Ventilation: Total, Alveolar, and Dead Space

Lung diffusing capacity for nitric oxide measured by two commercial devices: a randomised crossover comparison in healthy adults

In Europe, two commercial devices are available to measure combined single-breath diffusing capacity of the lung for nitric oxide (D_LNO) and carbon monoxide (D_LCO) in one manoeuvre. Reference values were derived by pooling datasets from both devices, but agreement between devices has not been established.

We conducted a randomised crossover trial in 35 healthy adults (age 40.0±15.5 years, 51% female) to compare D_LNO (primary end-point) between MasterScreen™ (Vyaire Medical, Mettawa, IL, USA) and HypAir (Medisoft, Dinant, Belgium) devices during a single visit under controlled conditions. Linear mixed models were used adjusting for device and period as fixed effects and random intercept for each participant.

Difference in D_LNO between HypAir and MasterScreen was 24.0 mL·min⁻¹·mmHg⁻¹ (95% CI 21.7–26.3). There was no difference in D_LCO (−0.03 mL·min⁻¹·mmHg⁻¹, 95% CI −0.57–0.12) between devices while alveolar volume (V_A) was higher on HypAir compared to MasterScreen™ (0.48 L, 95% CI 0.45–0.52). Disparity in the estimation of V_A and the rate of NO uptake (K_NO=D_LNO/V_A) could explain the discrepancy in D_LNO between devices. Disparity in the estimation of V_A and the rate of CO uptake (K_CO=D_LCO/V_A) per unit of V_A offset each other resulting in negligible discrepancy in D_LCO between devices. Differences in methods of expiratory gas sampling and sensor specifications between devices likely explain these observations.

These findings have important implications for derivation of D_LNO reference values and comparison of results across studies. Until this issue is resolved, reference values, established on the respective devices, should be used for test interpretation.

Large discrepancies between commercial devices to measure single-breath diffusing capacity of the lung for nitric oxide in healthy subjects caution against pooling or direct comparison of measurements obtained using different protocols and deviceshttps://bit.ly/3vKyF7U

Clinical use of volumetric capnography in mechanically ventilated patients

Capnography is a first line monitoring system in mechanically ventilated patients. Volumetric capnography supports noninvasive and breath-by-breath information at the bedside using mainstream CO₂ and flow sensors placed at the airways opening. This volume-based capnography provides information of important body functions related to the kinetics of carbon dioxide. Volumetric capnography goes one step forward standard respiratory mechanics and provides a new dimension for monitoring of mechanical ventilation. The article discusses the role of volumetric capnography for the clinical monitoring of mechanical ventilation.

Unevenness of ventilation assessed by the expired CO(2) gas volume versus V(T) curve in asthmatic patients

Recently, we have shown that the expired CO2 gas volume versus tidal volume (VCO2-VT) curve is a useful tool for assessing unevenness of ventilation because it allows the separation of tidal volume into three functional compartments: (a) the CO2-free expired air (V0), (b) the transitional volume (Vtr), (c) the alveolar volume (VA) and the measurement of alveolar FCO2 during resting breathing in normal subjects and patients with COPD. In this paper, we have investigated whether changes pertaining to unevenness of ventilation taking place immediately after the administration of methacholine can be assessed using the VCO2-VT curve in asthmatic patients. The VCO2-VT curve was obtained during tidal breathing from 16 stable asthmatic patients who underwent a methacholine challenge test. It has been found that the Vtr, and hence Bohr's dead space (VD,Bohr = V0 + Vtr), over tidal volume ratios were significantly increased immediately after the methacholine administration, whilst the V0 over tidal volume ratio was not affected. The change of the above ratios was not related to the percentage decrease of FEV1.0 following methacholine administration.

Noninvasive measurement of mean alveolar carbon dioxide tension and Bohr's dead space during tidal breathing

The lack of methodology for measuring the alveolar carbon dioxide tension (PA,CO2) has forced investigators to make several assumptions, such as that PA,CO2 is equal to end-tidal (PET,CO2) and arterial CO2 tension (Pa,CO2). The present study measured the mean PA,CO2 and Bohr's dead space ratio (Bohr's dead space/tidal volume (VD,Bohr/VT)) during tidal breathing. The method used is a new, simple and noninvasive technique, based on the analysis of the expired CO2 volume per breath (VCO2) versus the exhaled VT. This curve was analysed in 21 normal, healthy subjects and 35 chronic obstructive pulmonary disease (COPD) patients breathing tidally through a mouthpiece apparatus in the sitting position. It is shown that: 1) PA,CO2 is similar to Pa,CO2 in normal subjects, whilst it is significantly lower than Pa,CO2 in COPD patients; 2) PA,CO2 is significantly higher than PET,CO2 in all subjects, especially in COPD patients; 3) VD,Bohr/VT is increased in COPD patients as compared to normal subjects; and 4) VD,Bohr/VT is lower than the "physiological" dead space ratio (VD,phys/VT) in COPD patients. It is concluded that the expired carbon dioxide versus tidal volume curve is a useful tool for research and clinical work, because it permits the noninvasive and accurate measurement of Bohr's dead space and mean alveolar carbon dioxide tension accurately during spontaneous breathing.