BRITISH JOURNAL OF ANAESTHESIA

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that of MHS muscle. There was no change in the relative concentrations of the metabolites during the first 1.75 h in either control or MHS muscle. Neither 3% ...
BRITISH JOURNAL OF ANAESTHESIA

664 that of MHS muscle. There was no change in the relative concentrations of the metabolites during the first 1.75 h in either control or MHS muscle. Neither 3% halrhanff nor caffeine 2mmollitre~ l induced an observable change in the metabolite profile of control porcine muscle. However, both these drugs caused a change in the spectrum of MHS muscle. The phosphccreatine peak was decreased in area, while the inorganic phosphate resonance showed a comparable increase in area. The three ATP resonances did not change. The results of these experiments support the suggestion that continued synthesis and hydrolysis of ATP plays an important role in the increase in temperttuxe which occurs during fulminant MH. The supply of ATP is replenished by the creatine phojphokinase reaction: ADP+PCr

ATP + Cr

If the demand for ATP is greater than the ability of glycolyta and of oxidative phosphory larion to replenish the energy reserves of the cell, then the concentration of PCr decreases. The ATP concentration remains constant. G. J. GALLOWAY M. A. DENBOROUGH

Canbtrra REFERENCES

Hoult, D. I., Busby, S. J. W., Gadian, D. G., Radda, G. K., Richards, R. E., and Seeby, P. J. (1974). Observation of tissue metabolites using !1 PNMR. Naturt (Lowf.), 252,285. Okumura, F., Crocker, B. D., and Denborough, M. A. (1979). Identification of susceptibility to malignant hyperpyrezia in swine. Br. J. Anaath., 51,171. I.V. LABETALOL AND CORONARY ARTERY SURGERY

Sir,—With reference to the article "I. v. labetalol in the treatment of hypertension following coronary artery surgery" (Morel, ForsterandSuter, 1982), we would like to point out that, for a number of reasons this assessment of the value of i.v. labetalol must be viewed with circumspection. Labetalol i.v. has predominantly beta-blocking activity, the ratio of beta- to alpha-blockade being 7:1 (3:1 via the oral route). Droperidol and chlorpromazine, both of which have alphablocking properties were used concomitantly in these patients, thus augmenting, to an unknown degree, the weak alphaadrenergic blocking action of the labetalol. The haemodynamic variables measured in this study are insufficient to assess accurately the cardiovascular effects of the labetalol. The goal of haemodynamic manipulation is to optimize tissue perfusion. Arterial pressure ptru does not give an accurate indication of tissue perfusion and, in fact, arterial pressure may be greatest when tissue perfuuon is poorest. In patients with left ventricular dysfunction the measurement of left-tided tilling pressures, cardiac output and the calculation of systemic vascular TttitTancf are mandatory for the assessment and manipulation of haemodynamic variables. The measurement of CVP does not always express a reliable reflection of left-sided pressures, espedalry in patients with some left ventricular dysfunction. The fact that the CVP increased after the injection of labetalol suggests that left-sided pressures also increased, possibly more so than the CVP. The beta-receptor blockade probably accounted for this The preoperative rate - pressure product (RPP) cannot be com-

pared with the postoperative RPP. Many patients in the carry period after operation following coronary artery surgery have better haemodynamic fnn