Tidal Volume Change and Gas Mixing in the Lung

Abstract

Changing the depth and the frequency of breathing affects the efficiency of ventilation. This has been studied in eight normal subjects using the technique of nitrogen washout whilst breathing a mixture of 79% argon and oxygen. The signals were converted to digital data at 50 Hz and all calculations were then done with the computer. The size of the dead space of the conducting airways (series dead space, VdS) and of the alveolar dead space (VdA) has been measured for carbon dioxide (CO₂) and oxygen (O₂). The nitrogen (N₂) decay curve was computed allowing the calculation of first breath (AMEsb) and multi breath (AMEmb) alveolar mixing efficiency. The increase in VdS per 100 ml increase in tidal volume (Vt) was 3.7 ml for CO₂ and 3.3 for O₂. As VdS increased, VdA also increased by 11.1 ml for CO₂ and 19.1 ml for O₂, for each 100 ml increase in Vt. Whilst VdA and VdS increase with increasing Vt, the proportion of VdA+VdS in each breath diminishes with such an increase, the net result is that, for each 100 ml increase in Vt, alveolar ventilation increases by 86 ml for CO₂ and 78 ml for O₂. The increase of absolute values and the different behaviour of N₂, AMEsb and AMEmb show a progressive decrease of the parallel component of the intra-pulmonary ventilation distribution with increasing Vt. It appears that the pattern of ventilation, as well as minute ventilation, plays a role in the effectiveness of ventilation.

Validation of the MetaMax II Portable Metabolic Measurement System

“…In the present study the percentage of the extra dead-space volume (DB breathing valve) to total ventila- tion [(0.1 l times fb)/VE] was at 100 W 5.4 % and at 200 W 3.9 % (data not shown), supporting a smaller influence of the extra dead-space at a higher physical load. In addition, a higher VT has been shown to increase effective ventilation [4]. Further, validation studies with a non-simultaneous set-up did not display these non-linear patterns in V Ç O 2 and V Ç CO 2 [11,13,14].…”