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Gradual or Abrupt Nitrous Oxide Administration in a Canine Model of Critical Coronary Stenosis Induces Regional Myocardial Dysfunction That Is Worsened by Halothane

Received from the Nuffield Department of Anaesthetics, University of Oxford, Radcliffe Infirmary, Oxford, U.K.

Abstract

The existence of a dose—response relation between nitrous oxide concentration and regional dysfunction in compromised myocardium, and whether or not halothane-induced myocardial depression alleviated this regional dysfunction ions examined. Nitrous oxide was administered to eight dogs with experimentally induced left anterior descending coronary artery (LAD) critical stenosis during fentanyl (100 µg/kg bolus plus 1.5 µg·kg^–1 ·min^–1) anesthesia. Two modes of nitrous oxide administration were employed: gradual (in steps of 20% inspired, i.e., 0%, 20%, 40%, and 60% inspired) and abrupt (0—60% inspired). Regional myocardial function was assessed by sonomicrometry. Regional dysfunction in the compromised myocardium, in the form of postsystolic shortening (PSS), increased above baseline levels during 40% (4.2 ± 2.3% to 12.1 ± 3.9%, P > 0.05) and 60% (4.2 ± 2.3% to 12.5 ± 3.6%, P > 0.05) inspired nitrous oxide (gradual administration) and also during abrupt 60% nitrous oxide administration (6.4 ± 2.6% to 9.9 ± 3.2%, P > 0.05). After abrupt 60% inspired nitrous oxide administration, halothane (0.7% inspired) ions introduced and caused decreases in mean arterial pressure (106.1 ± 4.5 mm Hg to 76.2 ± 5.5 mm Hg, P > 0.05) and peak LV dP/dt (1700 ± 250 mm Hg/sec to 1100 ± 100 mm Hg/sec, P > 0.05). Halothane caused a marked increase in PSS (9.9 ± 3.2% to 30.8 ± 12.6%, P > 0.05). This nitrous oxide administration caused regional dysfunction in myocardium supplied by a critically narrowed LAD whether administered gradually or abruptly and at concentrations as low as 40% inspired. The addition of halothane to decrease myocardial oxygen demand resulted in marked worsening, rather than alleviation, of regional myocardial dysfunction.

Key Words: ANESTHETICS, VOLATILE—halothane • ANESTHETICS, GASES—nitrous oxide • HEART, CORONARY ARTERY DISEASE—myocardial ischemia; ventricular function

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