

“…However, it was not the goal of the present study to specifically compare these methods, because it has been done previously. 17,[19][20][21] In our current study, the V CO 2 measurements between the CCM Express indirect calorimeter and the NM3 volumetric capnography device were in agreement despite technological differences in flow and gas concentration measurement.…”

The Whisper Game

“…1 While I am reluctant to criticize such a good paper, some minor mathematical errors crept in that reminded me of the whisper game. 1 While I am reluctant to criticize such a good paper, some minor mathematical errors crept in that reminded me of the whisper game.…”

The Ability of Volumetric Capnography to Distinguish between Chronic Obstructive Pulmonary Disease Patients and Normal Subjects

“…VCap is considered a method for testing lung function; it uses infrared light of a specific wavelength to measure the concentration of carbon dioxide in exhaled air [4][5][6]. Many indices, including the slopes of phases II and III and estimates of dead space [7][8][9], can be deduced using VCap, and it is of proven value in the clinical assessment of a number of diseases, including pulmonary thromboembolism [10][11][12][13], interstitial lung disease [14], and acute lung injury [15,16]; it is also used to assess patients undergoing mechanical ventilation [17][18][19][20][21]. The shape of the VCap curve in patients with obstructive lung diseases such as COPD and asthma differs from those of normal subjects and patients with restrictive lung disease [22][23][24][25].…”