Venous Admixture to the Pulmonary Circulation in Human Subjects Breathing 100 Per Cent Oxygen*

Abstract

At any given time, the alveolar-arterial oxygen partial pressure difference (AaD) may be due to one or more of the three following mechanisms (1-5): 1) failure of pulmonary capillary blood to come to complete equilibrium with alveolar gas; 2) uneven ventilation perfusion ratios; and 3) admixture of venous blood by direct shunting.The first mechanism causes the diffusion component of the AaD, relating to diffusion across the alveolar-capillary membrane as well as chemical reaction rates of oxygen with hemoglobin (6, 7). The second mechanism accounts for the "distribution" component, and the third is spoken of as the "true," "pure," or "anatomical" shunt, or the "direct" venous admixture component.When the inspired oxygen tension is low, as at high altitude or during breathing of hypoxic mixtures, particularly if either condition is combined with exercise, the diffusion component increases, whereas those due to direct venous admixture and to uneven ventilation-perfusion ratios diminish. These changes are used to determine the pulmonary diffusing capacity for oxygen (1-3, 8).On the other hand, breathing 100% oxygen increases the AaD due to direct venous-arterial shunting, and virtually eliminates all other components (9).The role of diffusion impairment in the causation of arterial hypoxemia has been recently reevaluated ( 10), and a number of newer approaches have permitted a more precise definition of the distribution of ventilation-perfusion ratios and of its importance in gas exchange (11)(12)(13)(14)(15).

Criteria for Pulmonary and Respiratory Failure in COPD Patients – A Theoretical Study Based on Clinical Data

“…Right-to-left shunt of our patients is in close agreement with the previous re ports in normal subjects (1.6 ± SD 0.7 or less than 7% [12,13]) and patients with pulmonary emphysema (7.4 ± SD 4% [13] ). However, these figures together with ours probably overestimated true anatomi cal shunt on air.…”

Mechanisms of Postoperative Hypoxaemia

“…Although there are many factors which might account for such a difference, it is of interest that 'titration' of narcotic against pain has been shown to be as efficacious as extradural block (Muneyuki et al 1968). (2) Obesity: The preoperative FRC in obese subjects is below normal. They are specially prone to airways closure and exhibit increased intrapulmonary shunting (Said & Banerjee 1963 , Spence et al 1970.…”

The response of patients with respiratory failure and cardiopulmonary disease to different levels of constant volume ventilation.

“…Such diffusion/perfusion limitations are not evident in normal lungs when cardiac output is changed (30). Moreover, the physiologic shunt in patients with normal lungs is too small (12) for reductions in the percentage physiologic shunt to be easily measurable.…”

“…The use of 3The "physiologic shunt" during pure oxygen breathing measures that proportion of the cardiac output which bypasses functioning alveoli (11). Under these circumstances the physiologic shunt has been called the "true," "pure," or "anatomical" shunt, or the "direct" venous admixture component (12). 4 In the first measurement in Patient 3 and in the first four measurements in Patient 11 (Table III) more than 2% nitrogen was present in mixed expired gas (due to entrainment of air into the inspired 100% oxygen), and PAo2 was assumed to equal PEO2 + PFCo2-superior vena caval samples to estimate mixed venous blood oxygen content is a compromise dictated by the practicalities of clinical care.…”