Roger Fletcher

Clonidine and the sympatico-adrenal response to coronary artery by-pass surgery.

Clonidine was administered intravenously in an attempt to limit sympatico‐adrenal activity and thereby reduce the incidence of arterial hypertension associated with coronary artery by‐pass graft surgery (CABG). Forty patients scheduled for CABG were assigned to two groups. Twenty patients received clonidine 4μg kg‐1 before surgery, 2 μg kg‐1 after cardiopulmonary by‐pass and 1 μg kg‐1 when the skin was sutured. The other 20 patients served as controls. All patients were anesthetized with fentanyl, droperidol, nitrous oxide and alcuronium. During surgery 5 min after sternotomy, mean arterial pressure was 13 mmHg lower (P<0.01) in the clonidine group, while after operation the difference between the groups was negligible. Both during and after surgery the plasma catecholamine concentrations were significantly lower in the clonidine group (P<0.01). The greatest difference between the groups was seen 90 min after operation, when plasma noradrenaline and plasma adrenaline concentrations in the clonidine group were less than 1/ 3 of those in the control group (P<0.01). As judged by catecholamine concentrations clonidine was effective in attenuating sympatico‐adrenal hyperactivity during and after surgery. Postoperative arterial hypertension was not reduced, however, and it is concluded that other factors besides sympatico‐adrenal hyperactivity must be important.

On-line measurement of gas-exchange during cardiac surgery

This paper describes an on‐line system for continuously monitoring expired CO2 during controlled ventilation. Signals from a Servo ventilator 900B or C and a CO2 Analyzer 930 are processed and corrected by the computer to produce a CO2 single breath test (SBT‐CO2). This is the tracing of expired CO2 concentration or fraction against expired volume, from which the computer calculates the airway deadspace (VDaw). If a value for arterial PCO2 is supplied, the computer will calculate the physiological deadspace (VDphys) and the alveolar deadspace (VDalv) for each breath. The system was used to make measurements at four stages during coronary artery by‐pass grafting in 13 male patients. When the sternum was opened there was a 32% increase in VDaw, and the physiological deadspace fraction therefore increased. There were reductions in VDaw after extra‐corporeal circulation and again after sternal suture. By the end of surgery, the alveolar deadspace fraction had increased significantly. VDaw at this stage was smaller than pre‐operatively, and so there was no net change in the physiological deadspace fraction at the end of surgery. Arterial Po2 was, however, reduced at this stage.

Monitoring of physiological parameters during high frequency ventilation (HFV)

Non‐biological descriptors such as jet feeding pressure and oscillator stroke volume are often used to describe HFV. This results in confusion and hinders acceptance of HFV. The goal of this paper is to show how physiological parameters which are valid during HFV can be monitored. Airway pressure measured in narrow tubes with high linear flow rates is underestimated. A relevant airway pressure must be measured well below the tracheal tube. Pressure measured higher up should be validated against peripheral pressure measurements. Minute ventilation and expired CO2 concentration can be determined with a ServoVentilator and a CO2 analyzer arranged at its exit port. Minute ventilation and CO2 elimination can thereby be continuously monitored during high frequency jet ventilation or so‐called “combined high frequency jet ventilation” to prevent undetected disturbance of ventilation and perfusion. Physiological dead space can be studied for optimization of ventilatory pattern. The principle of gas analysis at the exit port of the ventilator may be used for FRC determinations with sulfur‐hexafluoride.

Anaesthetic management of the malignant hyperthermia susceptible parturient

R E P L Y To my knowledge the single segment combined subarachnaid epidural ( CSE ) block has noi been reported for Cesarean section previously. The mod~ficatlon recommended by N&kalls and Dennison whereby the spinal needle is clamped to maintain its position in the dura appears interesling. CSE block avoids one of the ms)or disadvantages of suharachnoid block in the pregnant patient, i.e., the difficulty in controlling the upper level of analgesia l f Dr, Dennison can consistently achieve a T2-Ta block with 1.5-1.6 ml isobaric subaracbnaid bupivocaine and keep the incidence of hypotenslon down to an impressive 10-15 per cent it is arguable i f a n epidural catheter is necessar3 at all. In contrast to Dr. Dennisons techniqae, our aim with the CSE technique is to achieve a Ts subarachnoid block followed by extension of the block to To by injecting bupivacaine in the epidural catheter. The less extensive subarachnoid block combinad with the slower onset of epiduraf block allows more time for compensatory mechanisms to be effective and thereby minimizes the risk of precip#ous hypotension with the two stage CSE technique. We do not use prophylactic vasopressors since these drags may have undesirable fetal and maternal effects. J Thus the differences in the spread of subarachnoid blocks in spite of similar doses is due to differences in the techniques. Dr. Dennisons patients received isobaric bupivacaine while our patients were given hyperbaric buplvctcaine in the sitting posiaon. For the surgical procedure Dr. Dennison apparently i~sea the conventional subarachaoid technique while we use the CSE technique. For postoperative analgesia with epidural opiates our experience is similar to that of Dr, Dennison.

Investigation of malignant hyperthermia in Sweden

One hundred and thirty patients from 52 families were investigated for susceptibility to malignant hyperthermia (MH). The diagnosis of MH susceptibility (MHS) was made by in vitro exposure of muscle to halothane and to caffeine, according to the protocol established by the European malignant hyperpyrexia group. In addition, 13 normal control biopsies were obtained from the same muscle and with the same anaesthesia as in the MH patients. These control results agree with and confirm the criteria formulated in the European MH group. The results are compared with our first 85 investigations performed prior to the establishment of the protocol. The main difference compared to the earlier material is the addition of an equivocal group (MHE), whose muscle reacted in vitro to either halothane or caffeine, but not both. All the families referred because of a fulminant MH reaction contained MHS or MHE members. There was a greater incidence of MH‐negative families, suggesting an increased suspicion of MH amongst the clinicians.

Isocapnic high frequency jet ventilation: dead space depends on frequency, inspiratory time and entrainment

Twelve healthy pigs were ventilated with high frequency jet ventilation via a Mallincrodt HiLo jet tube. The expired gas was led to a conventional ventilator and CO2 analyzer which were used to measure CO2 climination. There was no bias flow, so that the jet entrained only expired gas, i.e. rebreathing occurred. Frequency was varied between 2 and 11 Hz and the duration of inspiration, as a fraction of the ventilatory cycle (Ti/Ttot), from 5 to 20%. The minute ventilation, Vjet, delivered by the jet ventilator was adjusted to maintain a constant Paco2. At 2 Hz and a Ti/Ttot of 5%, Vjet was of the same magnitude as ventilation during conventional intermittent positive pressure ventilation, and the total dead space fraction, VD/VT was 0.32. Both increasing frequency at a constant Ti/Ttot, and increasing Ti/Ttot at a constant frequency, increased VD/VT which was maximal (0.8) at 11 Hz and a Ti/Ttot of 20%. When entrainment was blocked, tidal jet volume had to be greatly increased. The continuous measurement of CO2 climination was found to be useful for maintaining isocapnia when the jet ventilator setting was changed.