Willis A. Tacker

Myocardial perfusion pressure: A predictor of 24-hour survival during prolonged cardiac arrest in dogs

Myocardial perfusion pressure, defined as the aortic diastolic pressure minus the right atrial diastolic pressure, correlates with coronary blood flow during cardiopulmonary resuscitation (CPR) and predicts initial resuscitation success. Whether this hemodynamic parameter can predict 24-h survival is not known. We examined the relationship between myocardial perfusion pressure and 24-h survival in 60 dogs that underwent prolonged (20 min) ventricular fibrillation and CPR. Forty-two (70%) animals were initially resuscitated and 20 (33%) survived for 24 h. Myocardial perfusion pressure was significantly greater when measured at 5, 10, 15 and 20 min of ventricular fibrillation in the resuscitated animals than in the non-resuscitated animals (P less than 0.01). Likewise, the myocardial perfusion pressure was also greater in the animals that survived 24 h than in animals that were resuscitated, but died before 24 h (P less than 0.02). Myocardial perfusion pressure measured after 10 min of CPR was 11 +/- 2 mmHg in animals never resuscitated, 20 +/- 3 mmHg in those resuscitated that died before 24 h and 29 +/- 2 mmHg in those that survived 24 h (P less than 0.05). A myocardial perfusion pressure at 10 min of CPR of 20 mmHg or less is an excellent predictor of poor survival (negative predictive value = 96%). Myocardial perfusion pressure is a useful index of CPR effectiveness and therefore may be a useful guide in helping to optimize resuscitation efforts.

Comparison of the resistance to infection of intestinal submucosa arterial autografts versus polytetrafluoroethylene arterial prostheses in a dog model

PURPOSE Prosthetic graft infection represents a most challenging complication to the vascular surgeon. Although expanded polytetrafluoroethylene (ePTFE) grafts have an acceptable patency rate, especially in the large-diameter arterial location, bacterial contamination of this material usually requires surgical removal of the graft. METHODS We compared the resistance of large-diameter ePTFE grafts and grafts constructed of small intestinal submucosa (SIS) to deliberate infection with Staphylococcus aureus. Eighteen dogs were divided into two equal groups, and the infrarenal aorta was replaced with either ePTFE or SIS graft material. One hundred million S. aureus organisms were deposited directly on the graft at the time of surgery, and the dogs were observed for 30 days. RESULTS One dog with an ePTFE graft died of hemorrhage from anastomosis site at 21 days. Of the remaining eight dogs with ePTFE grafts, four had positive culture results from the removed graft material, and all had histologic evidence for persistent infection. These dogs also had chronic fever, and the average white blood cell count at day 30 was 15,600/mm3. All nine dogs with SIS grafts had patent grafts, were afebrile after the first week, had an average white blood cell count of 11,500/mm3 at 30 days (p value = NS), had negative culture results, and had the histologic appearance of graft remodeling with collagen that was free of active inflammation. CONCLUSIONS We conclude that large-diameter arterial SIS grafts are more resistant to persistent infection with S. aureus than ePTFE grafts in this dog model of deliberate bacterial inoculation.

Internal cardiac defibrillation in man: pronounced improvement with sequential pulse delivery to two different lead orientations.

Wider applicability of an implantable automatic defibrillator depends on achieving internal cardiac defibrillation consistently with the lowest possible energy. In animal studies, we have found that the cardiac defibrillation threshold could be reduced when sequential shocks separated in time and spacially arranged were delivered to the heart. We compared internal cardiac defibrillation using a single pulse shock delivered through an intravascular catheter with this new method for internal cardiac defibrillation in patients undergoing cardiac surgery for the correction of arrhythmias. For the single pulse shock and the first pulse of the sequential pulse shock, current was passed through an intravascular catheter with the catheter cathode at the apex of the right ventricle and the anode at the superior vena cava-atrial junction region. The second pulse of the sequential pulse countershock was delivered between the catheter cathode in the right ventricular apex and an oval plaque electrode secured on the laterobasal left ventricular epicardium as anode. With the single pulse alone for shock delivery, 12 patients could be defibrillated with an average of 20.1 +/- 16.8 J, with a corresponding leading-edge peak voltage and current of 836 +/- 319 V and 9.4 +/- 4.5 A, respectively. However, two of the patients could not be defibrillated with energies below 50 J. With the sequential pulse shock delivery, a significant reduction in all values were recorded. Mean total energy for defibrillation averaged 7.7 +/- 6.0 J. Leading-edge peak voltage and current from the catheter averaged 430 +/- 148 V and 5.0 +/- 2.8 A, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Endotracheal versus intravenous epinephrine during electromechanical dissociation with CPR in dogs.

The dose-response curves of epinephrine given either IV or endotracheally (ET) were compared during resuscitation from electromechanical dissociation (EMD). Ten anesthetized dogs were subjected to a two-minute period of electrically induced ventricular fibrillation (VF) followed by defibrillation without CPR to produce EMD. Mechanical CPR was followed by injection of either ET or IV epinephrine. Successful response was defined as a return of pulsatile blood pressure within two minutes of drug administration. Using log-dose increments of epinephrine, experimental trials were repeated in each animal. The IV and ET median effective doses were 14 and 130 micrograms/kg, respectively. When the trials were successful, the time between drug administration and either arterial blood pressure increases or return of spontaneous circulation did not differ significantly for the ET and IV groups. These results show that the dosage for epinephrine delivered ET must be higher than the IV dosage to achieve the same response during CPR.

Regional blood flow during cardiopulmonary resuscitation in dogs

To determine differences in regional blood flow during CPR versus normal cardiac function, the authors measured regional blood flow to several organs in 19 pentobarbital-anesthetized dogs (6–12 kg). Regional blood flow was measured during sinus rhythm in five dogs and during electrically induced ventricular fibrillation with CPR in the other 14 dogs. Regional blood flow and cardiac output were measured using radioactively labeled polystyrene microspheres of 15 ± 3 μ diameter, injected into the left ventricle. Adequacy of microsphere mixing at low cardiac outputs was verified by comparing flow rates to paired organs. Cardiac output was 175 ml/kgμmin during sinus rhythm versus 47 ml/kgμmin during CPR. Flow to all organs sampled was less during CPR, but the relative decrease varied widely. The ratios of regional blood flow during CPR to regional blood flow during sinus rhythm were 90% for brain, 35% for heart, 15% for kidneys, 17% for adrenal glands, 14% for pancreas, 3% for spleen, and 33% for small intestine. These results provide baseline values for regional blood flow during CPR which can be used to evaluate alternative CPR techniques and/or drugs which may improve perfusion of vital organs during CPR.

Elevation of ventricular defibrillation threshold in dogs by antiarrhythmic drugs

Effects of antiarrhythmic drugs upon the threshold delivered energy (TDE) and threshold peak current (TPC) for electrical ventricular defibrillation by damped sinusoidal shocks were investigated in 25 pentobarbital-anesthetized dogs. TDE and TPC were increased by the three antiarrhytmic drugs tested. Bolus injections produced a transient rise and continuous infusions produced a steady rise in difibrillation threshold. The maximal percent elevations in mean defibrillation threshold during the 60 minutes after intravenous drug treatment in groups of n = 5 dogs were: (formula: see text). Accordingly, individuals receiving antiarrhythmic drugs who nonetheless fibrillate may require greater electric shock strength for defibrillation.

Changes in expired end-tidal carbon dioxide during cardiopulmonary resuscitation in dogs: A prognostic guide for resuscitation efforts

Expired end-tidal carbon dioxide (PCO2) measurements made during cardiopulmonary resuscitation have correlated with cardiac output and coronary perfusion pressure when wide ranges of blood flow are included. The utility of such measurements for predicting resuscitation outcome during the low flow state associated with closed chest cardiopulmonary resuscitation remains uncertain. Expired end-tidal PCO2 and coronary perfusion pressures were measured in 15 mongrel dogs undergoing 15 min of closed chest cardiopulmonary resuscitation after a 3 min period of untreated ventricular fibrillation. In six successfully resuscitated dogs, the mean expired end-tidal PCO2 was significantly higher than that in nine nonresuscitated dogs only after 14 min of cardiopulmonary resuscitation (6.2 +/- 1.2 versus 3.4 +/- 0.8 mm Hg; p less than 0.05). No differences in expired end-tidal PCO2 values were found at 2, 7 or 12 min of cardiopulmonary resuscitation. A significant decline in end-tidal PCO2 levels during the resuscitation effort was seen in the nonresuscitated group (from 6.3 +/- 0.8 to 3.4 +/- 0.8 mm Hg; p less than 0.05); the successfully resuscitated group had constant PCO2 levels throughout the 15 min of cardiac arrest (from 6.8 +/- 1.1 to 6.2 +/- 1.2 mm Hg). Changes in expired PCO2 levels during cardiopulmonary resuscitation may be a useful noninvasive predictor of successful resuscitation and survival from cardiac arrest.

Long-term survival with open-chest cardiac massage after ineffective closed-chest compression in a canine preparation.

The ultimate goal of cardiopulmonary resuscitation (CPR) is long-term, neurologically intact survival. This study examined whether open-chest cardiac massage could improve 7 day survival and neurologic function when instituted after the failure of standard closed-chest compression CPR. Twenty-nine mongrel dogs were anesthetized and then instrumented with catheters to monitor right atrial and ascending aortic pressures. Ventricular fibrillation was induced and after 3 min standard CPR was begun. Standard CPR was performed with a Thumper programmed for 2 inch chest compressions at 60/min with a 50% duty cycle. External defibrillation was attempted twice after 15 min of ventricular fibrillation. Unsuccessfully defibrillated animals were randomly assigned to either an additional 2 min of continued closed-chest compressions, or 2 min of open-chest cardiac massage. All animals underwent a period of advanced cardiac life support and were followed until they were resuscitated or died. Follow-up care, including scoring of neurologic deficit, was performed for 7 days. In dogs receiving open-chest cardiac massage there was significantly more immediate resuscitation success (14/14 vs 5/14; p less than .005), 24 hr survival (12/14 vs 4/14; p less than .005), and 7 day survival (11/14 vs 4/14; p less than .02) than in those receiving continued closed-chest compression. Open-chest cardiac massage significantly improved long-term outcome when instituted after 15 min of ineffective closed-chest compression.

The effect of newer antiarrhythmic drugs on defibrillation threshold.

This study was conducted to determine the effects of clofilium phosphate and bretylium tosylate on ventricular defibrillation threshold. Dogs were anesthetized with pentobarbital and subjected to repeated fibrillation-defibrillation episodes. Defibrillation thresholds were determined at 15-min intervals, using underdamped 5–6 msec sinusoidal current shocks, from 30 min before drug injection to 120 min after injection. Eight dogs were given clofilium phosphate (0.34 mg/kg, iv). Another 10 dogs were given bretylium tosylate (10.0 mg/kg. iv). Both drugs lowered defibrillation threshold from 15–90 min after injection. The maximum clofilium effect was a 31% decrease in threshold current and a 54% decrease in threshold energy. The greatest decrease in defibrillation threshold produced by bretylium was 16% for current and 31% for energy. These drug induced changes in defibrillation threshold are of potential clinical benefit if they occur in human subjects at doses which are effective for control of ventricular arrhythmias.

Bipolar catheter defibrillation in dogs using trapezoidal waveforms of various tilts

The choice of defibrillating waveform is critical in determining the size, battery life, and effectiveness of an automatic implantable defibrillator (AID). The trapezoidal (truncated exponential) waveform is well suited for the AID and its use can be optimized by the selection of appropriate values of pulse duration and tilt. The purpose of this study was to determine the dependence of the threshold peak current (the minimum peak current necessary to defibrillate the ventricles) on pulse duration and tilt for a bipolar catheter electrode configuration. Successive fibrillation-defibrillation trials were performed in 30 dogs anesthetized with sodium pentobarbital (30 mg/kg). The defibrillating pulse was applied via a bipolar-electrode catheter positioned such that the electrodes were located in the right ventricle at the apex and in the superior vena cava. The threshold peak current was determined in each dog for trapezoidal waveforms with 80%, 65%, 50%, and less than 5% tilt and with pulse durations of 2, 5, 10, 15, and 20 milliseconds. From a total of 600 threshold peak-current values, a strength-duration curve was derived for each value of tilt. The threshold peak current dose (peak current divided by body weight) increased with increasing tilt and decreasing duration. The threshold average current dose (average current over the duration of the defibrillating pulse divided by body weight) was IAV = 0.26 + 0.47/d, where d is the pulse duration in milliseconds and IAV is the average current in amperes per kilogram. If catheter apparent impedance is known, the minimum capacitance and output voltage necessary for defibrillation can be inferred from the strength-duration curves. From these data one can quantitatively assess the effect of trapezoidal waveform shape on the design criteria for the AID.