David E. Leith

Stewart's quantitative acid-base chemistry : applications in biology and medicine

We review P.A. Stewarts quantitative approach to acid-base chemistry, starting with its historical context. We outline its implications for cellular and membrane processes in acid-base physiology; discuss its contributions to the understanding and analysis of acid-base phenomena; show how it can be applied in clinical problems; and propose a classification of clinical acid-base disturbances based on this general approach.

Tracheal insufflation of O2 (TRIO) at low flow rates sustains life for several hours.

The emergency management of respiratory arrest can be a difficult problem. The authors hypothesized that tracheal insufflation of O2 (TRIO) at low flows could provide adequate oxygenation and sufficient CO2 elimination to sustain life until more definitive (but more difficult to implement) measures could be applied. Therefore, 10 anesthetized, paralyzed dogs (15.9–48.2 kg), initially ventilated with conventional mechanical ventilation (CMV) using room air, were studied. CMV was stopped and a 1− or 5-mm id catheter with a constant flow (V) of O2 ranging from 0.2 to 3.0 1/min was inserted to within 1 cm of the carina. With all flow rates, Pao2 and Paco2 initially increased with time; the rate of increase of Pao2 was greater and that of Paco2 was less, with increasing V. In three dogs studied at flow rates of 2.0 or 3.0 1/min, arterial blood gases reached a plateau after about 2 h: pH = 6.87; Pco2 = 164 mmHg; and Po1 = 363 mmHg (mean values). These studies were stopped at between 4 and 5 h, with no dogs showing any signs of cardiovascular or other decompensation. Results in which catheter position was studied indicated that as long as the catheter tip was at or past the carina, gas transport was similar to that observed when the catheter tip was 1 cm proximal to the carina. The authors conclude that TRIO at low flow rates can produce sufficient gas exchange to support life for prolonged periods in apneic dogs.

Maximal oxygen consumption and pulmonary diffusing capacity: A direct comparison of physiologic and morphometric measurements in canids

The purpose of this study was to check the validity of the morphometric model for estimating physiological conductances for gases, DL. We make a direct comparison between the lungs conductance for carbon monoxide, measured physiologically using the single breath method, DLCO (sb), and that measured morphometrically using the previously published model, DLCO(mm). We also make a direct comparison between the maximum rate of oxygen uptake by the lung during exercise, VO2max, and the lungs conductance for oxygen DLO2(mm). We made these measurements on four species of canids (foxes, coyotes, dogs and wolves). We find a direct proportionality between morphometric and physiologic DLCO measurements, the morphometric being consistently larger by a factor of two. We also find that both DLCO and DLO2 increase more steeply with body mass than VO2max, the difference between the allometric slopes being the same as we had found previously in a wide range of mammalian species ranging from 2 g to 700 kg, although the slopes themselves were different. We conclude that the discordant scaling of DLO2 and VO2max with respect to body mass is not an artifact of the model for calculating DLO2 from morphometric data.

Ventilatory and P0.1 response to hypercapnia in quadriplegia

Unlike individuals with comparable degrees of respiratory muscle weakness from other causes, quadriplegic patients have a blunted ventilatory and P0.1 response to hypercapnia. This suggests that the diminished response in quadriplegia is due, in part, to an alteration in respiratory drive. We measured the hypercapnic response in 9 subjects with chronic quadriplegia (Q) and 8 normal controls (N). Ventilatory muscle strength, maximum voluntary ventilation (MVV), and lung volumes were measured in all subjects. The ventilatory response (HCVR) in Q was significantly less than in N (0.73 +/- 0.37 vs 2.95 +/- 0.4 L.min-1.mmHg-1; P less than 0.001), even when normalized for indices of respiratory muscle performance (e.g., vital capacity, MVV). There was no significant change in the HCVR in Q after the administration of naloxone. We also serially studied 2 subjects with acute quadriplegia, and found that despite progressive improvement in respiratory muscle performance, there was no accompanying increase in the response to hypercapnia. These data suggest that muscle weakness alone cannot explain the blunted hypercapnic response in quadriplegia, and are consistent with the hypothesis that these subjects have a reduced ventilatory drive.

Cardiac output but not high pulmonary artery pressure varies with FIO2 in exercising horses

Horses have high mean pulmonary artery pressure (Ppa) both at rest and during exercise (approximately 30 and > or = 80 mmHg, respectively). The mechanisms are unknown. To see if hypoxic pulmonary vasoconstriction (HPV) plays a role, we compared pulmonary artery pressure-flow (Ppa-Q) curves when inspired O2 fraction (FIO2) was 0.16, 0.21, and 0.30, in 5 normal Thoroughbred horses standing quietly and while galloping at 10 and 14 m/sec on a level treadmill. We calculated O2 consumption (VO2) from measurements of respired gas composition and flow, and calculated Q from VO2 and measurements of oxygen content in arterial and mixed venous blood (CaO2 and CVO2). VO2 was 3.8, 74 and 128 ml.min-1.kg-1, at rest and at 10 and 14 m/sec, and did not vary with FIO2 at any speed. At 14 m/sec only, when FIO2 was lowered to 0.16, CaO2 fell (to 14.7 from 20 ml/dl on air), Q increased (to 0.86 from 0.66 L.min-1.kg-1 on air), and stroke volume increased (to 4.1 from 3.2 ml.kg-1 on air). Slopes and intercepts of Ppa-Q curves did not vary with FIO2. We conclude that HPV does not contribute to the high Ppa of exercising horses breathing air near sea level.