Ventilation and blood gases in anaesthetized patients

Abstract

RECENTLY the safety of intermittent positive pressure breathing (IPPB) from the standpoint of oxygenation has been questioned. An increase in physiologic dead space (VD (phys)) and ventilation-perfusion inequality have been demonstrated during IPPB in both awake and anaesthetized subjects. 1-n An anatomical shunt with venous admixture may result from ventilation with constant volumes. The degree of this disturbance, the time relationships between onset and maximum hypoxaemia, and its clinical significance remain to be evaluated.The purpose of this study was to evaluate the significance of blood gas changes during anaesthesia and IPPB. Although evidence is available to the contrary, we wanted to test the validity of the assumption that during the awake, spontaneously breathing state and the anaesthetized IPPB state similar inhaled oxygen tensions (Pro_o), tidal volumes (VT), and respiratory rates (f) should provide similar blood gas values in the same patient. Results proved that the assumption was not valid, although in many instances higher arterial oxygen tensions occurred at tidal volumes considerably less than are used during the management of clinical anaesthesia. EXPERIMENTAL METHODSurgical patients were placed on the operating table one hour after the int~:amuscular injection of 100 mg. pentobarbital and 0.5 mg. atropine. A Courhand needle was placed in the brachial artery and an intravenous infusion of 5 per cent dextrose and water initiated. A mouthpiece was inserted and a nose clip applied. After a tight seal was ensured, a recently calibrated Wrigh.t respirometer was attached to measure expiratory VT and minute volume (VE). Two arterial samples were withdrawn in the conscious state, the first while the patient was breathing air. A non-rebreathing valve and reservoir bag with 100 per cent oxygen were then attached. This was followed by 5 minutes of denitrogenation and, in turn, succeeded by 5 minutes of inhalation of 50 per cent oxygen in nitrogen through the same breathing syste~.n. The second arterial sample (the awake control) was then withdrawn while VE and f were again measured.Thiopental was then injected intravenously followed by succinylcholine. The trachea was then intubated. Fifty per cent 02 in N.oO-halothane was inhaled and the patients' respirations were controlled with the same pressure-regulated ventilator during all subsequent measurements. This 02 concentration was maintained and monitored at a point distal to the inspiratory valve in a circle CO2 absorption

Effect of chest physiotherapy on blood gases of neonates treated by intermittent positive pressure respiration

“…DISCUSSION As found in the previous study (Holloway et al, 1966) the Pao2 of neonates receiving I.P.P.R. was significantly below that of a comparable group of normals, thus substantiating what is now a wellknown fact ; patients receiving artificial ventilation oftein have hypoxaemia unless oxygen enrichment of inspired air is provided (Nealon et al, 1966;Gold and Helrich, 1967).…”

“…In common with others (Nealon, Prorok, Gosin, Fraimow, 1966;Gold and Helrich, 1967;Bendixen, Hedley-Whyte, and Laver, 1963), we have found a wide alveolar to arterial oxygen pressure gradient (PAao2) and failure to maintain normal arterial oxygenation among patients ibeing ventilated on air by intermittent positive pressure respiration (I.P.P.R.) (Holloway, Desai, Kelly, Thambiran, Strydom, and Adams, 1966).…”