ABSTRACTS FROM PURDUE CONFERENCE ON CPR

Maximum CPR blood flow is achieved at a duty cycle of 0.4; however, Plvtm continued to rise as the duty cycle de- creased, suggesting an increasing contribution to CPR blood flow by the LV stroke volume. Furthermore, the Pplmax increased as the duty cycle decreased.

Thus, it was concluded that the LV may act as a conduit with little volume change under conditions of long duty cy- cles, whereas the LV stroke volume may contribute the major portion of the CPR blood flow when the duty cycles are short. This occurs even in a system in which the pressure change surrounding the heart is not different from the pres- sure change surrounding the other thoracic vascular struc- tures.

Internal Ventricular Defibrillation: Lead Orientation and Twin Pulse Delivery. D. L. Jones, G. J. Klein. University of Western Ontario, London, Ontario N6A 5Cl.

Catheter electrode defibrillation in animals and human beings requires 10% to 25% of the normal transthoracic en- ergy. However, higher energy delivery may be accompanied by cardiac arrhythmias, pacing failure, and unsuccessful car- dioversion. In addition, energy delivery greater than 5 J is painful to patients. Therefore, alternate lead orientations and twin pulse delivery were examined in an attempt at reduction of energy delivery and enhancement of defibrillation suc- cess.

Fibrillation was induced in open-chest, halothane anes- thetized Yorkshire pigs weighing 18.9 ? 0.5 kg. Ten seconds after the onset of fibrillation, defibrillation was attempted by delivery of a single or a twin-pulse countershock from custom-designed trapezoidal external Defibrillation, at 65% tilt and fixed outputs (2.5, 5.0, 7.5, 10.0, 15.0, 20.0, 25.0, 35.0, and 50.0 J). All fibrillation bouts were terminated within 1 minute, and at least 10 minutes separated defibril- lation attempts. Defibrillation attempts were repeated until threshold was determined or 50 J failed to defibrillate.

An average of 2.32 * 0.16 J/kg passed through an in- dwelling catheter electrode (Medtronic 6880) defibrillated normal pig hearts. However, over 50% of animals could not be defibrillated with less than 50 J. When the countershock was passed between the right ventricular apical electrodes of the catheter and a constructed mesh electrode, located on the epicardium of high left ventricle, the energy required for defibrillation was reduced significantly to 1.26 ? 0.09 J/kg (P < O.Ol), and all animals could be successfully defibrillated with less than 50 J total energy. When twin pulses were delivered first to the catheter and subsequently to the right ventricular apical electrode paired with a mesh plaque elec- trode (Medtronic TX-7) on the high left epicardium, the en- ergy requirement was also reduced, the magnitude of the threshold energy being dependent on the separation time between the two countershocks: 1.96 + 0.23J/kg at 100 mil- lisecond separation, 1.51 2 O.l6J/kg at 10 millisecond sep- aration, and 0.93 +- O.llJ/kg at 1 millisecond separation. It was concluded that twin pulse delivery provides a reduction in energy necessary for defibrillation and also significantly improves the defibrillation success.

The Relation of Defibrillatory Waveform to Cardiac Damage. Mitchell Kase, David V. Woo, George Lumb,

Jacqueline Emrich, Gary J. Anderson. Hahnemann Uni- versity, Philadelphia, PA 19102.

The purpose of this study was to assess the potential for cardiac damage of two different defibrillatory waveforms at comparable energy levels. Twenty-four dogs were shocked with three consecutive transthoracic energy pulses of 10 JI kg given at 30-second intervals. Group A consisted of 13 dogs shocked with damped capacitive waveforms (10.1 t 0.2 J/kg); Group B consisted of 11 dogs shocked with a trun- cated trapezoidal waveform (9.6 2 1.0 J/kg; P = NS).

Damage was assessed by ECG changes, technetium 99m py- rophosphate (Tc-Pyp) imaging at either 24 hours (n = 18) or 72 hours (n = 6), and histopathological study immediately after Tc-Pyp imaging. In Group A, 8113 (61%) dogs had ven- tricular tachycardia (VT) immediately after the shock, and transient heart block occurred in lo/13 (77%); one (8%) had VT 24 hours later. All Group B dogs remained in sinus rhythm with no arrhythmias after shock. Visible lesions were observed in 12/13 Group A dogs and correlated with Tc-Pyp uptake in I l/12 (92%); in Group B, l/l 1 animals showed minimal Tc-Pyp uptake (P < 0.05). Technetium 99m pyrophosphate imaging substantiated localization of regions of damage: 11113 (85%) of Group A dogs had damaged right ventricular free walls, 7 (64%) of which were transmural as determined by gross and microscopic observation. Eight of 13 (61%) had lesions on the left ventricular anterior surface; 3 (37%) were transmural. Lesion/normal myocardial uptake of Tc-Pyp ranged from 2/l to 32/l (mean = 10.2 * 8.1/l, n = 19 samples) in Group A. In epicardial lesions, Tc-Pyp uptake averaged 4.2 compared with 15.8 (P < 0.05) in areas of transmural damage. Histopathoiogical examination by light and electron microscopy confirmed myocardial damage in the regions of increased Tc-Pyp uptake. Myocytolysis, histiocytic and leukocytic infiltration, contraction bands, and mitochondrial swelling with calcium deposits appeared prominently. It was concluded that significant differences exist in the frequency and severity of myocardial damage produced by these two clinically used waveforms when shock energy content is comparable.

Automated Detection of an Compensation for High Trans- thoracic Impedance in Defibrillation: Experimental and Clinical Studies. Richard E. Kerber, David McPherson, Robert Kieso, Pamela Hite, Francis Charbonnier. Univer- sity of Iowa, Iowa City, IA 52242.

Low-energy defibrillation has been advocated to reduce shock-induced myocardial damage. However, if transtho- racic impedance is high, the current generated by low-energy shocks may be inadequate to achieve defibrillation. Knowl- edge of transthoracic impedance in advance of attempts to defibrillate would permit more appropriate energy selection. The authors approached this problem in two ways. First, a method to determine transthoracic impedance before de5 brillation was validated in human beings A 31 KHz current was applied to the chest via defibrillator electrodes. The current flow was determined by the transthoracic imped- ance; the flow was monitored by a microprocessor, and the impedance was determined by calibration against known re- sistance values. By use of this technique, the predicted (preshock) impedance was compared with the actual imped-

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