Tidal volume change and gas mixing in the lung

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.