The effect of pco2 on hypoxic pulmonary vasoconstriction

Abstract

Lung areas with a low V/Q ratio cause hypuxaemia. The low alveolar oxygen concentration may cause local hypoxic pulmonary vasoconstriction (HPV) which reduces peffusion, raises the V/Q ratio, and hence reduces the tendency to a low Paul. By changing Pco2, the HPV response can be altered. We examined this relationship in anaesthetized dogs by using a tracheal divider to separate hypoxic (nitrogen ventilated) from oxygenated (100 per cent oxygen ventilated) lung. Relative perfusion was assessed from total 1~3Xe exhaled from each lung area after intravenous infusions. When Paco~ was changed by changing ventilation, we found that an increasing Paco, increased HPV and also Paul. At a Paco~ of 3.3 kPa, HPV was abolished and Pao, fell. We also changed Paco2 by altering Plco, to one or both lung areas while ventilation remained constant throughout the experiment. Again as Paco, increased, HPV and Pao2 increased. When Pacoz fell and end tidal carbon dioxide in the hypoxic lung (PETco~) remained eleyated (by maintaining Ptco, in the hypoxic lung and removing COz from the oxygenated lung) HPV was maintained. Thus it is the alveolar concentration of COz in the hypoxic lung which is important in modifying HPV.We conclude that in this model a low PETco, (3.3 kPa) in hypoxic lung will reduce HPV, and will result in more severe hypoxaemia. This may have relevance in both anaesthetized and intensive care unit patients when a higher Pao, may be obtained by increasing hypoxic lung PETco v The effect of PETco 2 on Pao, will be influenced by other variables, but when hypoventilated or hypoxic lung areas exist, increasing PETco ~ may reinforce hypoxic pulmonary vasoconstriction and thus may increase Paul.KEY WOADS: LUNG, hypoxic vasoconstriction, carbon dioxide.RESPIRATORY GAS EXCHANGE depends on the ratio of ventilation to perfusion. A low VIQ ratio and hypoxaemia will result from poorly ventilated lung unless perfusion is also reduced in the same region. Alveolar oxygen concentration helps to match regional perfusion to ventilation through a mechanism of hypoxic pulmonary vasoconstriction (HPV).I-3The mechanism of HPV is still not understood, but several events which occur under anaesthesia are reported to reduce HPV (and therefore to create hypoxaemia). Sykes, et al.,[4][5][6][7] Benumof and Wahrenbrock, s and Bjertnaes 9"1~ have found that inhaled anaesthetics reduce HPV. Benumof, et al. I1 have found that increased pulmonary vascular pressure can reduce HPV. Pc02 is also known to affect HPV.II-~3 Pc02 during anaesthesia may be increased, decreased, or normal. If HPV were present and a low Pc02 reduced HPV, the low Pc02 could create hypoxaemia by increasing perfusion to hypoxic lung. Pc02 may also alter Paoz through its effect in the alveolar gas equation or by changing the position of the oxyhaemoglobin dissociation curve, cardiac output, or R values. In order to document which effect was dominant we assessed HPV in dogs and observed the effect of altering ventilation and therefore Paco2 on both HPV and Pao~ when one t...

L’hypercapnie augmente la tension en oxygène du tissu cérébral chez des rats anesthésiés

“…Previous experimental studies have characterized hypercapnia induced elevation in PaO 2 which may result from improved ventilation perfusion matching in the lung. [15][16][17][18] The significant increase in PaO 2 observed during hypercapnia would be expected to enhance cerebral tissue oxygenation, as has been demonstrated in this and other studies. 29,30 However, the relative magnitude of the increase in PBrO 2 achieved during hypercapnia was threefold higher than that obtained when a comparable increase in PaO 2 was achieved by increasing the percentage of inspired O 2 .…”

“…Although not predicted by the alveolar gas equation, elevated PaCO 2 has been demonstrated to increase arterial PaO 2 , possibly by improved ventilation perfusion matching in the lung. [15][16][17][18] In addition to augmenting PaO 2 , hypercapnia has been shown to increase arterial blood O 2 carrying capacity 19,20 and increase cardiac output. 15,21 Hypercapnia might also improve tissue O 2 delivery due to a right shift in the O 2 hemoglobin dissociation curve.…”

Right ventriculo–arterial uncoupling and impaired contractile reserve in obese patients with unexplained exercise intolerance

“…Increased sympathetic nervous system activation and metabolic dysregulation may also drive higher filling pressures in obesity (Ketabchi et al 2009; Noble et al 1981). Both obese groups in our study developed higher right atrial pressure with exercise suggesting excess RV preload.…”

“…The net result is a lowering of the VE/VCO 2 slope in obesity allowing for preserved VCO 2 at a lower level of alveolar ventilation. We did not undertake an isoWork analysis to enable meaningful gas exchange comparisons between groups; however, higher arterial and, by implication, alveolar pCO 2 levels have been shown to provoke greater pulmonary vasoconstriction (Barer and Shaw 1971; Kregenow and Swenson 2002; Nishio et al 2001; Noble et al 1981; Sweeney et al 1998; Viitanen et al 1990) which may have added to RV afterload in obesity. Dempsey and Wagner have previously highlighted the influence of a raised PaCO 2 on exercise-induced hypoxaemia during maximal exercise (Dempsey and Wagner 1999).…”

Arterial oxygenation and one-lung anesthesia

“…For example, hypercarbia may lead to increased PAP [60] with subsequent diversion of blood away from the dependent, ventilated lung toward the nonventilated lung, thereby increasing the shunt. Nonetheless, hypercarbia associated with hypoventilation has not been shown to decrease arterial oxygenation during OLA [60]; firstly, because hypercarbia potentiates HPV [87] and, secondly, because lower airway pressures are associated with a higher cardiac output [61].…”