The effect of ischemia on ventricular fibrillation as measured by fractal dimension and frequency measures

INTRODUCTION

Most animal studies of ventricular fibrillation (VF) waveform characteristics involve healthy animals with VF initiated by electric shock. However, clinical VF is usually the result of ischemia. The waveform characteristics in these two types of VF may differ. The angular velocity (AV), frequency ratio (FR) and median frequency (MF) are three frequency-based measures of VF. The scaling exponent (ScE), the logarithm of the absolute correlations (LAC) and the Hurst exponent (HE) are three measures of the fractal dimension of VF.

HYPOTHESIS

We hypothesized that these quantitative measures would differ between ischemic and electrically initiated VF.

METHODS

VF was induced in 14 swine by electric shock and in 12 swine by ischemia. For ischemia induced VF animals, an angioplasty catheter was positioned in the mid-LAD and the balloon inflated. A mean of 891+/-608 (S.D.)s later, VF occurred. For electrically induced animals, an AC current was passed through a catheter in the RV. Following initiation by either method, VF was recorded for 7min. Sequential 5s epochs were analyzed for AV, FR, MF and fractal dimension measures.

RESULTS

Ischemic VF demonstrated a significantly higher fractal dimension as estimated by the ScE for the first 0-90s (p=0.021) and for 90-180s (p=0.016). The Hurst exponent was significantly higher for ischemic VF for both 0-90s (p<0.0001) and 90-180s (p<0.0001). The fractal dimension as estimated by the LAC method was not significantly different for 0-90s (p=0.056) but was highly significant for 90-180s (p=0.001). During the initial 90s the groups did differ in all measures of frequency as follows: AV (p<0.001), FR (p<0.001), MF (p<0.001). These differences did not persist beyond 90s except for a mild elevation of the FR after 270s (p<0.02).

CONCLUSION

Fractal based measures indicate an increase in the fractal dimension of ischemia induced VF for the first 180s when compared to electrically induced VF. Frequency-based measures uniformly demonstrate a pattern of higher frequencies for electrically induced VF for the first 90s. The increased fractal dimension and decreased frequencies associated with ischemia induced VF may reflect changes in the underlying myocardial physiology that can be used to guide therapies.

Is all ventricular fibrillation the same? A comparison of ischemically induced with electrically induced ventricular fibrillation in a porcine cardiac arrest and resuscitation model

OBJECTIVES

The standard porcine cardiac arrest model uses electrical induction of ventricular fibrillation. Reported restoration of spontaneous circulation and survival rates in this model are as high as 90% for ventricular fibrillation durations of 7-10 mins, values substantially greater than rates in the clinical population (i.e., 20% to 30%). A high first shock success rate, infrequent refibrillation, and short times for restoration of spontaneous circulation are typical of the model. The purpose of this study was to determine whether ischemic induction of ventricular fibrillation in swine followed by standard advanced cardiac life support would result in short-term outcomes approximating those observed in human victims of out-of-hospital ventricular fibrillation.

DESIGN

Randomized comparative trial.

SETTING

Translational research laboratory.

SUBJECTS

Domestic swine (n = 40, mean weight 40 +/- 4 kg, range 34-47 kg) of both genders.

INTERVENTIONS

Swine were instrumented and randomized to either electrical ventricular fibrillation induction or ischemic ventricular fibrillation, produced by balloon occlusion of the mid-left anterior descending coronary artery (n = 20 per group). Transthoracic impedance was measured and 30 Omega added in series for all animals. The balloon remained inflated during resuscitation efforts in ischemic ventricular fibrillation animals. After 7 mins of ventricular fibrillation, cardiopulmonary resuscitation was initiated and defibrillation was attempted 1 min later. Epinephrine and antiarrhythmics were administered as per guidelines. Resuscitation was terminated if restoration of spontaneous circulation had not occurred after 15 mins of advanced cardiac life support.

MEASUREMENTS AND MAIN RESULTS

Although the number of countershocks required to initially terminate ventricular fibrillation was not different (electrical ventricular fibrillation 1.9 +/- 1.6, ischemic ventricular fibrillation 2.4 +/- 2.0), the refibrillation rate was higher in the ischemic ventricular fibrillation group (4.9 +/- 4 vs. 0.8 +/- 1 episodes/animal, p < .001), resulting in a greater number of shocks before restoration of spontaneous circulation (total shocks for ischemic ventricular fibrillation 9.4 +/- 5.6 vs. electrical ventricular fibrillation 2.7 +/- 2.2, p < .001). Time to restoration of spontaneous circulation was longer in the ischemic ventricular fibrillation group (430 +/- 234 secs vs. 149 +/- 120 secs, p < .001). Restoration of spontaneous circulation rates were not different (electrical ventricular fibrillation 90% vs. ischemic ventricular fibrillation 65%). However, survival to 6 hrs was greater in the electrical ventricular fibrillation group (18 of 20, 90%) than in the ischemic ventricular fibrillation group (8 of 20, 40%, p = .002).

CONCLUSIONS

Resuscitation from ischemic ventricular fibrillation is more difficult than electrical ventricular fibrillation and is characterized by greater time to restoration of spontaneous circulation, frequent refibrillation, greater number of countershocks, higher epinephrine dose during resuscitation efforts, profound cardiac dysfunction, and a short-term survival rate approaching clinical experience. Ischemically induced ventricular fibrillation is a more clinically relevant model for the evaluation of resuscitation interventions.

A percutaneous catheter-based system for the measurement of potential gradients applicable to the study of transthoracic defibrillation

BACKGROUND

The local electric (E) field or potential gradient produced by a shock reliably predicts VF termination. In this study we evaluated a multiple electrode, catheter-based device for closed-chest 3D measurements of E field from transthoracic defibrillation shocks.

METHODS

Catheters with multiple electrodes on the tip were placed in intracardiac locations in anesthetized swine. An empirically derived calibration matrix and custom microprocessor was used to transform simultaneously measured voltages into orthogonal E field vector components. E fields produced in six intracardiac locations by 30 and 300 J shocks were compared in eight animals. Correlations were determined for measured current and E field at various shock strengths at two different transthoracic impedances in five additional animals. VF was induced in 12 animals and E field measured during defibrillation attempts.

RESULTS

The E field measurements resulting for 30 J transthoracic shocks were not significantly different among different intracardiac sites. At 300 J, however, significant differences were observed between sites with the greatest intensities recorded in the coronary sinus and right ventricle. Within animals, the variability of the measurement at each site was small, ranging from 2.8 +/- 1.6% to 5.7 +/- 4.5%. Significant correlations (P < 0.001) between measured E field and peak current were observed at native impedance (34 +/- 4 Omega, r = 0.81) and at adjusted impedance (76 +/- 4 Omega, r = 0.78) with transthoracic shocks of 200, 300, and 360 J. In VF studies, the probability of defibrillation was closely fit by a sigmoidal dose response curve in the coronary sinus E field with an approximate threshold of 4.7 V/cm with 50% defibrillation success at 9.3 V/cm.

CONCLUSIONS

The measured intracardiac E field variability within animals and at a specific site was small, exhibiting a median value of 5.1%, contrasted to median variabilities across animals of 5-11% suggesting the capacity of this measurement system to provide subject specific information on the distribution of E fields. The measured E field magnitudes across animals in the coronary sinus were linearly correlated with applied shock current with a very strong linear relation to effective shock voltage observed in vitro in a saline tank. When evaluated as a predictor of shock success, the observed values were consistent with previously reported critical fields. This technique may be of value in evaluating waveforms for transthoracic defibrillation as well as electrode size, placement, and composition.

A new device producing manual sternal compression with thoracic constraint for cardiopulmonary resuscitation

OBJECTIVE

Blood flow during conventional cardiopulmonary resuscitation (CPR) is usually less than adequate to sustain vital organ perfusion. A new chest compression device (LifeBelt) which compresses both the sternum and the lateral thoraces (compression and thoracic constraint) has been developed. The device is light weight, portable, manually powered and mechanically advantaged to minimize user fatigue. The purpose of this study was to evaluate the mechanism of blood flow with the device, determine the optimal compression force and compare the device to standard manual CPR in a swine arrest model.

METHODS

Following anesthesia and instrumentation, intravascular contrast injections were performed in four animals and the performance characteristics of the device were evaluated in eight animals. In a comparative outcome study, 42 anesthetized and instrumented swine were randomized to receive LifeBelt or manual CPR. Ventricular fibrillation (VF) was induced electrically and was untreated for 7.5 min. After 7.5 min, countershocks were administered and chest compressions initiated. Pulseless electrical activity (PEA) was observed after one to three shocks in all animals. CPR was continued until restoration of spontaneous circulation (ROSC) or for 10 min after the first shock. If ROSC had not occurred within 5 min of beginning CPR, 0.01 mg/kg of epinephrine (adrenaline) was administered. During CPR, peak systolic aortic pressure (Ao), diastolic coronary perfusion pressure (CPP-diastolic aortic minus diastolic right pressure) and end-tidal CO(2) were measured.

RESULTS

Angiographic studies demonstrated cardiac compression as the mechanism of blood flow. Optimal performance, determined by coronary perfusion pressure, was observed at a sternal force of 100-130 lb (45-59 kg). In the comparative trial, significant differences in the measured CPP were observed between LifeBelt and manual CPR both at 1 min (15+/-8 mmHg versus 10+/-6 mmHg, p<0.05) and 5 min (17+/-4 mmHg versus 13+/-7 mmHg, p<0.02) of chest compression. A greater (p<0.05) ETCO(2), a marker of cardiac output and systemic perfusion, was observed with LifeBelt CPR (20+/-7 mmHg) than with manual CPR (15+/-5 mmHg) at 1 min. Peak Ao pressures were not different between methods. With the device, 86% of animals were resuscitated compared to 76% in the manual group.

CONCLUSIONS

Blood flow with the LifeBelt device is primarily the result of cardiac compression. At a sternal force of 100-130 lb (45-59 kg), the device produces greater CPP than well-performed manual CPR during resuscitation from prolonged VF.

Post-resuscitation plasma catecholamines after prolonged arrest in a swine model

OBJECTIVE

A dramatic increase in plasma catecholamines has been demonstrated consistently following cardiac arrest and during CPR. The time course of this initial catecholamine surge after successful resuscitation has not been well studied. The purpose of this study was to measure plasma catecholamines after successful resuscitation and to determine their relationship to post-resuscitation hemodynamics.

METHODS

VF cardiac arrest was induced in eight anesthetized and instrumented swine. After 5 min of VF, conventional CPR was initiated followed 2 min later by transthoracic defibrillation. Restoration of spontaneous circulation (ROSC) was achieved in six animals. Following resuscitation, hemodynamic variables and plasma catecholamines were measured at intervals.

RESULTS

Myocardial contractility (peak systolic dP/dt), stoke volume, left ventricular stroke work (LVSW), and mean arterial pressure (MAP) were significantly decreased from pre-arrest values within 15 min of ROSC and remained depressed during 60 min of observation. Systemic vascular resistance (SVR) was significantly increased within 15 min and remained elevated. Significant negative correlations were observed between SVR and plasma epinephrine (adrenaline) (r=-0.72, p<0.001) and norepinephrine (noradrenaline) (r=-0.76, p<0.001). Significant negative correlations were also observed between MAP and these catecholamines. A negative correlation was also observed between norepinephrine and LVSW (r=-0.50, p=0.039). Catecholamine levels were not related to other indices of cardiac function.

CONCLUSIONS

A post-resuscitation adrenergic state is driven by a decline in MAP and PVR. Although seemingly compensatory, it may also contribute to the observed decline in cardiac function.

Prediction rules for estimating neurologic outcome following out-of-hospital cardiac arrest

BACKGROUND

No valid model has been developed to predict survival following out-of-hospital cardiac arrest. The purpose of this study was to develop a prediction model for meaningful survival following out-of-hospital cardiac arrest using variables available during resuscitation.

METHODS

This was a retrospective cohort study. Consecutive adult cardiac arrest patients were studied between 1994 and 2001. Variables included age, sex, race/ethnicity, arrest location, whether the arrest was witnessed, initial rhythm, whether CPR was performed, patient downtime, paramedic response time, survival to hospital discharge, and Glasgow Coma Score (GCS) at hospital discharge. Classification and Regression Tree analysis was used to develop decision rules to predict meaningful survival, as defined by the patient's discharge GCS.

RESULTS

Of the 754 patients, 16 (2%) survived with a GCS > or =13, 15 (2%) survived with a GCS = 14, and 5 (0.7%) survived with a GCS = 15. The decision rule for survival with a GCS > or = 13 incorporated whether the arrest was witnessed and the patient's age, resulting in a negative predictive value (NPV) of 99.8%. The rule for survival with a GCS > or = 14 incorporated the initial arrest rhythm, whether the arrest was witnessed, and the patient's age, resulting in a NPV of 99.6%. The rule for survival with a GCS = 15 incorporated only the interval between collapse and the initiation of life support, resulting in a NPV of 99.8%.

CONCLUSIONS

This study reports decision rules for potential meaningful survival following out-of-hospital cardiac arrest with high NPVs for each. Future studies need to be performed to prospectively validate these models.

Intracardiac voltage gradients during transthoracic defibrillation: implications for postshock myocardial injury

In-vitro studies indicate that the electric-field intensity, or voltage gradient (VG), generated by a defibrillation shock is a determinant of defibrillation success as well as potential shock-induced cardiac injury. It is not clear how common descriptors of shock dose, e.g., joules (J), relate to VGs.

OBJECTIVES

To assess the relationships between shock energy descriptors and VG.

METHODS

One monophasic and three biphasic waveforms were compared using transthoracic shocks and standard electrodes in five swine. VG measurements via intracavitary multielectrode-tipped catheters were compared with delivered energy and peak current. Shock variables were recorded at native transthoracic impedance and an adjusted impedance approximating that of typical humans.

RESULTS

For shocks at the same energy setting, peak current and VGs varied widely among the four defibrillators. At simulated human impedance (75 Omega), shocks at each device's maximum energy setting produced similar VGs among the three biphasic defibrillators, despite different delivered energies. For both native impedance (35 Omega) and at 75 Omega, VG correlated with peak current (r = 0.81 and 0.77, respectively) but not energy setting (r = 0.61, 0.52) or delivered energy (r = 0.58, 0.56). At the maximum energy setting of each device, maximum recorded VGs for both monophasic (33 V/cm) and biphasic (24 V/cm) defibrillators were less than those reported to cause myocardial injury (>60 to 80 V/cm).

CONCLUSIONS

Energy descriptors correlate poorly to actual shock intensities. When compared with reported VG thresholds of myocardial injury, this study suggests that risk of injury from critically strong VGs is low for all of these defibrillators and equivalent among tested biphasic waveforms.

Tumor necrosis factor-alpha is associated with early postresuscitation myocardial dysfunction

OBJECTIVE

Left ventricular dysfunction after successful cardiopulmonary resuscitation contributes to early death following resuscitation. The stress-induced proinflammatory cytokines, particularly tumor necrosis factor-alpha and interleukin-1beta, are known to depress myocardial function. We hypothesized that tumor necrosis factor-alpha and interleukin-1beta, synthesized and released in response to the stress of global ischemia accompanying cardiac arrest, play a role in development of postresuscitation left ventricular dysfunction.

METHODS

Hemodynamic variables, tumor necrosis factor-alpha , interleukin-1beta, interleukin-6 (enzyme-linked immunosorbent assay method), and ionized calcium were measured in ten anesthetized swine before and after 7 mins of cardiac arrest and during the early postresuscitation period (60-90 mins).

RESULTS

Tumor necrosis factor-alpha increased three-fold within 15 mins of restoration of circulation and remained elevated throughout the observation period. A significant negative correlation was observed between tumor necrosis factor-alpha and left ventricular systolic change in pressure over time (r = -.54, p <.001). Interleukin-1beta was undetectable before and after resuscitation, and interleukin-6 was detectable in only two animals after resuscitation. Although a significant decline in ionized calcium was observed and correlated with left ventricular systolic change in pressure over time, an independent role for ionized calcium in postresuscitation left ventricular dysfunction was not demonstrated.

CONCLUSION

Tumor necrosis factor-alpha increases during the early postresuscitation period and may play a role in postresuscitation myocardial dysfunction.

A model of ischemically induced ventricular fibrillation for comparison of fixed-dose and escalating-dose defibrillation strategies

OBJECTIVES

Fixed- and escalating-dose defibrillation protocols are both in clinical use. Clinical observations suggest that the probability of successful defibrillation is not constant across a population of patients with ventricular fibrillation (VF). Common animal models of electrically induced VF do not represent a clinical VF etiology or reproduce clinical heterogeneity in defibrillation probability. The authors hypothesized that a model of ischemically induced VF would exhibit heterogeneous defibrillation shock strength requirements and that an escalating-dose strategy would more effectively achieve prompt defibrillation.

METHODS

Forty-six swine were randomized to fixed, lower-energy (150 J) transthoracic shocks (group 1) or escalating, higher-energy (200 J-300 J-360 J) shocks (group 2). VF was induced by balloon occlusion of a coronary artery. After 1 or 5 minutes of VF, countershocks with a biphasic waveform were administered. The primary endpoint was successful defibrillation (termination of VF for 5 seconds) with < or =3 shocks.

RESULTS

VF was induced with occlusion or after reperfusion in 35 animals. Only five of 17 group 1 animals (29%, 95% CI = 10 to 56) could be defibrillated with < or =3 shocks; 15 of 18 group 2 animals (83%, 95% CI = 59 to 96) were defibrillated with < or =3 shocks (p < 0.002 vs. group 1). Nine of the group 1 animals (75%) that could not be defibrillated with 150-J shocks were rescued with < or =3 shocks ranging from 200 to 360 J.

CONCLUSIONS

In this ischemic VF animal model, defibrillation shock strength requirements varied among individuals, and when defibrillation was difficult, an escalating-dose strategy was more effective for prompt defibrillation than fixed, lower-energy shocks.

Milrinone Facilitates Resuscitation From Cardiac Arrest and Attenuates Postresuscitation Myocardial Dysfunction

Background— Left ventricular (LV) dysfunction with a low cardiac index after successful CPR contributes to early death attributable to multiorgan failure, and an effective treatment has not been identified. The purpose of this study was to investigate the use of milrinone, a selective phosphodiesterase III inhibitor, as treatment for LV dysfunction after resuscitation.

Methods and Results— Ventricular fibrillation (VF) was induced electrically in 32 swine. After 5 minutes of VF, CPR was initiated and animals were randomized to receive either saline (control group, n=16) as a bolus and infusion or milrinone 50 μg/kg as a bolus and then 0.5 μg/kg per min for 60 minutes (treatment group, n=16). After 2 minutes of CPR (total VF time, 7 minutes), countershocks were given. Coronary perfusion pressures during CPR were similar for the groups (24±2 versus 21±4 mm Hg). All animals were defibrillated; 6 of 16 control animals developed refractory postcountershock pulseless electrical activity compared with 0 of 16 treated animals ( P =0.018). At 30 minutes after restoration of spontaneous circulation, stroke volume (16±3 versus 26±7 mL, P <0.01) and LV dp/dt (793±197 versus 1108±316 mm Hg/s, P <0.02) were higher in the treatment group. Similar differences were observed 60 minutes after restoration of spontaneous circulation. Significant differences in heart rates between groups were not observed, and peripheral vascular resistance was significantly greater in the control group 30 and 60 minutes after resuscitation.

Conclusions— Milrinone facilitates resuscitation from prolonged VF and attenuates LV dysfunction after resuscitation without worsening major determinants of myocardial oxygen demand.

Is the ACLS score a valid prediction rule for survival after cardiac arrest?

The ACLS (advanced cardiac life support) Score was previously developed to predict survival from out-of-hospital cardiac arrest. Whether the arrest was witnessed, initial cardiac rhythm, performance of bystander cardiopulmonary resuscitation (CPR), and the response time of the paramedic unit were determined to be predictive of survival. However, the ACLS Score has not been validated in other emergency medical services systems.

OBJECTIVES

The purpose of this study was to externally validate the ACLS Score in one patient population.

METHODS

This was a retrospective cohort study performed at an urban county teaching hospital. The study population consisted of consecutive adult patients treated for out-of-hospital, nontraumatic cardiac arrest, and transported to the authors' institution between November 1, 1994, and September 30, 2001. Patient records for all cardiac arrests during the study period were reviewed. Study variables included witnessed arrest, initial arrest rhythm, bystander CPR, paramedic response time, and survival to hospital discharge. Predicted probability of survival to hospital discharge was calculated for each patient using the ACLS Score. The overall predicted and observed survival rates were compared using Flora's Z score. The Hosmer-Lemeshow test was used to evaluate the model's goodness-of-fit over a range of survival probabilities.

RESULTS

Of 754 cardiac arrest patients enrolled in the study period, 575 (76%) patients had documentation that allowed scoring using the ACLS Score. Twenty-five (4%) patients survived to hospital discharge. The predicted number of survivors based on the ACLS Score was 104 (18%), yielding a Flora's Z statistic of -4.46 (p < 0.0001). After categorizing predicted survival probabilities into four categories, the resulting Hosmer-Lemeshow statistic was 210 (p << 10(-6)). Both goodness-of-fit statistics demonstrated extremely poor fit of the model. A receiver operating characteristic (ROC) curve was created, yielding an area under the ROC curve of 0.33 (95% CI = 0.19 to 0.47), signifying extremely poor discrimination.

CONCLUSIONS

The previously published ACLS Score was not valid when applied to an external cohort of out-of-hospital cardiac arrest patients. An externally valid model is needed to predict survival to hospital discharge following out-of-hospital cardiac arrest.

Transthoracic impedance does not decrease with rapidly repeated countershocks in a swine cardiac arrest model

STUDY PURPOSE

Successful defibrillation is dependent upon the delivery of adequate electrical current to the myocardium. One of the major determinant of current flow is transthoracic impedance. Prior work has suggested that impedance falls with repeated shocks during sinus rhythm. The purpose of this study was to evaluate changes in transthoracic impedance with repeated defibrillation shocks in an animal model of cardiac arrest due to ventricular fibrillation (VF).

METHODS

VF was electrically induced in anesthetized swine. After 5 min of untreated VF, monophasic or biphasic waveform defibrillation was attempted using a standard sequence of 'stacked shocks' (200, 300, then 360 J, if necessary) administered via adhesive electrodes. If one of the first three shocks failed to convert VF, conventional CPR was initiated and defibrillation (360 J) attempted 1 min later. Strength-duration curves for delivered voltage and current were measured during each shock and transthoracic impedance calculated. Animals requiring a minimum of four shocks were selected for study inclusion. Impedance data from sequential shocks were analyzed using mixed linear models to account for the repeated-measures design and the variability of the initial impedance of individual animals.

RESULTS

Thirteen animals (monophasic waveform, n=7, biphasic waveform, n=6) required at least four shocks to terminate VF (range 4-6). Transthoracic impedance did not change from the first shock in the 13 animals (46+/-8 Omega) to the fourth shock (46+/-9 Omega). In animals receiving more than four shocks, transthoracic impedance likewise did not change significantly from the first to the last shock, which terminated VF. The lack of a significant change in impedance was also observed when animals were analyzed according to defibrillation waveform.

CONCLUSION

Transthoracic impedance does not change significantly with repeated shocks in a VF cardiac arrest model. This is likely due to the lack of reactive skin and soft tissue hyperemia and edema observed in non-arrest models.

Left ventricular function after monophasic and biphasic waveform defibrillation: the impact of cardiopulmonary resuscitation time on contractile indices

Previous work has suggested that low-energy biphasic waveform defibrillation (BWD) is followed by less post-resuscitation left ventricular (LV) dysfunction when compared with higher-energy monophasic waveform defibrillation (MWD). To the best of the authors' knowledge, the effect of cardiopulmonary resuscitation (CPR) duration and total ischemia time on LV function after countershock, controlling for waveform type, has not been evaluated.

OBJECTIVE

To determine the effect of CPR duration on LV function after MWD and BWD.

METHODS

VF was electrically induced in anesthetized and instrumented swine. After 5 minutes of VF, the animals were randomized to MWD (n = 22) or one of two BWDs (n = 46). If countershock terminated VF but was followed by a nonperfusing rhythm, conventional manual CPR without drug therapy was performed until restoration of spontaneous circulation (ROSC), defined as a systolic arterial pressure >60 mm Hg for 10 minutes without vasopressor support. Systolic LV pressure (LVP), LV dP/dt (first derivative of pressure measured over time), and cardiac output (CO) were measured at intervals for 60 minutes postresuscitation. CPR times (times to ROSC) and hemodynamic variables for the three groups were compared. Multivariable linear regression was performed to assess the contribution of defibrillation waveform, total joules, and CPR time on LVP, LV dP/dt, and CO at 15, 30, and 60 minutes postresuscitation.

RESULTS

When analyzed as groups, significant differences in median number of shocks to terminate VF, total joules, or CPR time were not observed between waveform groups. Regression analysis demonstrated that increasing CPR time was associated with a significant effect on indices of LV function at 15 and 30 minutes postresuscitation. Global LV function was not influenced by waveform type or total joules.

CONCLUSIONS

Adjustment for CPR time, a determinant of total myocardial ischemia time, is necessary when defibrillation waveforms are compared for their effect on postresuscitation cardiac function and short-term outcome.

Endotracheal drug administration during out-of-hospital resuscitation: where are the survivors?

BACKGROUND

Drugs administered endotracheally are effectively absorbed during normal spontaneous cardiac activity. However, animal cardiac arrest studies and limited clinical investigations do not support either the use of endotracheal (ET) drugs in doses currently recommended for adults or the method of direct endotracheal instillation. The purpose of this study was to compare the effect of intravenous (IV) and ET drug therapy on outcome from out-of-hospital cardiac arrest secondary to all cardiac arrest rhythms.

DESIGN

Five and one-half year retrospective cohort study.

SETTING

Municipal, university affiliated hospital.

PATIENTS

Consecutive patients >18 years of age in nontraumatic out-of-hospital cardiac arrest who received advanced cardiac life support (ACLS) medications by only the ET or IV route were included.

INTERVENTIONS

None.

RESULTS

Five hundred and ninety-six patients met inclusion criteria (IV drugs=495, ET drugs=101). There was no difference between groups in the rate of witnessed arrest and the frequency of bystander cardiopulmonary resuscitation (CPR). In the ET drug group, a significantly greater number of patients had an initial documented arrest rhythm of asystole compared to the IV drug group (56 vs 37%, P=0.01). The rate of return of spontaneous circulation (27 vs 15%, P=0.01) and survival to hospital admission rate (20 vs 9%, P=0.01) were significantly greater in the IV drug group. No patient who received ET drugs survived to hospital discharge compared to 5% of those receiving IV drugs (P=0.01).

CONCLUSION

For our out-of-hospital advanced rescuer system, ET drugs at recommended doses (twice the IV dose) injected into an ET tube during cardiac arrest and CPR were of no benefit.

Reconsideration of proximate Utstein-style end points

The Utstein-style template defines core and supplementary data for reporting out-of-hospital cardiac arrest information. The primary outcome statistic of the Utstein template is survival to hospital discharge (SHD). The SHD statistic is dependent on Utstein-defined out-of-hospital variables and multiple in-hospital variables that are undefined and uncontrolled. An example of one of these undefined in-hospital variables is the decision to place a patient on do-not-resuscitate status. At our municipal teaching hospital, 418 patients who had out-of-hospital cardiac arrest presented over a 4-yr period; 79 (19%; 95% confidence interval [CI], 15% to 23%) survived to hospital admission, with 54 (68%; 95% CI, 57% to 78%) subsequently being placed on do-not-resuscitate status. When patients on do-not-resuscitate status were included in the SHD calculation, the SHD rate was 5.3% (95% CI, 3.3% to 7.8%), and when patients on do-not-resuscitate status were excluded from the SHD calculation, the SHD rate was 6.1% (95% CI, 3.8% to 9.0%). These data show a relative 15% change in SHD resulting from a single in-hospital variable. Cardiac arrest survivors represent a small proportion of a total population; therefore, large numbers of study subjects are required for a statistically significant interpretation of the SHD statistic. This requirement for large study populations has resulted in recent studies that report results by using end points proximate to SHD when assessing the effect of individual interventions. It is logical that success of a specific intervention should be determined by the ability of the intervention to accomplish its purpose rather than the ability to improve SHD that is dependent on multiple variables. Furthermore, because in-hospital care is not standardized and uncontrolled variables exist, the primary Utstein end point of SHD should be reconsidered when evaluating cardiac arrest interventions.

Outcome of out-of-hospital postcountershock asystole and pulseless electrical activity versus primary asystole and pulseless electrical activity

OBJECTIVE

In the prehospital setting, countershock terminates ventricular fibrillation (VF) in about 80% of cases. However, countershock is most commonly followed by asystole or pulseless electrical activity (PEA). The consequences of such a countershock outcome have not been well studied. The purpose of this investigation was to compare the outcome of prehospital VF victims shocked into asystole or PEA with that of patients whose first documented rhythm was asystole or PEA (primary asystole or PEA).

DESIGN

Observational, retrospective study conducted over 5 yrs (1995-1999).

SETTING

A municipal hospital with a catchment area of >200,000.

PATIENTS

Consecutive adult patients with out-of-hospital nontraumatic cardiopulmonary arrest of cardiac origin. Patients found in VF who developed asystole or PEA after countershocks (group 1) and patients found in asystole or PEA (primary asystole or PEA) (group 2) were included if the reported downtime was <10 min.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Study end points included restoration of circulation (defined as a pulse for any duration), survival to hospital admission, and survival to hospital discharge. Ratios were determined, 95% confidence intervals were calculated, and observed differences were compared. For group 1 patients (n = 101), 61% of patients had a bystander-witnessed collapse and 34% received bystander cardiopulmonary resuscitation. For group 2 patients (n = 140), collapse was bystander witnessed in 71% and 45% received bystander cardiopulmonary resuscitation. These differences were not statistically significant. Restoration of circulation was significantly more frequent in group 2 than group 1 (42% vs. 16%, p <.001) as was survival to hospital admission (36% vs. 11%, p =.001). Survival to hospital discharge was greater in group 2 patients, but the difference failed to achieve statistical significance (10% vs. 3%, p =.062).

CONCLUSIONS

Countershock of prolonged VF followed by a nonperfusing rhythm has a worse prognosis than primary asystole or PEA and may be related to myocardial electrical injury.

The Utstein template and the effect of in-hospital decisions: the impact of do-not-attempt resuscitation status on survival to discharge statistics

BACKGROUND

Variables for reporting outcome of pre-hospital cardiac arrest have been delineated in the Utstein style template. The primary outcome statistic is survival to hospital discharge (SHD). The template allows comparisons of pre-hospital care systems and has been used to determine the benefit of pre-hospital interventions. Post-resuscitation care has not been standardized and in-hospital events that affect SHD are not considered in the template.

STUDY PURPOSE

To determine the frequency and timing with which do-not-attempt resuscitation (DNAR) status is conferred following resuscitation from pre-hospital cardiac arrest and to assess the impact of this action on SHD.

METHODS

A 4-year retrospective, observational cohort study of all adult patients successfully resuscitated from nontraumatic pre-hospital cardiac arrest and admitted to a single municipal teaching hospital. Study variables included age, witnessed arrest, bystander cardiopulmonary resuscitation (CPR), initial rhythm documented by paramedics, hospital admission rate, frequency and time at which DNAR status was conferred, and SHD.

RESULTS

Four hundred and eighteen adult patients experienced pre-hospital arrest and received standard advanced cardiac life support interventions during the study period. Seventy-nine patients (19%; 95% confidence interval (CI), 15-23%) survived to be admitted to the hospital. Fifty-four of these patients (68%; 96% CI, 57-78%) were subsequently placed in DNAR status. Only one of these patients had a living will or advanced directive prior to cardiopulmonary arrest. In 37 DNAR patients (68%; 95% CI, 54-81%), DNAR status was conferred within 24 h of hospital admission. For patients made DNAR within 24 h of admission, 38% had a witnessed arrest, 22% had ventricular fibrillation as the first documented arrest rhythm, and 29% received bystander CPR. When patients made DNAR are included in the calculation of SHD rate, the SHD rate for the study period was 5.3% (95% CI, 3.3-7.8%). If DNAR patients are excluded, the SHD was 6.1% (95% CI, 3.8-9.0%), representing a 15% increase in SHD rate.

CONCLUSION

In-hospital care and medical decision making are not considered in the Utstein template and can have a significant effect on reported survival statistics. When assessing the benefit of pre-hospital interventions, it may be preferable to consider survival to hospital admission as the primary outcome statistic until such time as post-resuscitation care after hospital admission is rigidly standardized.

Immediate countershock versus cardiopulmonary resuscitation before countershock in a 5-minute swine model of ventricular fibrillation arrest

STUDY OBJECTIVE

Prior laboratory and clinical studies demonstrate that cardiopulmonary resuscitation (CPR) preceding countershock of prolonged ventricular fibrillation (VF) increases the likelihood of successful cardiac resuscitation. The lower limit of VF duration at which time preshock CPR provides no benefit has not been specifically studied. The purpose of this study was to compare countershock and cardiac resuscitation outcome between immediate countershock of VF of 5-minute duration and CPR without drug therapy before countershock in a swine model.

METHODS

VF was induced in anesthetized and instrumented swine. After 5 minutes of VF, animals received 1 of 2 treatments. Animals in group 1, a "historical" control group (n=20), received immediate countershock followed by CPR and repeated shocks if needed. Group 2 animals (n=11) received CPR for 90 seconds preceding countershock, then continued CPR and repeated countershock if necessary. Drugs were not administered to either group, and resuscitation efforts were discontinued if a perfusing rhythm was not restored within 10 minutes of the first countershock. First shock success rate (defined as termination of VF), the number of shocks required to terminate VF, and the cardiac resuscitation rate were compared between groups.

RESULTS

The first shock terminated VF in 13 of 20 group 1 animals and 2 of 11 group 2 animals (P =.023). All but 1 animal in group 1 developed pulseless electrical activity after countershock. All but 1 animal in group 1 were eventually successfully resuscitated with CPR and repeated shocks if necessary. Four group 2 animals could not be resuscitated (P =.042).

CONCLUSION

Although effective in improving outcome of prolonged VF, CPR preceding countershock of VF of 5-minute duration does not improve the response to the first shock, decrease the incidence of postshock pulseless electrical activity, or the rate of return of circulation. In this study, CPR preceding countershock resulted in a significantly lower cardiac resuscitation rate.

Experimental studies on precordial compression or defibrillation as initial interventions for ventricular fibrillation

Countershock of prolonged ventricular fibrillation is usually followed by asystole or a nonperfusing rhythm. Data from three laboratory investigations indicate that administration of epinephrine and cardiopulmonary resuscitation (CPR) preceding countershock of prolonged ventricular fibrillation significantly improves cardiac resuscitation outcome compared with immediate countershock (relative risk reduction of failed resuscitation, 0.61). Preliminary investigations indicate that a similar improvement is not observed when the ventricular fibrillation period is of shorter duration, e.g., 5 mins. This time interval is probably at the lower limit at which CPR preceding shock of ventricular fibrillation provides benefit in terms of cardiac resuscitation. A single clinical trial of "CPR first" supports the use of a brief period of CPR before countershock of prolonged ventricular fibrillation. Additional trials with and without epinephrine are anticipated.

Transthoracic monophasic and biphasic defibrillation in a swine model: a comparison of efficacy, ST segment changes, and postshock hemodynamics

OBJECTIVE

Biphasic waveforms for transthoracic defibrillation (DF) have been tested extensively after brief (15 s) episodes of VF in animal models and in patients undergoing electrophysiologic testing. The purpose of this study was to compare the effects mono- and biphasic waveforms for DF on postdefibrillation ST segments and left ventricular pressure, markers of myocardial injury, after more extended periods of VF (30 and 90 s).

METHODS

21 anesthetized and instrumented swine were randomized to truncated exponential monophasic or biphasic waveform DF. VF was induced electrically and 30 s later, DF with the designated waveform was attempted with a shock dose of 200 J. If unsuccessful, 300 J and then 360 J were administered if necessary. Following return to control hemodynamic values and normalization of the surface ECG, VF was again induced and, after 90 s, DF was attempted as in the 30 s VF period. CPR was not performed during VF and each animal was countershocked with only one waveform for both VF episodes. Waveforms were compared for frequency of first shock defibrillation success, surface ECG indicators of myocardial injury (ST segment changes at 10, 20, and 30 s after countershock) and time to return to pre-VF hemodynamics after successful DF, an indicator of postshock ventricular function.

RESULTS

Successful first shock conversion rates at 30 and 90 s were 60 and 63% for monophasic and 64 and 82% for biphasic (NS). Biphasic DF after 30 s produced ST segment changes (measured 10 s after DF) in 1/10 animals while six of eight animals in the monophasic group showed ST segment changes (P=0.013). After 90 s of VF, ST segment changes were observed in 6/8 in the monophasic group and 2/10 in the biphasic group (P=0.054). Differences in the time to hemodynamic recovery (return to control peak left ventricular pressure) were not observed between biphasic and monophasic waveforms after 30 or 90 s of VF.

CONCLUSIONS

Monophasic and biphasic transthoracic defibrillation are equally effective in terminating VF of 30 and 90 s duration and restoring a perfusing rhythm. The biphasic waveform produced less ECG evidence of transient myocardial injury. However, there was no difference in the rate of return to control hemodynamics. ST segment changes following countershock of VF of brief duration are transient and of questionable significance.

Monophasic versus biphasic transthoracic countershock after prolonged ventricular fibrillation in a swine model

OBJECTIVE

We sought to compare the defibrillation efficacy of a low-energy biphasic truncated exponential (BTE) waveform and a conventional higher-energy monophasic truncated exponential (MTE) waveform after prolonged ventricular fibrillation (VF).

BACKGROUND

Low energy biphasic countershocks have been shown to be effective after brief episodes of VF (15 to 30 s) and to produce few postshock electrocardiogram abnormalities.

METHODS

Swine were randomized to MTE (n = 18) or BTE (n = 20) after 5 min of VF. The first MTE shock dose was 200 J, and first BTE dose 150 J. If required, up to two additional shocks were administered (300, 360 J MTE; 150, 150 J BTE). If VF persisted manual cardiopulmonary resuscitation (CPR) was begun, and shocks were administered until VF was terminated. Successful defibrillation was defined as termination of VF regardless of postshock rhythm. If countershock terminated VF but was followed by a nonperfusing rhythm, CPR was performed until a perfusing rhythm developed. Arterial pressure, left ventricular (LV) pressure, first derivative of LV pressure and cardiac output were measured at intervals for 60 min postresuscitation.

RESULTS

The odds ratio of first-shock success with BTE versus MTE was 0.67 (p = 0.55). The rate of termination of VF with the second or third shocks was similar between groups, as was the incidence of postshock pulseless electrical activity (15/18 MTE, 18/20 BTE) and CPR time for those animals that were resuscitated. Hemodynamic variables were not significantly different between groups at 15, 30 and 60 min after resuscitation.

CONCLUSIONS

Monophasic and biphasic waveforms were equally effective in terminating prolonged VF with the first shock, and there was no apparent clinical disadvantage of subsequent low-energy biphasic shocks compared with progressive energy monophasic shocks. Lower-energy shocks were not associated with less postresuscitation myocardial dysfunction.