Components of the inspiratory-arterial isoflurane partial pressure difference.

We have measured the partial pressure of isoflurane simultaneously in inspired gas (PIiso), end-expired gas (PE'iso), mixed-expired gas (PEiso), arterial (Paiso) and mixed venous blood (Pviso) in six patients (aged 57-79 yr) anaesthetized with nitrous oxide, oxygen and isoflurane before surgery and after PE'iso had been stable for at least 15 min. We related these changes to the various indices of pulmonary maldistribution to determine if they were sufficient to explain reported differences between PE'iso and Paiso. Alveolar deadspace dilution of end-expired gas was calculated for carbon dioxide and this dilution factor used to calculate the "ideal" alveolar Piso (PAiso) from the observed inspired and end-expired concentrations. Shunt fraction was measured for oxygen and then used to calculate the partial pressure of isoflurane in the pulmonary end-capillary blood (Pc'iso) from the partial pressure in arterial and mixed venous blood. Mean (SE) values were: PIiso 0.69 (0.05) kPa; PE'iso 0.52 (0.04) kPa; PAiso 0.50 (0.04) kPa; Pc'iso 0.38 (0.04) kPa; Paiso 0.35 (0.03) kPa and Pviso 0.22 (0.02) kPa; Paiso: PE'iso 0.66 (0.02) kPa. The mean "ideal" alveolar to pulmonary end-capillary Piso difference was 0.12 (0.01) kPa and highly significant (P < 0.001). Paiso was substantially less than PE'iso but, for isoflurane, the difference was reasonably constant (range 0.14-0.22 kPa). The difference was attributable in part to the effects of shunt and deadspace, but also a failure of equilibration of isoflurane between the alveolar gas and pulmonary end-capillary blood. It is likely to be different for other anaesthetics. We conclude that, while PE'iso may adequately reflect Paiso for isoflurane, it cannot be assumed that the relation between end-expiratory gas and arterial partial pressures is the same for all anaesthetics.

Studies on the haemopoietic toxicity of nitrous oxide in man.

Nitrous oxide inactivates vitamin B12 and in man can produce a megaloblastic anaemia. Haematological and biochemical changes were studied in nine surgical patients ventilated with 70% N2O for up to 24 h and in three control patients. There was a rise in the numbers of hypersegmented neutrophils in peripheral blood following N2O. Serial bone marrow aspirates showed gross megaloblastic change after 24 h of N2O which had reverted to normoblastic but dyserythropoietic haemopoiesis by 1 week. Giant forms of early myeloid precursors were also seen after 24 h ventilation with N2O but by 1 week abnormalities were evident in more mature cells, metamyelocytes and segmented neutrophils. Megaloblastosis was associated with abnormal dU suppression which showed a correction pattern similar to that seen in vitamin B12 deficiency. Administration of N2O was also associated with a progressive rise in serum folate and fall in serum methionine levels. No similar patterns were seen in the three control patients.

Pseudomonas aeruginosa colonisation in an intensive therapy unit: role of cross infection and host factors.

Despite the sparsity of Pseudomonas aeruginosa in the environment colonisation and infection with this organism was found at several sites by selective culture in 20 out of 46 patients in an intensive therapy unit. Three patients developed Ps aeruginosa pneumonia. Serial serogrouping and phage typing identified multiple strains in the unit and in the same patient. Rectal carriage occurred in 16 patients but rectal strains did not subsequently appear in tracheal aspirates; strains varied in their affinity for the upper respiratory tract. Colonisation was not directly related to length of stay and was detected in 16 of those colonised within 24 hours of admission. In intubated patients, who were colonised more frequently than those not intubated, upper respiratory tract colonisation correlated strongly with low initial arterial pH values. Personnel were probably responsible for cross infection among patients when the unit was busy. Strain differences and the susceptibility of patients also influenced colonisation and infection. Elimination of major reservoirs of Ps aeruginosa and compliance with procedures to control cross infection remain essential if patients in hospital are to escape colonisation by the organism.

Mandatory minute volume. A new concept in weaning from mechanical ventilation.

The new concept of mandatory minute volume (MMV) is described. The system provides a preset minute volume to the patient, who breathes spontaneously from it as much as he is able, the remainder being delivered to him via a ventilator. The necessary apparatus has been constructed and has the additional facility of PEEP and/or CPAP up to a level of 15 cmH2O pressure. With possible exceptions, the apparatus allows simpler and more direct control over the patient's PaCO2 than with the IMV system and it ensures, without adjustment, a constant minute volume of fresh gas breathed by the patient, despite minute-to-minute changes in his ability to breathe. Use of the system in conjunction with two commonly used ventilators is described. It should not be difficult to build the facility for MMV into new versions of artificial ventilators.

Functional residual capacity during anaesthesia. I: Methodology.

The helium dilution technique for the measurement of functional residual capacity (FRC) is reviewed with special reference to anaesthesia. A modification is described which permits measurements to be made during intermittent positive pressure ventilation. This modification causes minimal interference with the circuit as it is used for spontaneous respiration. The measuring circuit may be alternated with an open circuit without disturbance of the pattern of breathing (spontaneous or artificial). Potential errors in the measurement of FRC during anaesthesia are considered and assessment of linearity, reproducibility and accuracy is described.

Functional residual capacity during anaesthesia. II. Spontaneous respiration.

Functional residual capacity has been measured by helium dilution in 26 spontaneously breathing patients before and immediately after anaesthesia, which was induced with thiopentone and maintained with halothane. The mean reduction was 390 ml (16.1% or pre-induction value) and the change was highly significant (P less than 0.001). The decrease in FRC correlated with age (r equals 0.41: P less than 0.005) and less (P less than 0.001). The decrease in FRC correlated with age (r equals 0.41; P less than 0.005) and less satisfactorily with the weight/height ratio (r equals 0.31; P less than 0.05) which itself correlated with age. Inspired oxygen concentration, expiratory reserve volume and the presence of phasic expiratory muscle activity bore no significant relationship to the decrease in FRC. There was no evidence of any progressive change in FRC between 6 and 20 min after the induction of anaesthesia.

Functional residual capacity during anaesthesia III: Artificial ventilation.

The helium dilution technique has been used to measure the FRC of 13 patients before and during anaesthesia with paralysis and artificial ventilation, and also in 5 conscious subjects during spontaneous and artificial ventilation without paralysis. After induction of anaesthesia the mean reduction in FRC was 297 ml or 15.4% of the preoperative value and the change was highly significant (P is smaller than 0.005). In the conscious subjects, the FRC was reduced in every case during artificial ventilation by a mean value of 99 ml or 4.1% of original value. This change was also significant. The anaesthetized patients breathed 35% oxygen before and during anaesthesia. The (A-a) PO2 difference increased from a mean value of 49.1 mm Hg before anaesthesia to a mean of 79.7 mm Hg during anaesthesia, the change being highly significant (P is smaller than 0.001). Individual changes correlated with the reduction in FRC (P is smaller than 0.005).