risk ex-premature infants in not free of risk. For elective .... thesia, 4th ed., Baltimore: Williams & Wilkins, 1993: 85-177. ... Pharmacology & Physiology in Anesthetic Practice,. 2nd Ed. Philadelphia: J.B. Lippincott, 1991; 264--79. 5 Critchley LA ...
362 and the FIO2 adjusted to maintain peripheral oxyhaemoglobin saturation (SpO2) >90% and < 100%. After the procedure, IPPV was continued and residual neuromuscular blockade was reversed. With return of limb movement and phonation, the LMA was removed, the pharynx aspirated, and ventilation was assisted by bag/ mask until judged to be adequate by rate, depth and pulse oximetry. In six patients (37.5%), oxyhaemoglobin desaturation < 80% immediately after insertion of the LMA was corrected when the mask was partially withdrawn. The mask was then unstable. When held in the optimum position as indicated by capnography, adequate IPPV was mainrained. Eleven infants (68.75%) had apnoeic spells during the first 12 hr after anaesthesia; in four, two of whom after propofol infusion, these ocurred so frequently that tracheal intubation and mechanical ventilation in the NICU for 2.75 5= 1.5 days were required. In 12 infants tracheal intubation was avoided. Chest x-ray within 24 hr of surgery revealed no evidence of aspiration. Desaturation following insertion of the LMA may be attributed to inexperience, inadequate control of airway reflexes, or to respiratory obstruction. Improvement when the mask was partially withdrawn, and its subsequent instability suggests that its size and shape do not conform to the small infant's upper airway as effectively as larger sizes in older patients. Tracheal intubation in small, highrisk ex-premature infants in not free of risk. For elective anaesthesia, the #1 LMA appears to be capable of providing a satisfactory alternative. A . C . W e b s t e r MD FRCA FRCPC W.D. R e i d MD FRCPC
L.E Siebert MDFRCPC M.D. Taylor MD Departments of Anaesthesia, Neonatal and Perinatal Medicine and Ophthalmology University of Western Ontario St. Joseph's Hospital
Unusual regional block To the Editor: Recently I had occasion to administer regional anaesthesia for Caesarean section to two women with somewhat unexpected results. The first was for an elective, repeat Caesarean section in a healthy woman with marked scoliosis. She had an S-shaped curve, (thoracic curve of 45 degrees) with marked prominence of her right scapula, due to rotation. I elected to administer spinal anaesthesia
CANADIAN JOURNAL OF ANAESTHESIA
with 1.5 ml of hyperbaric 0.75% bupivacaine, 10 ~g intrathecal fentanyl and 250 l~g of intrathecal morphine. The subarachnoid injection was performed easily with the patient in the sitting position and she was then positioned supine with left uterine displacement using a wedge. The block had a rapid onset and five minutes following injection there was marked asymmetry with a T 6 level on the right and a Tt2 level on the left. The operating room table was therefore tilted to the left. Five minutes later the block was symmetrical with a T3 level bilaterally prior to surgery. I believe that the initial asymmetry of the block was secondary to the rotation of the thoracic spine and neural elements, such that the portion supplying the right side was situated more posteriorly. With the hyperbaric solution this meant that the right side was preferentially blocked. Altering the flit of the table allowed the left side to be exposed to the hyperbaric solution, producing a bilateral block. The second case was that of a morbidly obese primipara who presented for labour analgesia. She had had gastric bypass surgery in 1986 for morbid obesity (> 180 kg). This resulted in a weight loss of 68 kg and she subsequently had an abdominoplasty with removal of excess skin. She was now pregnant with her first child and her pregnant weight was 118 kg. She had a morbid fear of needles and had initially expressed a wish for general anaesthesia, if Caesarean section was required. During labour she required analgesia and requested an epidural. Following application of an EMLA| patch for 60 min, an epidural needle was inserted following multiple attempts, with the patient in the left lateral position. The sitting position was not used due to fetal heart rate abnormalities. The epidural provided effective labour analgesia and the patient was willing to have the block extended for Caesarean section because of failure to progress. In the operating room, the level of the block was Ttm and the patient was comfortable. Incremental doses of carbonated lidocaine with 1:200,000 epinephrine were administered to a total of 10 ml. There was a decrease in the diastolic pressure of 10 mmHg, evidence of motor block and increase in the temperature of her feet (signs which generally indicate a block to ice of T8 or above). However, abdominally the block still measured T H bilaterally. A further five ml of the localanaesthetic, administered incrementally, resulted in a cutaneous level of T4. The surgery then proceeded uneventfully. I believe that the discrepancy between the clinical signs and the level of the block to ice was related to her abdominoplasty. Removal of redundant skin meant that the skin now surrounding the umbilicus was likely innervated by T8 and above. Therefore the cutaneous level did not change until the block extended above that level. Thus,
CORRESPONDENCE
the abdominoplasty masked the level of block, as tested over the abdomen, and could have led to a mistaken belief that the epidural was not working or that a larger increment of local anaesthetic was required. These two cases are unusual in the presentation of their regional block. However, knowing the anatomy of the one case and the past surgical history in the other it is easy to postulate a mechanism for the clinical picture. M. Joanne Douglas MDFRCPC Department of Anaesthesia British Columbia's Women's Hospital and Health Centre Society
Vasopressors and hypotension To the Editor: We read with interest Dr. P. Morgan's review entitled "The role of vasopressors in the management of hypotension induced by spinal and epidural anaesthesia" (Can J Anaesth 1994: 41; 404-13) and would like to make the following comments. There was no mention of the role played by the venous system in the genesis of hypotension during spinal blockade. Greene and Brull, l Macrae and Wildsmith2 and our own findings 3 all report that venous capacitance is increased during spinal blockade. Greene and Brulls'l view was that venodilation causes a reduction in venous return to the heart and this is one of main factors causing hypotension. They strongly recommend the use of headdown tilt during spinal blockade to improve venous return. Our experience with elderly patients receiving subarachnoid block is that a moderate decrease in central venouss pressure occurs, on average 2.5 cm H20, and this can be adequately reversed, in most cases, by giving 8 ml. kg -I colloid during the induction phase of the block. 3 Dr. Morgan contradicts himself when discussing the use of ephedrine. He mentions that ephedrine has both a and 13-adrenergic agonist actions and the "vasoeontriction is almost balanced by vasodilatation, and peripheral resistance is usually little changed." Later he states that ephedrine is capable of increasing cardiac output and of increasing peripheral resistance. We think that Dr. Morgan alludes to the fact that agonists with both a and 13 simulating activity, such as ephedrine, are poor vasoconstrictors becausse of the opposing effects of simultaneous a (vasoconstriction) and 13 (vasodilatation) stimulation on the arterial beds. 4 We found the same results in treating hypotension during subarachnoid block with ephedrine infusion in 30 elderly patients. 5
363
Dr. Morgan refers to metaraminol as having both and 13stimulatory effects. This is misleading as the clinical effects of metaraminol are predominantly alpha. We have used metaraminol during subarachnoid block for several years and consider the dose of 5 mg/v to be excessive, despite being frequently quoted. 6'7 In the non-obstetric elderly patient undergoing subaraehnoid block, our current practice, after giving a moderate volume of colloid (8 ml-kg-l), is to treat hypotension with a bolus of 0.5-1.0 mg (0.1-0.2 mg-kg -l) followed by an infusion of 10 mg in 20 ml of normal saline run at 5-10 ml. hr -~. Our experience with this dose is that metaraminol is highly effective at restoring preload and afterload during subarachnoid block but has little demonstrable effect on heart rate or contractility.S L.A.H. Critchley MBChBFFARCI F. Conway MB ChB FFARCI T.G. Short MBChBMDFANZCA Department of Anaesthesia and Intensive Care Prince of Wales Hospital The Chinese University of Hong Kong Shatin, N.T., Hong Kong
REFERENCES 1 Greene NM, Brull SJ. The cardiovascular system. In:
2
3
4
5
6
7
8
Greene NM, Brull SJ (Eds.). Physiologyof Spinal Anaesthesia, 4th ed., Baltimore: Williams & Wilkins, 1993: 85-177. McCrae AF, Wildsmith JA. Prevention and treatment of hypotension during central neural block. Br J Anaesth 1993; 70: 672-80. Critchley LA, Stuart JC, Short TG, Gin T. The haemodynamic effectsof subarachnoid block in elderly patients: measurement by tramthoracic bioimpedance. Br J Anaesth (in press). Stoelting RK. Sympathomimetics.In: Stoelting, R.K. (Ed.). Pharmacology & Physiologyin AnestheticPractice, 2nd Ed. Philadelphia: J.B. Lippincott, 1991; 264--79. Critchley LA, Stuart JC, Conway F, Short TG. Hypotension during subarachnoid anaesthesia: haemodynamiceffects of ephedrine. Br J Anaesth (in press). Hoffman B, Lefkowitz R. Catecholaminesand sympathomimetic drugs. In: Gilman A, Rall T, Nies A, Taylor P (Eds.). The Pharmacological Basis of Therapeutics, 8th ed., Singapore: Pergamon Press, 1991: 213-4. Sympathomimetics.In: Reynolds JE, Parfitt K (Eds.). Martindale, the Extra Pharmacopoeia, 29th ed., London: The Pharmaceutical Press 1989: 1468-89. Critchley LA, Short TG, Gin T. Hypotension during subamchnoid anaesthesia:hacmodynarnic analysisof three treatments. Br J Anaesth 1994; 72: 151-5.
