The regulation of $$Pa_{CO_2 } $$ during controlled ventilation of children with a T-piece

Abstract

A lung model study described the factors determining the carbon dioxide tension during controlled ventilation with a T-piece system (either a Bain circuit or the classical Ayre’s T-piece). When minute ventilation was large with respect to fresh gas flow and a standard ventilatory pattern was utilized the important variables were fresh gas flow (FGF) and carbon dioxide production $$\left( {\dot V_{CO_2 } } \right)$$ . Under these circumstances the influence of minute ventilation $$\left( {\dot V_E } \right)$$ , dead space to tidal volume ratio (VD/VT), waveform and $$a - AD_{CO_2 } $$ on carbon dioxide tension were minimized. Eight healthy patients (11–64 kg) were studied during controlled ventilation to determine how much of the fresh gas coming from the flowmeters actually participates in gas exchange. With a T-piece system when minute ventilation to fresh gas flow ratio is at least 1.5 the patient’s effective alveolar ventilation equals 72 per cent of the total fresh gas flow. This value and levels of carbon dioxide production from the literature were used to derive a simple two part formula of fresh gas flow requirements for two levels of arterial carbon dioxide in children. Children weighing between 10 and 30 kg require fresh gas flows of 1000 + 100 ml · kg-1 · min-1 and 1600 + 100 ml · kg-1 · min-1 to achieve arterial carbon dioxide tensions of 4.9 kPa (37 torr) and 4.0 kPa (30 torr) respectively. When body weight is between 30 and 70 kg the requirements increased to 2000 + 50 ml · kg-1 · min−1 to achieve a $$Pa_{CO_2 } $$ of 4.9 kPa (37 torr) and 3200 + 50 ml · kg-1 · min-1 to achieve a $$Pa_{CO_2 } $$ of 4.0 kPa (30 torr). In all situations Ve must be at least twice the fresh gas flow requirements. p ]The formula was validated in a heterogeneous group of 26 patients. Mean values of $$Pa_{CO_2 } $$ were very close to those predicted, although individual values differed considerably from the mean. These deviations cannot be eliminated because of patient and equipment variability. Therefore blood gas analysis must be performed whenever the exact regulation of $$Pa_{CO_2 } $$ is required.