Preinspiratory Volume Research Articles

Ventilation heterogeneity in excised lobes: effect of tidal volume.

Although several factors are known to influence nonuniformity of ventilation, including lung mechanical properties (regional structure and compliance), external factors (chest wall, pleural pressure, heart), and ventilatory parameters (tidal and preinspiratory volume, flow rate), their relative contributions are poorly understood. We studied five excised, unperfused, canine right-middle lobes under varied levels of tidal volume (VT), thus eliminating many factors affecting heterogeneity. Multiple-breath washouts of N(2) were analyzed for anatomic dead space volume (VD(anat)), nonuniformity of N(2) washout, and nonuniformity between joined acinar regions vs. that occurring between larger joined regions. Approximately 80% of ventilation heterogeneity was found among joined acinar regions at resting levels of VT, but increasing VT reduced intra-acinar heterogeneity to about 25% of that found at resting levels. Increasing VT had essentially no effect on VD(anat) and heterogeneity among larger joined regions. The results indicate that the magnitude of VT is a major influence on the dominant intra-acinar component of ventilation heterogeneity and that VT effects on VD(anat) are likely due to perfusion and/or influences normally external to the lobar structure.

Influence of lung volume on the mixing defect for an inert gas

For quantitative assessment of how gas mixing in the lung compares to the ideal of perfect mixing, it is necessary to measure pre-inspiratory lung volume as well as to study the inhaled and exhaled gas. Normal young-adult persons in the seated position assumed various lung volumes and then took 1-L test breaths (11% Ar, 22% O2 in N2). Integration of concentration at the mouth against volume yielded amounts of Ar inhaled, exhaled and retainedl un-exhaled concentration in the lung after the single breath was calculated from amoung retained divided by lung volume (measured by rebreathing). Retention was 60% of the inhaled amount when pre-inspiratory volume was above FRC and less when it was below. Series dead space increased with lung volume. End-tidal Ar was the same as, or slightly above, the concentration predicted for perfect mixing and un-exhaled Ar was slightly below the perfect-mix concentration. Interpretation: three separable influences decrease the transfer of inspirate to the resident gas. (a) A dilution effect, predictable from the volumes involved, decreases retention at low lung volumes. (b) An enlarged series dead space decreases retention at high volume. (c) In normal persons, a mixing defect that is attributable to poor distribution or incomplete gas-phase diffusion is small and can be ascribed almost completely to the part of the expirate that gives rise to the slope of the alveolar plateau, except when breaths are taken at very low lung volumes.

Effects of preinsipiratory volume on the nitrogen closing volume test

The effect of varying the preinspiratory volume (V iair; range: 0–75% vital capacity, VC) on the nitrogen closing volume (CV) test was studied in twelve seated subjects, aged 24–62 years. When V iair was increased from 0 to about 12% VC, the height of phase IV, the amplitude of the cardiogenic oscillations, CV and the slope of phase III increased. The height of phase IV and the amplitude of the cardiogenic oscillations showed a maximum at V iair = 12% VC , although the average CV was about 18% VC. While the height of phase IV and the amplitude of the cardiogenic oscillations decreased when V iair was increased above 12% VC, CV did not change and the slope of phase III increased consistently. These results cannot be explained solely by the regional lung volume model of Sutherland et al. (1968). However, if that model is extended to include the assumption that within a region alveoli behind closed airways may be differently expanded, we predict CV to be underestimated at low V iair, independently of the upper to lower concentration difference, in agreement with present findings. This assumption would also explain why the maximal height of phase IV can be obtained at a V iair lower than CV.

The effect of preinspiratory lung volume on the result of the single breath O 2 test

The effect of preinspiratory lung volume on the slope of the nitrogen plateau after a single breath of 750 ml of oxygen has been studied in a group of normal subjects. The slope of the plateau expressed as the nitrogen concentration difference between points 750 ml and 1250 ml after the beginning of expiration is found to be a function of preinspiratory lung volume. There is a linear relationship between nitrogen difference and the preinspiratory volume above R.V. Change in shape of the plateau with diminishing lung volume appeared to be related to the age of the subjects. The significance of the results is discussed.