RESUMO
In obese subjects with chronic bronchitis hypercapnia by limitation of tidal volume regresses if a slow ventilatory frequency is imposed by lengthening of expiration time. But hypoxemia due to shunt effect can either be corrected by global improvement of alveolar ventilation VA or persist and become worse by low pulmonary volume ventilation which lowers VA/Q. The effect of bradypnoea on 11 elderly patients with moderate obesity has been evaluated by simultaneous measurements of blood gases, ventilatory output coefficients, gas flow rate and steady state transfer for CO. Although hypo-VA disappeared in bradypnoea, hypoxemia persisted in 5 cases, the increase of P(Aa)O2 was accentuated in 7 cases, VCO always remained in deficit compared with VCO2 (in healthy subjects at rest, VCO and VCO2 are interrelated by a proportionally constant whatever the respiratory regimen: VCO/VCO2 = specific VCO, or VCO Sp). The VCO/VA variation correlated negatively with the P(Aa)O2 variation. This study: 1) confirms the link between PaCO2 and VT and the persistence of shunt effect bradypnoea compatible with the deficit of VCO versus VCO2; 2) distinguishes the efficiency of bradypnoea in ventilation and in alveolo-capillary exchange; 3) compares the variations of VCO Sp with those of PaO2 in relation to VT; 4) defines the characteristics of respiratory insufficiency in the obese and bronchitic subjects examined, and 5) specifies the value of VCO Sp measurement in testing the controlled ventilation technique used.
