Pediatr. Res. 16: 357-361 (1982) The Effects of Ouabain in Lambs with Depressed Myocardial Function WILLIAM BERMAN, JR.,'"' STEVEN M. YABEK, AND RAMONA SHORTENCARRIER University of New Mexico, School of Medicine, Department of Pediatrics, Cardiology Division, Albuquerque, New Mexico, USA Summary mesh pouch, sewn to the skin. Animals were treated postopera- tively with 50,000 units/kg/day of penicillin and 15 mg/kg/day The effects of ouabain infusion were tested in six lambs before of Kanamycin, given intramuscularly. Studies were performed 48 and after depression of myocardial function by halothane anesthe- h or more after surgery. sia. Halothane reduced the left ventricular rate of pressure rise (dp/dt), stroke work, and stroke volume; the ratio of preejection RESTING STUDIES period to left ventricular ejection time rose. Heart rate and systemic vascular resistance did not change. Before halothane, Baseline measurements. Resting measurements of hemodynamic ouabain infusion did not alter the hemodynamic variables mea- function were made while the lambs rested quietly, blind- sured. After myocardial depression, ouabain infusion returned dp/ folded in an open cage. Physiologic data were recorded on a dt, stroke work, stroke volume and the ratio of preejection period Beckman R-611 direct writing recorder at paper speeds of 2.5, 10, to left ventricular ejection time to control levels. Pacing studies 25, 100 and 200 mm/sec. Arterial pH, Pcoz, Pop and hematocrit showed a biphasic relationship between left ventricular dp/dt and were measured on each study day. Aortic and right atrial pressures heart rate. Maximal dp/dt occurred at a heart rate 42 beats/min were recorded with Statham P23DB strain gauges. The ECG was higher than the resting rate. recorded with an AC/DC coupler (Beckman 9806-A). Left ven- These studies suggest resting myocardial performance in the tricular pressure was recorded with the Millar catheter and used healthy, newborn lamb is at near maximal level. to generate its first derivative with respect to time (LV dp/dt) with a Beckrnan 9879 differentiating circuit, calibrated both internally and by application of an external triangular waveform. Heart rate The tolerance to and physiologic effects of cardiac glycosides was recorded from the left ventricular signal with a cardiota- differ in newborn and adult sheep (1,2). Studies have shown that chometer (Beckman 9857). Systolic time intervals, preejection age-related variations in the pharmacokinetics, metabolism, pro- period (PEP) and left ventricular ejection time (LVET), were tein binding or tissue penetration of digoxin do not explain the measured using the ECG, aortic and left ventricular tracings, as biologic differences documented (3-7). The differences in drug described previously (8). Cardiac output was measured using effect may relate to developmental variation in the effects of indocyanine green dye and an Electronics for Medicine DCCO- cardiac glycosides on the myocardial sodium-potassium ATP-ase 044 densitometer/output computer with internal standardization. (Na-K ATP-ase) system. Recently, however, evidence has accu- Dye, 1.25mg. was injected into the right atrium; arterial blood was mulated that the resting level of myocardial function in healthy withdrawn sterilely at a rate of 38.2 ml/min for 20-30 sec to assure newborn lambs is much higher than that in mature sheep (8-10). linearity of the downslope of the curve. Four output determina- The high resting level of heart rate and myocardial function in tions were made on each study, the withdrawn blood reinfused the normal, nonfailing neonatal lamb heart may make inotropic after each determination and the results averaged. Systemic vas- stimulation with cardiac glycosides difficult to document. cular resistance and stroke work were calculated from the appro- We tested the effects of ouabain infusion on the hemodynamic priate measured variables. function of newborn lambs. Drug effects were measured in undis- Ouabain infusion. After baseline measurements had been made, turbed, chronically instrumented, awake animals; studies were lambs were given 50 &kg of ouabain, IV, and maintained on an repeated after myocardial depression by halothane anesthesia. We infusion of 0.05 pg/kg/min of ouabain for 45 min. The loading also studied the contribution of heart rate to resting myocardial and maintenance doses were modified from those used in adult function in newborn lambs by testing the effect of atrial pacing on dogs (1 I) in order to produce arrhythmias in the study animals. the hemodynamic variables measured. Subsequently, hemodynarnic measurements were repeated. MATERIALS AND METHODS HALOTHANE STUDY ANIMAL PREPARATION Twenty-four or more h after resting studies, measurements were made again while animals were udisturbed in the open cage. Studies were performed on six lambs ranging in age from 5-26 Thereafter, halothane anesthesia (0.5-1%) was administered via days and in weight from 3.8-12.5 kg. Animals were prepared face mask for 20 min before repeating hemodynamic rneasure- surgically using 0.5% halothane anesthesia, administered by face ments. Halothane anesthesia was continued during the adminis- mask, and pentobarbital sedation (5 mg/kg, IV). Polyvinyl cath- tration of ouabain by the same technique used previously. Mea- eters (0.050 inches ID; 0.090 inches OD) were inserted into the surements were then repeated before discontinuation of halothane right atrium via the jugular vein and into the descending abdom- anesthesia and ouabain infusion. inal aorta via the femoral artery. A #5 French Millar catheter tip manometer was positioned in heleft ventricle via the left carotid PACING STUDIES artery. Three electrocardiographic (ECG) electrodes'were sewn to the chest wall. Catheters, wires and the Millar transducer cable Twenty-four or more h after the halothane study, each of the were tunnelled subcutaneously to the left flank and encased in a six animals underwent tracheostomy and halothane anesthesia 358 BERMAN ET AL. Table 1. Baseline hemodynamic data Systemic Right Cardiac vascular re- Aortic atrial output sistance Hema- mean mean Heart rate dp/dt (ml- Stroke Stroke (mm Hg- Animal Age Weight tocrit Potassium Calcium pressure pressure (beats. (mm Hg. -PEP min-I- volume work (mm min. kg. (No.) (days) (kg) (%) (meq. L-I) (mg.dl-I) (mm Hg) (mm Hg) min-I) sec-') LVET kg-') (ml) Hg.ml) ml-I) I 8 5.2 28 3.9 9.1 80 I 170 3000 0.312 387 11.8 944 0.204 2 10 4 3 1 4.0 8.9 79 2 135 4000 0.257 375 11.1 877 0.205 3 11 6.8 30 4.2 9.5 85 5 220 4400 0.282 444 13.7 1164 0.180 4 5 4.5 29 4.4 9.0 84 3 190 5400 0.355 406 9.6 806 0.200 5 26 13 34 4.0 9.6 85 4 185 3200 0.310 279 18.6 1666 0.290 6 18 9 32 4.1 9.1 88 5 195 3000 0.322 325 15 1320 0.255 - x 13 7.1 31 4.1 9.2 84 3 182 3833 0.306 369 13.3 1130 0.222 S.D. 7.7 3.4 2 0.2 0.3 3 2 28 958 0.034 59 3.2 325 0.041 Table 2. Change in hemodynamic variables relative to baseline were less than 2 wk old. The variation in heart rate, dp/dt, stroke measurements after ex~erimentalintervention volume and stroke work between animals was moderate; values Quabain infu- Halothane anes- Halothane and for mean aortic and right atrial pressure, PEP/LVET, cardiac sion thesia ouabain output per kg and systemic vascular resistance were more consist- ent. dp/dt (mm Hg.sec-I) 3975 (3.8)' 2758 (-27.21~ 3817 (0.4) The effects of experimental intervention on six variables reflec- Stroke volume (ml) 14.2 (5.4) 10.0(-25.6)2 13.6(1.0) tive of myocardial function are shown in Table 2. Because of the Stroke work (mm Hg. 1181 (5.4) 766 (-30. I)~ 1082 (-3.3) moderate variation in baseline values of dp/dt, stroke volume and ml) stroke work between animals, changes after intervention were PEP/LVET 0.280(-8.5) 0.41 1 (34.3)2 0.294(-3.9) examined by a two way analysis of variance. Heart rate (beats. min-I) 174(-4.4) 186 (1.6) I85 (1.6) Ouabain infusion caused no significant change in left ventric- Systemic vascular resist- 0.227 (3.3) 0.238 (7.5) 0.224 (0.9) ular dp/dt, stroke volume, stroke work, heart rate, systemic vas- ance (mm Hg.min. cular resistance or the PEP/LVET ratio from baseline values. kg. ml-I) Although 5 of 6 animals showed some arrhythmia during drug I Mean absolute value (mean % change from control). infusion (ventricular premature contractions in 3, junctional pre- Differs from control, by analysis of variance, P < 0.01. mature contractions in 2), physiologic measurements were made during periods of stable sinus rhythm. Hemodynamic data measured prior to halothane anesthesia did before right thoracotomy. Resting measurements were made be- not differ from measurements defined as baseline. Halothane fore further manipulation. Next, pacing electrodes were sewn to anesthesia depressed myocardial function, as evidenced by sig- the superior vena cava-right atrial junction and the right atrial nificant reductions in left ventricular dp/dt, stroke volume and lateral wall. The sinus node region was crushed with a hemostat stroke work, and caused a significant elevation in the PEP/LVET to lower resting heart rate. The pacing electrodes were connected ratio. Halothane anesthesia caused no consistent change in heart to a modified, electrically isolated pulse generator (Grass SD9) so rate or systemic vascular resistance, as reported previously (14, that impulses of varying rate, 2 msec in duration, and 2-7 mA in 15).