INTRAPULMONARY GAS MIXING AND DEAD SPACE IN ARTIFICIALLY VENTILATED DOGS

In this study we have investigated the effects of breath holding and o f the physical properties of gases on four different respiratory dead spaces (V-D): the Fowler, the physiological, the washout and the inert gas dead space. The experiments were performed with dogs which were v entilated artifically with breathing patterns with different post-insp iratory breath holding times (t(a)) of 0, 0.5, 1.0 and 2.0 s. Tracer a mounts of acetone, ether and enflurane were infused continuously into a peripheral vein and a bolus of a mixture of krypton, Freon12 and SF6 was introduced into the peritoneal cavity. After reaching steady stat e, samples of arterial blood, mixed venous blood and mixed expired air were taken simultaneously. From the partial pressures (P-a, <P((V))ov er bar> and <P((E))over bar> respectively) we determined the excretion ((E) over bar = <P((E))over bar>/<P((V))over bar>), retention (R = P- a/<P((V))over bar>) and the physiological dead space fraction (V-D,V-p hys/<V((T))over bar> = (1-<P((E))over bar>/P-a)) for each gas, where V -T is tidal volume. Further. we recorded the expirograms of the six tr acer gases and of CO2 from which the Fowler dead space fractions (T/(D ,Fowler)/V-T) of the different gases were determined. Also the washout dead space fractions (V-D,V-washout/V-T) for He and SF6 were determin ed as well as the inert gas dead space fraction (V-D,V-MIDGET/V-T) Wit h the use of the multiple inert gas elimination technique (MIGET). Wit h the exception of V-D,V-phys/V-T for SF6, all dead space fractions de creased with increasing t(a). V-D,V-phys/V-T for the poorly soluble ga s SF6 was considerably larger than V-D,V-phys/V-T for the remaining ga ses. For the highly acetone V-Fowler/V-T was considerably smaller than V-D,V-Fowler/V-T for the other gases. V-D,V-washout,V-SF6/V-T was alw ays larger than V-D,V-washout,V-He/V-T and V-D,V-Fowler,V-SF6/V-T. Fur ther, V-D,V-phys/V-T was larger than V-D,V-Fowler/V-T for SF6 and acet one. However, for gases with intermediate solubility in blood V-D,V-ph ys/V-T tended to be smaller than V-D,V-Fowler/V-T. We conclude that th e respiratory dead spaces are affected by the breathing pattern and by the physical properties of gases, i.e. their diffusivity in alveolar gas and their solubility in blood or lung tissue.