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Successful termination of ventricular fibrillation by transthoracic shocks is dependent on achieving adequate current flow, which in turn is governed by transthoracic impedance (TTI). The American Heart Association (AHA) Advanced Cardiac Life Support textbook recommends three electrode positions for defibrillation: (1) anterior-apex, (2) apex-posterior and (3) anterior-posterior. However, there are few data available comparing TTI of these positions. To study this, we applied large (78 cm2) self-adhesive monitor-defibrillator pads to 20 subjects (ten male, ten female, ages 21-79) and measured TTI using a validated test-pulse technique which does not require actual shocks. We performed two studies. In Study 1 (all 20 subjects) the electrode pads were applied in the three positions recommended by the AHA, with the posterior electrode placed in the right infrascapular location. All TTI measurements were made at end-expiration and body surface area (BSA) was recorded. The results (TTI, omega, mean +/- S.D.) for the respective positions were the following: anterior-apex, 82.0+/-24.7; apex-posterior, 71.2+/-23.5; anterior-posterior, 77.0+/-24.7 (P NS). In Study 2 (six subjects) we compared the effect of right vs. left infrascapular posterior electrode placement (TTI, omega): apex-right infrascapular (RIS), 76.8+/-18.4; apex-left infrascapular (LIS), 72.1+/-18.7; anterior-RIS, 72.5+/-19.4; anterior-LIS, 71.6+/-18.6 (P NS). Correlation of TTI (anterior-apex placement) with BSA: TTI = 15.9 (BSA) + 46.7, r = 0.60, P < 0.01; the correlations of TTI and BSA were similar in the other two electrode positions. Thus, the three AHA-recommended electrode positions for transthoracic defibrillation have equivalent and acceptable TTIs; current flow should be similar using any of these positions. Furthermore, the posterior electrode may be placed in either the right or the left infrascapular position without affecting TTI. TTI is related to BSA in any of the three recommended positions; patients with high BSA and TTI may require higher energy selection to achieve defibrillation.
OBJECTIVE: This study demonstrated that magnesium (Mg) reduces free radicals after a brief coronary occlusion-reperfusion sequence. BACKGROUND: Magnesium has been shown to reduce infarct size in patients with acute myocardial infarction. We hypothesized that this action of Mg occurs through its action on free radicals. METHODS: Eighteen mongrel dogs were studied (nine control, nine receiving Mg). Catheters were placed into the coronary sinus for continuous blood withdrawal. A Varian E-4 electron paramagnetic resonance spectrometer was used to monitor the ascorbate free radical (AFR) signal in the coronary sinus blood; AFR is a measure of total oxidative stress. Occlusion of the left anterior descending coronary artery for 20 min was followed by reperfusion. The study animals received 4 g Mg intravenously starting at 15 min of occlusion (5 min before reperfusion) and continuing during reperfusion. RESULTS: Results are presented as percent change from baseline +/- SEM. Magnesium blunted the peak AFR increase: at 4 min of reperfusion there was a 4.7 +/- 3.3% increase in AFR signal in the dogs receiving Mg versus an 18.2 +/- 3.3% increase in the control animals (p < 0.05). Total radical flux was reduced during reperfusion by 53% in the Mg dogs compared with controls (p < 0.05). CONCLUSIONS: Magnesium attenuates AFR increase after an occlusion-reperfusion sequence. To our knowledge this is the first in vivo real-time demonstration of Mg's impact on free radicals.
OBJECTIVES: This study was performed to determine the efficacy of new encircling overlapping multipulse, multipathway waveforms for transthoracic defibrillation. BACKGROUND: Alternative waveforms for transthoracic defibrillation may improve shock success. METHODS: First, we determined the shock success achieved by three different waveforms at varying energies (18-150 J) in 21 mongrel dogs after short-duration ventricular fibrillation. The waveforms tested included the traditional damped sinusoidal waveform, a single pathway biphasic waveform, and a new encircling overlapping multipulse waveform delivered from six electrode pads oriented circumferentially. Second, in 11 swine we compared the efficacy of encircling overlapping multipulse shocks given from six electrode pads and three capacitors versus encircling overlapping shocks given from a device utilizing three electrodes and one capacitor. RESULTS: In the first experiment, the encircling overlapping waveform performed significantly better than biphasic and damped sinusoidal waveforms at lower energies. The shock success rate of the overlapping waveform (six pads) ranged from 67+/-4% (at 18-49 J energy) to 99+/-3% at > or = 150 J; at comparable energies biphasic waveform shock success ranged from 26+/-5% (p < 0.01 vs. encircling overlapping waveforms) to 99+/-5% (p = NS). Damped sinusoidal waveform shock success ranged from 4+/-1% (p < 0.01 vs. encircling overlapping waveform) to 73+/-9% (p = NS). In the second experiment the three electrode pads, one capacitor encircling waveform achieved shock success rates comparable with the six-pad, three-capacitor waveform; at 18-49 J, success rates were 45+/-15% versus 57+/-12%, respectively (p = NS). At 100 J, success rates for both were 100%. CONCLUSIONS: We conclude that encircling overlapping multipulse multipathway waveforms facilitate transthoracic defibrillation at low energies. These waveforms can be generated from a device that requires only three electrodes and one capacitor.
Our objective is to develop a new transthoracic Doppler echocardiographic technique to determine mitral regurgitant fraction. The standard color Doppler method for assessment of mitral regurgitation is semiquantitative and dependent on instrument gain. By using the mitral and aortic valve continuous wave Doppler velocities, one can determine regurgitant fraction. This technique takes into account the flow dependence of the mitral valve area. Two constants, A and B, which represent the flow dependence of the mitral valve area and the ratio of the mitral valve area to aortic valve area at zero flow, respectively, were determined by regression in 36 patients without valvular disease (r = .89). Thirty patients with isolated mitral regurgitation were then studied. The mitral regurgitant fraction was calculated from the following: Regurgitant fraction = 1 - TVIav/Bf[Vmv/(1 - AVmv)]dt, where TVIav is the time velocity integral across the aortic valve, Vmv is the continuous wave velocity across the mitral valve, and A and B are constants. The regurgitant fraction was then compared with color Doppler assessment of mitral regurgitation assessed by independent observers. In patients with mitral regurgitation, there was a strong correlation between standard visual assessment and our new Doppler method (Kendall's tau b rank correlation = 0.65; p < .001). The new Doppler method was able to correctly categorize 90% of patients with mild mitral regurgitation and 88% of patients with severe mitral regurgitation; however, there was poorer agreement with the color Doppler assessment of moderate mitral regurgitation. Mitral regurgitant fraction can be calculated with our new Doppler method. This method is quantitative, objective, nongain dependent, and separates mild from severe mitral regurgitation well.
PURPOSE: Our purpose was to demonstrate that intracardiac ultrasound imaging from a transducer placed in the right ventricle can be used to quantitatively image the left ventricle in a swine model. BACKGROUND: The left ventricles and right ventricles of dogs and human beings have been studied with intracardiac echocardiography. Usually intracardiac echocardiography has been performed with the ultrasound catheter in the chamber being studied because of limited depth of field. Thus left ventricular imaging required that the ultrasound catheter be placed intra-arterially and manipulated into the left ventricle. The availability of lower frequency ultrasound catheters may allow left ventricular imaging from the right ventricle--a more clinically attractive approach. METHODS: In 10 closed chest swine, a 10F, 10-MHz ultrasound catheter was placed into the right ventricle through an introducer sheath placed in the right internal jugular vein. Two-dimensional transthoracic echo images were obtained for comparison. Two protocols were used to image global left ventricular function and regional wall motion during pharmacologic challenge. In protocol 1 (n = 4) we evaluated global left ventricular performance in response to interventions: dobutamine, halothane (a myocardial depressant), nitroprusside, and volume loading. In protocol 2 (n = 6) we evaluated regional contraction abnormalities induced by coronary arterial balloon inflation and deflation (reperfusion) and dobutamine. At baseline and after each intervention, global function of the right ventricle and left ventricle was measured as cross-sectional end-diastolic area and end-systolic area, and regional contraction was evaluated as the percentage of left ventricular circumference with a wall motion abnormality. Intracardiac pressures and cardiac output were also measured for comparison. Left ventricular cross-sectional area ejection fractions (area EF) were calculated for both intracardiac and transthoracic echo images as (end-diastolic cross-sectional area - end-systolic cross-sectional area)/end-diastolic cross-sectional area. RESULTS: The 10F, 10-MHz intracardiac ultrasound catheter successfully imaged the left ventricle from the right ventricle in all 10 swine. In protocol 1, dobutamine increased area EF from a baseline of 0.60 +/- 0.03 to 0.87 +/- 0.04 (P < .05). When dobutamine was added to the myocardial depressant halothane, left ventricular area EF increased from a baseline of 0.55 +/- 0.03 to 0.68 +/- 0.04. In protocol 2, coronary occlusion resulted in a detectable regional wall motion abnormality (circumferential percentage) of 23% +/- 3%. After reperfusion and during dobutamine stimulation, the wall abnormality decreased to 12% +/- 4%. Transthoracic echocardiography correlated well with these intracardiac findings. CONCLUSIONS: The left ventricle can be quantitatively imaged from the right ventricle with the use of a 10F, 10-MHz intracardiac catheter in swine. This system can detect changes in global and regional left ventricular systolic function. This technique warrants evaluation in clinical applications.
Transition metals such as iron and copper are present in the myocardium and can act as catalysts for the formation of oxygen free radicals during reperfusion after myocardial ischemia. Previous studies suggested that transition metal chelators such as desferrioxamine reduce the production of such radicals and may thereby attenuate postischemic myocardial dysfunction. These studies used spin trapping agents, commonly nitrone compounds, which may themselves influence the severity of the ischemia and reperfusion events being studied. We evaluated two transition metal chelators, desferrioxamine, an iron chelator, and bathocuproine, a copper chelator, by using a new electron paramagnetic resonance technique that does not require the administration of spin traps. We measured ascorbate free radical, an index of free radical production, in the great cardiac vein effluent. Twenty-eight open-chest dogs underwent 20 min of coronary artery occlusion and 30 min of reperfusion. Ten dogs received no drug, 10 dogs received 750 mg bathocuproine, i.v., and eight dogs received 700 mg desferrioxamine, i.v. Both bathocuproine and desferrioxamine blunted the postreperfusion increase in ascorbate free radical generation: no drug, 36+/-8% increase; desferrioxamine, 13+/-5% increase; bathocuproine, 21+/-6% increase (p < 0.05 vs. baseline). Thus direct free radical measurements indicate that chelation of the transition metals iron and copper reduces free radical generation during reperfusion.
Motion of the left ventricular cavity center during the cardiac cycle was compared using transthoracic and intracardiac echocardiography. Rotation was comparable for the 2 methods, however, translation of the left ventricular cavity area center was greater with intracardiac echocardiography.
These recommendations are presented to enhance the safety and efficacy of AEDs intended for public access. The task force recommends that manufacturers present developmental and validation data on their own devices, emphasizing high sensitivity for shockable rhythms and high specificity for nonshockable rhythms. Alternative defibrillation waveforms may reduce energy requirements, reducing the size and weight of the device. The highest levels of safety for public access defibrillation are needed. Safe and effective use of AEDs that are widely available and easily handled by nonmedical personnel has the potential to dramatically increase survival from cardiac arrest.
The recently developed echocardiographic technology of color kinesis (CK) displays endocardial motion in color layers on a single end-systolic 2-dimensional echocardiographic frame. Previous work using this method is promising for quantitation of regional function, but there is limited experience in patients with severely reduced left ventricular function. Twenty patients (age 59 +/- 10 years) with dilated cardiomyopathy (left ventricular ejection fraction 22 +/- 8%) underwent CK imaging. Endocardial motion was quantitated by measuring the distance of endocardial motion during the systolic interval and also by calculating the endocardial velocity. CK measurements were compared among 4 wall motion grades (i.e., normal, hypokinetic, akinetic, and dyskinetic) assessed by qualitative wall motion scoring. There was a significant overall difference (p < 0.0001) in the mean systolic endocardial inward motion (i.e., contraction) and outward motion (i.e., expansion) among wall motion grades. The mean endocardial outward distance was significantly greater for the dyskinetic segments than for the other grades (p < 0.001). There were also differences in the mean velocity of endocardial motion among the wall motion grades. In the presence of left bundle branch block, there was no difference in the mean endocardial inward distance of the hypokinetic, akinetic, and dyskinetic septal segments. We conclude that in the absence of left bundle branch block, normal, hypokinetic, akinetic, and dyskinetic ventricular wall segments may be distinguished in patients with dilated cardiomyopathy on the basis of endocardial motion measured with CK.
Skin biopsies obtained 24 hours after elective cardioversion of 30 patients showed variable epidermal necrosis and upper dermal perivascular inflammation, most noticeably in patients receiving high individual peak (> or = 300 J) and cumulative (> or = 350 J) shock energies. Thus, damped sine wave shocks cause skin injury--first degree burns--the severity of which is a function of peak and cumulative shock energy.
STUDY OBJECTIVE: In transthoracic defibrillation, the American Heart Association (AHA) recommends wide separation of electrodes and avoidance of gel smearing between electrodes. Few data support this recommendation. Our objective was to determine the importance of electrode placement and gel-application technique on transcardiac defibrillation current and the effect of changes caused by postexercise vasodilation and sweating. METHODS: Our subjects were 10 normal adults, 5 men and 5 women, who ranged in age from 22 to 48 years. We determined interelectrode impedance (Z) using a validated test-pulse method that does not require shock delivery. Electrode placement/gel-application techniques were varied among four types: (1) AHA-recommended technique (apex-to-anterior electrode placement, no smearing of gel between electrodes); (2) parasternal-to-anterior placement, electrodes within 2 cm of each other, no smearing of gel between electrodes; (3) parasternal-to-anterior placement, electrodes within 2 cm of each other with smearing of gel between electrodes (worst-case scenario); and (4) apex-to-anterior placement, smearing of gel between electrodes. To assess the effect of cutaneous vasodilation and sweating on interelectrode impedance, we repeated these measurements after the subjects performed 12 to 18 minutes of treadmill exercise. The ratio of predicted transcardiac current of the AHA technique to that of the nonstandard technique was estimated with this formula: square root of Z, non-standard technique divided by square root of Z, AHA technique. RESULTS: Resting interelectrode impedance declined 38% from 58 +/- 10.3 omega (AHA-recommended technique) to 36 +/- 7.6 omega (electrode paddles adjacent, gel smeared between) (P < .01). Predicted transcardiac current ratio was reduced to .78 +/- .09 (P < .01), a 22% reduction. We noted no change in the results after exercise. CONCLUSION: Adjacent placement of electrodes and smearing of gel between electrodes creates a low-impedance pathway along the chest wall, which shunts current away from the heart. Thus improper application of electrodes and gel substantially degrades transcardiac current and may result in failed defibrillation. Sweating and vasodilation did not cause a similar problem.
We present a case of open-chest cardiac massage where ventricular fibrillation developed and a direct current shock was required. In the absence of 'surgical' electrode paddles, standard paddle electrodes were used; one small electrode was placed directly on the exposed epicardial surface and the second electrode was placed on the lateral chest wall. Defibrillation was achieved with a 100 J shock. This combined epicardial-transthoracic electrode paddle placement technique allows defibrillation to be accomplished when open chest cardiac massage is being performed and no 'surgical' electrode paddles are available.
OBJECTIVES: The present study was designed to assess the impact of direct current shocks on cardiac troponin I (cTnI), which has greater sensitivity and specificity than creatine kinase (CK) for the diagnosis of myocardial injury. BACKGROUND: Transthoracic direct current shocks can cause myocardial injury. They also cause elevations of total CK and CK-MB fraction (CK-MB). METHODS: We obtained measurements of cTnI total CK and CK-MB before and after elective cardioversions in 38 patients. Blood samples were drawn before and 8, 16, 24 and 48 h after cardioversion. Shock energy, current, impedance and number of shocks delivered were tabulated. RESULTS: Patients received a mean (+/-SD) of 2.1 +/- 1.2 shocks with a median cumulative energy of 300 J (range 50 to 1,580). Three patients had minimal elevations (1.5, 1.2 and 0.8 ng/ml, normal < or = 0.6 ng/ml) of cTnI. Two of these patients had impaired left ventricular contractility by echocardiography. Thirty-five of the 38 patients had no elevations of cTnI. Sixty-two percent of patients had an increase in CK after cardioversion, but CK-MB was elevated to an abnormal level of 12.7 ng/ml (normal < 6.7) in only one patient after cardioversion. CONCLUSIONS: Cardiac troponin I levels are either normal or minimally elevated after elective direct current cardioversion, suggesting that subtle myocardial injury can be caused by direct current transthoracic shocks. However, substantial elevations of cTnI after cardioversion suggest the presence of myocardial injury from causes unrelated to the direct current shocks administered for cardioversion.
Our objective was to demonstrate that right ventricular (RV) infarction could be demonstrated by intracardiac ultrasonography in a canine model. RV infarction is a common and important clinical condition in patients with myocardial infarction. Traditional methods for diagnosing RV infarction have limitations. Intracardiac echocardiography, in which an ultrasonic transducer on the tip of a catheter is placed intravenously into the RV chamber, should allow detection of RV infarction. Nine closed-chest dogs were studied. The animals were instrumented with a 10 MHz ultrasound catheter placed into the right ventricle. The right coronary artery was occluded with a balloon angioplasty catheter for 20 minutes and subsequently embolized with elemental mercury. Intracardiac ultrasound images were obtained at baseline, during balloon occlusion, and during embolization. RV cross-sectional end-diastolic and end-systolic areas were calculated and fractional area changes were calculated; RV wall motion abnormalities were also evaluated. The interventricular septal thickening was also calculated. The 10 MHz intracardiac ultrasound catheter allowed visualization of much of the RV chamber and interventricular septum. The RV cross-sectional area increased with mercury embolization, which was also associated with regional wall motion abnormalities. RV end-systolic area was 1.6 cm2 and end-diastolic area 3.9 cm2 at baseline; these increased to 4.8 cm2 and 6.5 cm2 after embolization (p < 0.05). Interventricular septal thickening remained unchanged. The echocardiographic features of RV infarction, which include RV dilation and RV regional wall motion abnormalities, could be demonstrated in a canine infarct model with a 10 MHz intracardiac ultrasound catheter.
Achieving defibrillation depends on adequate intracardiac current. The purpose of this study was to determine, in advance of administering shocks which parameters of body habitus can be used to select the electrode size that maximizes intracardiac current in transthoracic defibrillation. We administered direct current shocks to 18 mongrel dogs over a wide range of weight and size (weight 10-30 kg with chest circumferences 44-77 cm) using a damped sine wave defibrillator and self-adhesive electrode pairs of various diameters (4 cm, 5.8 cm, 8 cm and 10 cm), placed on the right and left lateral chest walls. The energy levels used were 50, 100, and 150 J. Intracardiac voltage gradient, a parameter of intracardiac current, was determined in three orthogonal planes using an intramyocardial electrode array placed in the interventricular septum. The relation between intracardiac voltage gradient magnitude magnitude of VG and various parameters (body weight, chest, circumference, chest volume, chest radius, and heart weight divided by chest radius) was determined. The correlation (r) with the smallest P value was between magnitude of VG and the heart weight divided by chest radius (HW/R) (r = 0.71). Intracardiac current was highest at intermediate pad sizes. The electrode pads that maximized magnitude of VG tended to be large for the larger HW/R dogs, and smaller HW/R dogs. In none of the HW/R groups did the largest electrode pads yield the highest intracardiac voltage gradient. We conclude that there is no simple way to determine in advance an electrode pad size that maximizes intracardiac current. The HW/R ratio influences but does not determine intracardiac intracardiac current.
Currently available transthoracic defibrillators use either a damped sinusoidal or truncated exponential (TE) waveform. Truncated exponential waveforms deliver a long pulse if the transthoracic impedance is high; it has been suggested that such a long pulse may be less effective for defibrillation. Our objective was to compare the ability of damped sinusoidal (DS) waveform shocks versus TE waveform shocks to terminate ventricular fibrillation (VF) and achieve survival from witnessed cardiac arrest. We retrospectively reviewed field-recorded electrocardiograms from 86 patients with witnessed VF, treated by prehospital personnel equipped with DS or TE waveform defibrillators. Forty-four patients received 130 shocks from TE defibrillators; 42 patients received 108 shocks from DS defibrillators. There were no significant differences in time from arrest to first shock (8.0 vs 8.1 minutes), nor were there any differences in the size of the communities involved. The shocks resulted in the following rhythms: organized rhythm: TE: 15 of 130 (12%), DS: 24 of 108 (22%), p = 0.10 (NS); persistent VF: TE: 85 of 130 (65%), DS: 45 of 108 (42%), p <0.01; asystole: TE: 30 of 130 (23%), DS: 39 of 108 (36%), p = NS; and survival to hospital discharge: TE: 5 of 44 (11%), DS: 8 of 42 (19%), p = NS. We conclude that DS waveforms terminated VF more frequently than TE, but there was no significant difference in resumption of an organized rhythm or survival. A prospective comparison of these 2 waveforms is needed.
This study examines in a prospective, multicenter trial the feasibility and advantage of current-based, transthoracic defibrillation. Current-based, damped, sinusoidal waveform shocks of 18, 25, 30, 35, or 40 amperes (A) were administered beginning with 25 A for polymorphic ventricular tachycardia (VT) and ventricular fibrillation (VF) or 18 A for monomorphic VT; success rates were compared with those of energy-based shocks beginning at 200 J for VF/polymorphic VT and 100 J for VT. The current-based shocks were delivered from custom-modified defibrillators that determined impedance in advance of any shock using a "test-pulse" technique; the capacitor then charged to the exact energy necessary to deliver the operator-selected current against the impedance determined by the defibrillator. Three hundred sixty-two patients received > 1 shock for VF, polymorphic VT, or monomorphic VT: 569 current- based shocks and 420 energy-based shocks. Current-based shocks of 35/40 A achieved success rates of up to 74% for VF/polymorphic VT; 30 A shocks terminated 88% of monomorphic VT episodes. Energy-based shocks of 300 J terminated 72% of VF/polymorphic VT; 200-J shocks terminated 89% of monomorphic VT. We could not demonstrate a significant increase in the success rate of current-based shocks over energy-based shocks for patients with high transthoracic impedance; this may be due to inadequate sample size. Thus, current-based defibrillation is clinically feasible and effective. A larger study will be needed to test whether current-based defibrillation is superior to energy-based defibrillation.