W.W. Mapleson

Evaluation of Frova, single‐use intubation introducer, in a manikin. Comparison with Eschmann multiple‐use introducer and Portex single‐use introducer*

In a randomised cross‐over study, 48 anaesthetists attempted to place a Frova single‐use introducer, an Eschmann multiple‐use introducer and a Portex single‐use introducer in the trachea of a manikin set up to simulate a grade 3 laryngoscopic view. The anaesthetists were blinded to success (tracheal placement) or failure (oesophageal placement). Successful placement (proportion, 95% confidence interval) of either the Frova introducer (65%, 50–77%) or the Eschmann introducer (60%, 46–73%) was significantly more likely than with the Portex introducer (8%, 3–20%). There were no significant differences between the success rates for the Frova and the Eschmann introducers. A separate experiment revealed that the peak force exerted by the Frova and Portex introducers was two to three times greater than that which could be exerted by the Eschmann introducer, p < 0.0001, indicating that the single‐use introducers are more likely to cause tissue trauma during placement.

A comparison of oxygen therapy devices used in the postoperative recovery period

Seventy‐one patients scheduled to undergo upper or lower abdominal surgical procedures were allocated at random to one of seven treatment groups: in the recovery room they were to receive oxygen via a 40% Ventimask with 10 litres/minute oxygen flow, or via either a Hudson mask or a nasal cannula with 3,6 or 9 litres/minute oxygen flow. The 40% Ventimask gave the most consistent, satisfactory postoperative values of PaO2 but the much cheaper nasal cannula at 6 or 9 litres/minute was generally adequate in conscious patients. The performance of the intermediately priced Hudson mask was similar to that of the nasal cannula at these flows. The unconscious state was associated with a 45% lower PaO2 than the rousable or awake states. Differences between the treatments with regard to postoperative PaCO2 were small and non‐significant. The nasal cannula with 6 litres, minute humidified oxygen flow is recommended for routine treatment, and the Ventimask for unconscious patients.

The effect of fluosol-DA on induction of inhalation anesthesia

The liquid/gas partition coefficients of three inhalation anesthetics in Fluosol-DA 20% (Fluosol), a perfluorocarbon blood substitute, were determined in vitro. The high values found (6.68 for halothane, 7.54 for enflurane, and 7.20 for isoflurane) suggested that induction with these agents would be prolonged in patients treated with Fluosol. Induction of isoflurane anesthesia (as a representative agent) at constant inspired concentration was studied in five mongrel dogs before and after replacement of about 25% of each animals blood volume with Fluosol. Inspired and end-tidal isoflurane and carbon dioxide concentrations were recorded breath by breath, together with cardiac output. There was a significant delay in rise of end-tidal isoflurane concentration after Fluosol infusion. However, because cardiac output could not be held constant during each experiment, and because cardiac output also affects the rate of rise of alveolar anesthetic concentration, a physiological computer model was used to compare the isoflurane blood/gas partition coefficients that must have existed to account for the observed end-tidal levels before and after Fluosol infusion, while taking cardiac output variation into account. Post-Fluosol blood/gas partition coefficients calculated in this way (2.59 ± 0.51 SD) were significantly different (P < 0.001) from pre-Fluosol levels (1.45 ± 0.15 SD) and were not significantly different from post-Fluosol partition coefficients calculated by volume-weighted averaging (2.91 ± 0.36 SD). This indicates that the delay observed was attributable in large part to increased solubility of isoflurane in blood after addition of Fluosol. Based on their similar liquid/gas partition coefficients in Fluosol, similar delays should occur with halothane and enflurane.

Why 5% risk of type error but 20% risk of type error?

adequacy of ventilation. It was judged adequate when the chest expanded with satisfactory compliance via the laryngeal tube, and was judged partially obstructed if the chest expanded insufficiently with a resistance to squeezing the reservoir bag. Airway pressure was then maintained at 20 cmH2O by squeezing the reservoir bag for 3 s and the presence or absence of an audible gas leak around the device, and of gastric insufflation (examined by placing a stethoscope over the egigastrium), were recorded. Before insertion of the laryngeal tube, ventilation via a face mask was judged easy in 32 patients, and moderately difficult in the remaining two patients. Insertion was easy in 31 patients, moderately difficult in two and difficult in one patient. Ventilation through the laryngeal tube was possible at the first attempt at insertion in 31 of 34 patients, at the second attempt in two patients, and failed in one patient. Ventilation was judged as adequate in 33 of 34 patients. A gas leak was present around the device in two of 33 patients (in both cases the chest expanded adequately) and no gastric insufflation was detected. In the one patient in whom ventilation via a laryngeal tube failed, ventilation via a face mask had been judged moderately difficult. After tracheal intubation, ventilation via a tracheal tube was also difficult, and a large amount of sputum was aspirated on suctioning. In conclusion, insertion of the laryngeal tube is generally easy for paramedic staff. We believe the device is potentially useful not only for anaesthetists but for paramedic staff too.

LONDON MEETING AT THE ROYAL POSTGRADUATE MEDICAL SCHOOL, HAMMERSMITH HOSPITAL, NOVEMBER 16, 1973 EFFECT OF FREQUENCY OF AUTOMATIC VENTILATION ON CARDIORESPIRATORY FUNCTION IN THE DOG

fall in Qs/Qt from 9.8 to 6.4%. With induced pulmonary orrlema, a fall in cardiac output from 2.11 ±0.24 l./min to 1.37 ±0.19 L/min was accompanied by a fall in Qs/Qt from 29% to 21.6%. In no instance was there a significant change in arterial oxygen tension despite a fall in mixed venous saturation from 75±4.5% to 59± 3.6% in dogs with normal lungs and from 62±3.4% to 49 ±3.0% in dogs with pulmonary oedema. In another series of experiments on 11 pentobarbitoneanaesthetized dogs, mixed venous oxygenation was varied independent of cardiac output by means of veno-venous jnnnring of blood through a disc oxygenator. Changing the composition of the gas flow through the oxygenator was without effect on pulmonary artery pressure, pulmonary artery wedge pressure, cardiac output or pulmonary vascular resistance. However, increasing Svo, from 50 to 70% in normal lungs caused a significant increase in Qs/Qt from 7.9 to 10.1%. In 7 dogs with pulmonary oedema, a change in Sv0, from 32.2 to 593% was without effect on pulmonary pressure or flow but caused a significant increase in Qs/Qt from 25.7 to 28.8%.