Hemodilution and hyperoxia locally change distribution of regional pulmonary perfusion in dogs

[Rational use of oxygen in anesthesiology and intensive care medicine]

Oxygen (O(2)) is the most frequently used pharmaceutical in anesthesiology and intensive care medicine: Every patient receives O(2) during surgery or during a stay in the intensive care unit. Hypoxia and hypoxemia of various origins are the most typical indications which are mentioned in the prescribing information of O(2): the goal of the administration of O(2) is either an increase of arterial O(2) partial pressure in order to treat hypoxia, or an increase of arterial O(2) content in order to treat hypoxemia. Most of the indications for O(2) administration were developed in former times and have seldom been questioned from that time on as the short-term side-effects of O(2) are usually considered to be of minor importance. As a consequence only a small number of controlled randomized studies exist, which can demonstrate the efficacy of O(2) in terms of evidence-based medicine. However, there is an emerging body of evidence that specific side-effects of O(2) result in a deterioration of the microcirculation. The administration of O(2) induces arteriolar constriction which will initiate a decline of regional O(2) delivery and subsequently a decline of tissue oxygenation. The aim of the manuscript presented is to discuss the significance of O(2) as a pharmaceutical in the clinical setting.

The Effects of Acute Isovolemic Hemodilution on Oxygenation During One-Lung Ventilation

Data on the effects of isovolemic hemodilution (IH) on oxygenation during one-lung ventilation (OLV) are lacking. We studied 47 patients with hemoglobin >14 g/dL who were scheduled for lung surgery (17 with normal lung function [group NL], 17 with chronic obstructive pulmonary disease [COPD] [group COPD], and 13 with COPD as control for time/anesthesia effects [group CTRL]). Anesthesia was standardized. The tracheas were intubated with a double-lumen tube. Ventilatory settings and fraction of inspired oxygen remained constant. The study was performed with patients in the supine position before surgery. OLV was initiated for 15 min. Two-lung ventilation was reinstituted, and IH was performed (500 mL); an identical volume of hydroxyethyl starch was administered. Subsequently, OLV was again performed for 15 min. In group CTRL, the same sequences of OLV were performed without IH. At the end of each period of OLV, pulmonary mechanics and blood gases were recorded. Data were analyzed by analysis of variance (mean +/- sd). In group NL and group CTRL, the arterial oxygen partial pressure remained constant, whereas it decreased in group COPD from 119 +/- 21 mm Hg before IH to 86 +/- 16 mm Hg after IH (P <0.01). Mild IH impairs gas exchange during OLV in COPD patients, but not in patients with normal lung function.

Hypoxia and hyperoxia both transiently affect distribution of pulmonary perfusion but not ventilation in awake sheep

Despite a remarkable gravity independent heterogeneity in both local pulmonary ventilation and perfusion, the two are closely correlated at rest and during exercise in the normal lung. These observations strongly indicate that there is a mechanism for coupling of the two so that local V/Q-ratio is kept fairly uniform throughout the lung. This is also necessary to achieve adequate gas exchange in the lung. It was recently suggested that oxygen-induced vasoconstriction has a slow and intense component that might contribute to the matching of ventilation and perfusion also under normal conditions (Vejlstrup & Dorrington 1993). We therefore simultaneously determined distribution of local ( approximately 1(1/2) cm3 lung pieces) ventilation and perfusion in eight sheep at normoxia (FiO2 21%) and after 10 min and 2(1/2) h exposure to hypoxia (FiO2 12%; four sheep) or hyperoxia (FiO2 40%; four sheep). We used a approximately 1 microm wet fluorescent aerosol and 15 microm radioactive microspheres i.v. to measure local ventilation and perfusion, respectively. Neither hypoxia nor hyperoxia caused changes in the distribution of ventilation. After 10 min exposure to hypoxia or hyperoxia, distribution of perfusion was altered so that the correlation between values for local ventilation and perfusion decreased. After 2(1/2) h exposure to either hypoxia or hyperoxia, distributions of perfusion and V/Q-ratio had returned to baseline. These results show that distribution of perfusion is influenced by acute changes in oxygen tension, so that local matching of ventilation and perfusion is affected. Apparently, some mechanism restores the matching during extended exposure to the altered oxygen tension.