The effects of potassium-ATP channel modulation on ventricular fibrillation and defibrillation in the pig heart

International Journal of Cardiology 76 (2000) 187–197 www.elsevier.com / locate / ijcard

The effects of potassium-ATP channel modulation on ventricular fibrillation and defibrillation in the pig heart Mark T. Harbinson MD MRCP,a , J. Desmond Allen MD b , A.A. Jennifer Adgey MD FRCP, a, FACC * a

Regional Medical Cardiology Centre, Royal Victoria Hospital, Belfast BT12 6 BA, N. Ireland, UK b Department of Physiology, The Queen’ s University, Belfast, N. Ireland, UK Received 19 April 2000; received in revised form 4 July 2000; accepted 4 September 2000

Abstract Background: Drugs acting on the cardiac ATP-sensitive potassium (K-ATP) channels may modulate responses to ischaemia and arrhythmogenesis. We investigated the effects of K-ATP channel modulation on frequency patterns of ventricular fibrillation (VF) and on defibrillation threshold (DFT). Methods and Results: Each group of 24 pigs randomly received intravenous levcromakalim (LKM) 40 mg / kg (K-ATP agonist), glibenclamide (Glib) 20 mg / kg (K-ATP antagonist), saline or vehicle. Firstly, QTc interval was measured before and after drug. VF was then induced by endocardial stimulation and its power spectra and dominant frequencies over 15 min determined by fast Fourier transformation. Secondly, transthoracic DFT was determined (step-up / step-down protocol) before and after each drug. LKM reduced QTc interval (e.g., lead II, 354–321 ms, P,0.05) and increased the dominant VF frequency between 6 and 8 min (9.560.5 Hz at 6.5 min compared with 7.260.6 Hz (saline), 7.460.8 Hz (vehicle), 6.860.5 Hz (Glib), P50.03). LKM reduced (to 57.262.1 mmHg) and Glib increased (to 107.866.1) mean arterial BP compared with saline (80.365.6) and vehicle (87.667.1; P,0.01). There was no significant difference in defibrillation threshold energy, current or voltage, after any drug. Conclusions: Activation of K-ATP channels reduced blood pressure and QTc interval. The lack of major effect on VF dominant frequency and DFT of either LKM or Glib suggests that prior administration of similar drugs to patients should not prejudice outcome from VF cardiac arrest.  2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Ventricular fibrillation defibrillation; Fourier analysis; Potassium–ATP channels

1. Introduction Acute myocardial ischaemia opens the ATP-sensitive potassium (K-ATP) channels due to a fall in intracellular concentrations of ATP, a rise in ADP and an increase in lactate concentration [1,2]. Drugs which act via the K-ATP channel are now in clinical use in patients with ischaemic heart disease. Nicoran-

*Corresponding author. Tel.: 144-28-90-240-503, ext. 3223; fax: 14428-90-312-907.

dil, a K-ATP agonist and nitric oxide donor, is used in the treatment of angina. Recently its benefits were demonstrated in the treatment of unstable angina, when its cardioprotective actions were attributed to effects on the K-ATP channel [3]. However, in experimental studies, K-ATP channel agonists, such as levcromakalim, may increase the tendency to arrhythmias [4]. Sulphonylurea drugs, used in the treatment of diabetes mellitus, are K-ATP channel antagonists. In some clinical studies sulphonylureas increased cardiovascular mortality [5], while in experimental studies they protected against the develop-

0167-5273 / 00 / $ – see front matter  2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S0167-5273( 00 )00378-8

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ment of ventricular fibrillation (VF) during myocardial ischaemia [6,7]. Clearly, these drugs have significant effects on cellular electrophysiology which may be important during cardiac arrest and its correction. As many patients with ischaemic heart disease and diabetes may be taking these agents, we investigated the effects of K-ATP channel modulation on VF and its correction by defibrillation. In the first part of the study, the dominant frequency of ventricular fibrillation was therefore studied after modulation of the K-ATP channel by the agonist levcromakalim and the antagonist glibenclamide. When VF is analysed by a fast Fourier transform method, the power spectra show distinct dominant frequencies, which may be influenced by potassium concentrations and various drugs active on ion channels [8–10]. Clinically, a low fibrillation frequency is associated with an adverse effect on resuscitation outcome [11,12]. Agents which act on cardiac membrane channels to alter cellular electrophysiology can terminate and prevent cardiac arrhythmias, and may alter the threshold for defibrillation [13]. Since it is likely that the intense ischaemia associated with ventricular fibrillation will activate the K-ATP channels, we studied, in a second experiment, the ease of defibrillation (measured by the defibrillation threshold, DFT), after activation and blockade of the K-ATP channel by the same two agents.

2. Methods This investigation was performed in two parts. In the first experiment, the power spectrum of VF was examined in 24 anaesthetised pigs after K-ATP channel activation or blockade. In the second experiment, the threshold energy for ventricular defibrillation was determined in a further 24 pigs after similar modulation of K-ATP channels. In each part of the study four groups each of six pigs (mean weight, 49.5 kg) were randomly assigned to a study drug. All procedures were performed in accordance with the Animals (Scientific Procedures) Act 1986. The pigs were sedated (azaperone, 200 mg i.m.), anaesthetised (pentobarbitone, 30 mg / kg i.v.), intubated and ventilated with room air. Pulse oxi-

metry displayed tissue oxygen saturation (90–100%) and end-tidal carbon dioxide concentration (35–40 mmHg; BOC Ohmeda Oxicap). Body temperature was maintained around 378C by a heating blanket (Harvard Inst.). Cannulae were inserted in the left jugular vein for blood sampling and a large peripheral vein for drug administration. Arterial blood pressure was recorded using a carotid artery cannula and transducer (Druck). Two pacing catheters (Elecath) were placed in the right ventricular apex via the right jugular vein under image intensification (Siemens). These allowed recording of the intracardiac (endocardial) ECG and the induction of ventricular fibrillation. Arterial pressure was displayed with the surface ECG (lead II), the endocardial ECG (during the frequency analysis experiments) and heart rate on a four-channel recorder (Gould 2400S).

2.1. Ventricular fibrillation frequency analysis After preparation, baseline venous blood samples (glucose and potassium) and measurements (ECG, arterial blood pressure, oxygen saturation and end tidal carbon dioxide) were obtained in a stable haemodynamic state. The animal was then randomised to receive an intravenous injection of saline (control), levcromakalim (a K-ATP channel agonist), glibenclamide (a K-ATP channel antagonist), or vehicle. The agents were given as an intravenous injection (final volume, 200 ml) over 5 min. Levcromakalim (40 mg / kg) and glibenclamide (20 mg / kg) were dissolved in the vehicle (equal parts of sodium hydroxide, ethanol and polyethylene glycol up to 30 ml in total) and gently warmed to encourage dissolution, before dilution to 200 ml with warm 0.9% saline. The vehicle group received 30 ml of the vehicle diluted to 200 ml with saline, and the saline group 200 ml of 0.9% saline alone. The test drug was injected into a large peripheral vein over 5 min and allowed to circulate for a further 10 min, before repeat estimations of haemodynamic variables and venous blood concentrations. After the samples were taken, ventricular fibrillation was induced via the right ventricular pacing catheter (square wave pulse train, 100 Hz, 2 ms duration; Grass stimulator). Ventricular fibrillation was recorded continuously from this point for 15 min. Further blood samples were withdrawn after 5

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and 15 min of ventricular fibrillation (Vacutainer tubes). On completion of the experiment the plasma was separated and analysed promptly to preclude artefactual changes in potassium or glucose concentration using a glucose oxidase technique and potassium ion-sensitive electrodes (Kodak Ektachem Analyser). Ventricular fibrillation was recorded from surface electrodes and from the catheter in the right ventricular apex (endocardial ECG), digitised (10 bits; 125 Hz) and recorded on cassette tape (Sony). The data were replayed through the analogue-digital recorder to a low frequency digital signal analyser (Bruel and Kjaer 2031) to resolve the fibrillation signal into its component frequencies by a fast Fourier transform. An anti-aliasing filter minimised artefact and the Hanning weighting limited frequency leakage. Linear averaging of the spectra over each time interval was used, as previously verified in this laboratory. In brief, the dominant frequency over the averaged time period, and the maximal power (in dB) of the signal were recorded at specified time intervals. The data were analysed in 8-s segments for the first 80 s, then in 20-s segments for the next 180 s, then in 40-s segments for the next 240 s and finally in 80-s segments until the termination of the experiment, after 15 min of ventricular fibrillation. At each period the power (which correlates with the square of the amplitude) of the dominant frequency was recorded. The QT interval and corrected QT interval (QTc) were determined during sinus rhythm, using the differential threshold method [14], for the control period and after drug administration prior to induction of ventricular fibrillation. At each time eight consecutive QRST intervals were averaged for both the surface (lead II) and endocardial ECGs. QTc interval was calculated using Bazett’s formula (QT interval / square root[R-R interval]).

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energy tested was 150 J. Shocks were delivered in expiration after approximately 10 s of VF using standard paddles and paste (Redux, Hewlett-Packard) at fixed positions on the praecordium. After successful defibrillation the next energy used was reduced by 10 J. A defibrillation failure was followed by a high energy rescue shock (some 200 J) and the subsequent energy used for the next sequence increased by 10 J. Each fibrillation / defibrillation sequence was separated by 5 min to allow adequate recovery of haemodynamics. The defibrillation threshold was defined as the lowest energy successfully converting VF to sinus rhythm. For each subsequent threshold determination, the starting energy was the defibrillation threshold of the previous series. After two pre-drug defibrillation thresholds had been obtained, the animal was randomised to receive saline (control), levcromakalim, glibenclamide, or vehicle, injected into a large peripheral vein over 5 min and allowed to circulate for a further 10 min. Levcromakalim (40 mg / kg) and glibenclamide (20 mg / kg) were dissolved in vehicle (dimethyl sulphoxide, DMSO). The vehicle group received 5 ml of the vehicle and the saline group 5 ml of 0.9% saline alone. Haemodynamic variables were recorded and the fibrillation and defibrillation thresholds obtained as before drug. Thus for each experiment two defibrillation thresholds were determined before and 2 after drug administration.

2.3. Statistics Comparison of data within groups and between groups was performed by a one-way analysis of variance (ANOVA; SPSS, Chicago, IL; DEC VAX 9600 computer). The level of statistical significance was defined as P#0.05. Data are given as mean6standard error of the mean (S.E.M.).

2.2. Defibrillation threshold determination 3. Results After preparation, baseline measurements (ECG, arterial blood pressure, oxygen saturation and end tidal carbon dioxide) were obtained in a stable state. Ventricular fibrillation was induced via the right ventricular pacing catheter, as before. The defibrillation threshold was then determined using a 10-J step-up / step-down protocol. The initial

3.1. Ventricular fibrillation frequency analysis There were no significant differences between the groups initially in terms of body weight, gender, baseline heart rate or arterial blood pressure (Table 1). Oxygen saturation and carbon dioxide concen-

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Table 1 Heart rate, blood pressure and QTc interval values before and after agent administration for each treatment group in the VF frequency study (mean6S.E.M.) Parameter

Saline

Vehicle

Levcromakalim

Glibenclamide

Heart rate pre-drug (beats / min) Heart rate pre-VF (beats / min) BP pre-drug (mmHg) BP pre-VF (mmHg) QTc interval pre-drug (lead II) QTc interval pre-VF (lead II)

111.5616.3 111.3615.3 74.863.2 79.063.7 404.9638.4 408.4632.0

104.0610.4 114.8613.8 70.565.5 66.369.3 373.0622.5 391.1623.4

136.2616.9 151.2617.9 73.365.3 52.064.5* 354.0624.6 321.5619.0†

105.565.3 125.2613.2 71.265.7 83.565.5 391.9611.1 408.2622.3

*P50.01 levcromakalim versus control mean arterial blood pressure; †P,0.05 pre-drug versus pre-VF QTc interval (lead II) for levcromakalim.

tration were also similar in each group, both before and after drug administration.

3.1.1. Saline group Following administration of saline, there was no significant change in heart rate, blood pressure, QT and QTc intervals (Table 1), or in blood glucose and potassium concentrations (Table 2). Examples of relatively early and late VF are shown (Fig. 1). Visual inspection indicates slower frequencies of depolarisations for later records compared with earlier. The power spectrum of ventricular

fibrillation followed a distinct and consistent pattern in each experiment (Figs. 2–4). In the power spectra, plotted against frequency on the x axis (Fig. 2), a clear peak, or dominant, frequency was identified in each signal, and formed the basis for the subsequent analysis. A plot of this dominant frequency against time formed the curves used for comparison of the agents (Figs. 3 and 4). Initially there was a high dominant frequency and a wide spread of frequency values. After 60 to 80 s of fibrillation the dominant frequency began to decline gradually. It then rose to a second peak between 180 and 300 s before declining

Table 2 Blood glucose and potassium concentrations before and after administration of each agent in the VF frequency study (mean6S.E.M.) Parameter

Saline

Vehicle

Levcromakalim

Glibenclamide

Glucose pre-drug (mmol / l) Glucose pre-VF (mmol / l) Potassium pre-drug (mmol / l) Potassium pre-VF (mmol / l)

7.160.5 7.961.1 3.660.1 3.660.1

6.660.9 6.560.9 3.760.1 3.660.1

5.760.7 6.160.8 3.560.2 3.460.2

5.960.4 6.760.6 3.660.2 3.760.2

Fig. 1. Simultaneous recording of endocardial and surface ECG (lead II) showing relatively early (left, 6 min duration) and late (right, 13 min) ventricular fibrillation.

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191

Fig. 2. Power spectra for VF of 2 min duration (left) and 14 min duration (right). Note the peak frequency of around 11 Hz after 2 min, and the lower peak frequency of 3.5 Hz after 14 min of fibrillation.

sharply. Frequency then remained low for the remainder of the experiment (Figs. 3 and 4). This pattern of frequency change was similar in the endocardial and surface (lead II) ECG recordings. Power was initially high and gradually declined with time.

3.1.2. Vehicle group Haemodynamic, ECG and respiratory parameters

remained unchanged after vehicle administration and were not significantly different from the other groups (Table 1), with no significant effect on blood chemistry (Table 2). After vehicle (Fig. 4), the dominant frequency of fibrillation followed a similar pattern to that observed in the saline group (Fig. 3), and was not significantly different from saline control at any duration of VF.

Fig. 3. Mean (6S.E.M.) dominant VF frequency during 15 min of VF after saline and levcromakalim (lead II).

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Fig. 4. Mean (6S.E.M.) dominant VF frequency during 15 min of VF after vehicle and glibenclamide (lead II).

Frequency was initially high and declined with time after 60–80 s before rising to a second peak after 3–5 min. This was followed by decline to a low level which persisted for the remainder of the experiment.

3.1.3. Levcromakalim group Levcromakalim reduced mean arterial blood pressure resulting in significantly lower pressures prior to the induction of ventricular fibrillation (5264.5 mmHg) than after saline or glibenclamide (P50.007; Table 1). No significant effects on respiratory parameters and heart rate (Table 1), or on blood potassium and glucose concentrations (Table 2) were observed. The administration of levcromakalim reduced QT and QTc intervals in both lead II (P,0.05; Table 1) and the endocardial ECG (mean QTc, 348.3–310.0 ms, P,0.05). As a result, the endocardial QTc interval prior to VF was lower in the levcromakalim group (e.g., mean 310.0 ms cf. 411.2 ms (saline), 411.2 ms (glibenclamide); P50.03). After levcromakalim, the initial dominant frequency during the first 60 s tended to be higher than in other groups, but the difference did not reach statistical significance. However the expected decline in frequency beginning after 5 min of VF was delayed and attenuated in the levcromakalim group

(Fig. 3). In lead II, dominant frequency after 440 s of VF was 9.460.5 Hz compared with 7.260.6 Hz (saline), 7.460.9 Hz (vehicle), 6.860.5 Hz (glibenclamide), P50.03. The dominant frequency remained higher for a further 2 min until 8 min VF duration, when it rapidly declined to the levels noted in the control groups. A similar association was apparent when analysing the endocardial ECG signals though the difference persisted for only 1 min (e.g., at 8 min, frequency 8.960.4 Hz after levcromakalim cf. 5.761.0 Hz (saline), 5.860.9 Hz (vehicle), 6.960.3 Hz (glibenclamide); P50.02).

3.1.4. Glibenclamide group Administration of glibenclamide caused a small and non significant rise in mean arterial blood pressure (Table 1). The remaining baseline variables were unchanged (Tables 1 and 2). Glibenclamide did not alter QT and QTc intervals in lead II or the endocardial ECG. Overall glibenclamide had little effect on the dominant frequency of VF and the typical changes with time were again observed in lead II and the endocardium (Fig. 4). While the frequency tended to be lower than the other groups throughout the first 30 s, this reached significance only after 24 s of fibrilla-

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193

Table 3 Heart rate, blood pressure and respiratory variables before and after administration of each agent in the defibrillation threshold study (mean6S.E.M.) Parameter

Saline

Vehicle

Levcromakalim

Glibenclamide

Heart rate pre-drug (beats / min) Heart rate post-drug (beats / min) Art. BP pre-drug (mmHg) Art. BP post-drug (mmHg) SaO 2 pre-drug (%) SaO 2 post-drug (%) ETCO 2 pre-drug (mmHg) ETCO 2 post-drug (mmHg)

136.5643.5 107.7617.0 89.067.0 80.365.6 91.560.5 90.860.9 35.563.5 33.561.4

122.0624.7 130.5621.6 78.066.8 87.667.1 92.861.3 93.561.3 33.361.1 33.061.3

99.269.6 104.768.9 91.368.1 57.262.1 92.561.1 92.260.7 34.861.2 33.861.3

119.3612.9 124.3615.9 83.865.6 107.866.3 92.760.6 90.561.1 35.861.2 31.860.8

tion (7.561.0 Hz cf. 10.260.9 Hz (saline), 11.660.9 Hz (vehicle), 11.160.6 Hz (levcromakalim), lead II; P50.01), and the effect was short-lived. After 32 s of VF there was no significant reduction in frequency in this group for the remainder of the experiment.

3). Three variables were used to characterise the threshold for defibrillation. The mean peak current and voltage of the threshold shocks were similar before and after saline (Table 4). The mean threshold stored energy for defibrillation showed no significant changes with time (Table 4).

3.2. Fibrillation and defibrillation thresholds Baseline sex, weight, heart rate, arterial blood pressure, oxygen and carbon dioxide concentrations were similar in all groups (Table 3). The voltage required to induce VF was similar in all groups both before and after drug administration. The effects of the agents on defibrillation threshold energy, current and voltage are shown in Table 4.

3.2.1. Saline Administration of saline had no significant effect on respiratory and haemodynamic variables (Table

3.2.2. Vehicle As in the saline group, all haemodynamic and respiratory variables remained stable. Although there was a small fall in DFT energy after the initial determination, the values were not significantly different from those for the other groups (Table 4). 3.2.3. Levcromakalim After levcromakalim there was again a significant fall in mean arterial blood pressure; other physiological parameters remained similar (Table 3). Although defibrillation threshold stored energies, peak currents

Table 4 Defibrillation threshold (DFT) as stored energy (J), peak current (A) and peak voltage (V) before and after administration of each agent (mean6S.E.M.): there was no significant difference in any variable between groups Saline Stored energy ( J) 1st DFT pre-drug 2nd DFT pre-drug 1st DFT post-drug 2nd DFT post-drug

121.7613.3 118.4614.7 111.7618.9 125.0619.3

Peak current (A) 1st DFT pre-drug 2nd DFT pre-drug 1st DFT post-drug 2nd DFT post-drug

38.761.9 38.062.4 37.063.0 39.463.4

Peak voltage ( V) 1st DFT pre-drug 2nd DFT pre-drug 1st DFT post-drug 2nd DFT post-drug

1180.8681.0 V 1156.7689.6 V 1078.3693.9 V 1121.76100.4 V

J J J J

A A A A

Vehicle

Levcromakalim

Glibenclamide

123.368.8 J 105.069.6 J 110.0612.4 J 110.0612.6 J

116.7616.5 J 103.3615.8 J 85.0611.5 J 93.367.6 J

120.0615.1 J 110.0618.8 J 90.0614.7 J 96.7617.5 J

38.761.5 35.762.1 37.762.3 36.662.3

37.762.7 36.262.9 33.062.4 34.361.6

38.262.3 36.263.1 33.362.5 33.463.0

A A A A

1295.0648.1 V 1206.7639.2 V 1170.0638.6 V 1219.2646.2 V

A A A A

1304.26108.5 V 1187.5683.2 V 1073.3661.5 V 1126.7639.9 V

A A A A

1249.2683.1 V 1177.5682.3 V 1099.2696.1 V 1109.26102.9 V

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and peak voltages were reduced, these changes did not reach statistical significance (Table 4).

3.2.4. Glibenclamide In contrast to levcromakalim, glibenclamide increased mean arterial blood pressure, with little significant effect on other haemodynamic parameters (Table 3). Glibenclamide did not cause a significant change in defibrillating current, voltage or energy compared with baseline and control values (Table 4).

4. Discussion Overall, these studies indicate that activation or blockade of the K-ATP channel did not greatly change the properties and characteristics of VF and ventricular defibrillation in the pig heart. Activation of the K-ATP channel by levcromakalim resulted in a fall in mean arterial blood pressure and a reduction in QT and QTc interval. The effects on the dominant frequency of VF occurred late and were modest. Levcromakalim caused a delay in the fall in fibrillation frequency observed after 6 min in the control groups. Blockade of the K-ATP channels by glibenclamide caused a small reduction in the early VF frequency, which was not sustained, and produced no other important effects. Blood glucose and potassium concentrations did not significantly change in any group. In the second experiment, activation of the K-ATP channel by levcromakalim again reduced mean arterial blood pressure but had little effect on other physiological parameters. It did not significantly change threshold current and energy for defibrillation. Conversely, while the K-ATP channel antagonist glibenclamide increased mean arterial blood pressure, it did not significantly alter defibrillation requirements.

4.1. The effects of potassium-ATP channel modulation Potassium-ATP channel agonists such as levcromakalim are potentially beneficial in several situations. During ischaemia, the rise in lactate levels, fall in pH, rise in ADP and fall in ATP levels combine to

open the K-ATP channels [1,2]. The channels are similarly opened by drugs such as levcromakalim and nicorandil. In animal models, K-ATP channel activation reduces experimental myocardial infarction size [15], promotes earlier recovery of post-ischaemic contractile dysfunction [16] and has a role in the phenomenon of ischaemic pre-conditioning [17]. In addition to these protective effects during ischaemia, K-ATP agonists also dilate coronary arteries and may therefore increase coronary blood flow. Against these benefits, the reduction in action potential duration [18] and rise in extracellular potassium concentration [19] following K-ATP activation mimic the local electrophysiological conditions seen during VF [20]. Activation of the potassium-ATP channel is antagonised by sulphonylureas such as glibenclamide, which antagonises the effects alluded to above. They may also cause coronary artery vasoconstriction [6]. Against this they prolong the action potential duration in some experimental models of ischaemic ventricular fibrillation and have an anti-fibrillatory effect [6]. The UGDP trial in patients with non-insulin-dependent diabetes mellitus demonstrated an increased mortality in patients taking sulphonylureas [5], though this study has been criticised. Many patients with diabetes mellitus are taking sulphonylurea drugs and the implications for ventricular fibrillation in this setting are not completely understood. Consequently it can be appreciated that agents active on the K-ATP channel may have several interactions with the electrophysiology of VF and its correction.

4.2. The ECG after K-ATP activation K-ATP channel agonists shorten the action potential duration. In the current experiment, levcromakalim reduced QTc interval by approximately 11%. This is a similar reduction to that observed in action potential duration in previous studies in resting sinus rhythm, though significantly greater reductions were observed during ischaemia [16]. In the present study glibenclamide had no significant effect on resting QT interval, confirming that K-ATP channels are mainly inactivated in the resting non-ischaemic state.

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4.3. Ventricular fibrillation frequency The analysis of the ECG signal of ventricular fibrillation by a fast Fourier method generates distinct power spectra with strongly dominant frequencies. The frequencies resulting tend to follow a consistent and distinct pattern as the high initial frequencies decline with time. This pattern was observed in all groups in the current experiment, as in other studies in dogs [8,9] and pigs [10] using the dominant frequency as the main descriptor of the power spectrum. Similar results have been described in the pig using the median frequency as the main marker of the power spectrum [21–24]. The underlying physiological reasons for the changes in VF frequency described above remain incompletely understood. The pattern does however seem to depend on certain variables. Firstly, the duration of the arrest determines the frequency pattern [8,21,23] as the dominant (or the median) frequency is high initially and then declines with time. For this reason the VF frequency has been proposed as a potential estimator of the duration of a VF cardiac arrest [21,23]. Clinical studies confirm this fall in frequency with the duration of VF [11,12] and indicate the prognostic significance of this for resuscitation. Secondly, the interaction of various ion channels during VF contributes to this pattern of frequency distribution. Administration of lignocaine prior to the induction of VF in dogs and pigs reduced the high initial frequencies usually seen [9,10], suggesting that the early high frequencies may be determined by the activation of fast sodium membrane channels. Similarly, verapamil prevented the usual decline in frequency seen after 1–2 min of fibrillation in dogs [9] and pigs [10]. Hence the fall in frequency after the initial 60–90 s may be caused by intracellular calcium accumulation and can be prevented by calcium channel blockade. In the current experiments pre-treatment with the K-ATP channel opener levcromakalim did not result in a dramatic change in the frequency pattern. However, the fall in fibrillation dominant frequency after 5–6 min was significantly reduced. K-ATP channel activation shortens action potential duration and relative refractory period [18] and re-activation times may therefore be shortened in VF, manifesting

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as a high dominant frequency. It has previously been reported that rapid activation patterns occur in intact hearts, with rates up to 10 Hz during ventricular arrhythmias [25]. Furthermore, early activation of the K-ATP channels may allow relative preservation of high energy intracellular phosphates resulting in less metabolic stress in the myocyte. Myocardial ATP declines progressively during untreated ventricular fibrillation [26], correlating with low resuscitation success. It is interesting to note that K-ATP channel blockade by glibenclamide had relatively little effect on VF frequency in this experiment. Haemodynamic measurements and QT interval after drug administration in sinus rhythm did not alter significantly confirming that the channels are closed at rest. There was a small early reduction in VF frequency at 30 s but thereafter frequencies remained the same as in the control groups. This is consistent with the reported insensitivity of the K-ATP channel to antagonists during severe metabolic stress [27].

4.4. Defibrillation threshold No clinically significant changes in defibrillation threshold energy or voltage were observed in this study after administration of either a K-ATP channel agonist or antagonist. The defibrillation threshold (DFT) has been used for some time as an indicator of defibrillation efficacy. In this study all shocks were delivered using the same paddles, ionic electrolyte paste and energy waveform (Lown) and during the same phase of the respiratory cycle to minimise changes in DFT related to these variables. The ionic channel model proposed by Babbs [13] suggests that drugs which increase potassium current will increase DFT, while those which reduce potassium current will accordingly reduce DFT. The model is limited in that drugs may have more than a single action on ion channels. The model therefore predicts that levcromakalim would increase DFT. In the opposite way, glibenclamide might be expected to prolong action potential duration and reduce DFT. However, during profound ischaemia, presumably like that encountered during VF, that K-ATP channel antagonists become significantly less active [27]. Furthermore, most drugs currently in use as antiar-

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rhythmic agents do not have dramatic effects on the DFT when tested in experiments [28–30]. Despite using doses of levcromakalim and glibenclamide with haemodynamic effects, no dramatic effects on defibrillation threshold were observed in this experiment with either agent. Interestingly, measurement of DFT during ischaemia has not been associated with a consistent change in defibrillation threshold in experimental studies [31].

5. Conclusions Potassium-ATP channel activation leads to reductions in mean arterial blood pressure and QT interval and to higher dominant frequencies of fibrillation between 6 and 8 min after the onset of cardiac arrest. Administration of levcromakalim and glibenclamide did not therefore result in major changes in the electrophysiology of the VF waveform as determined by Fourier analysis. Both fibrillation and defibrillation thresholds were not significantly altered by KATP channel modulation. These results suggest that activation or inactivation of the K-ATP channel does not play a large role in the electrophysiology of VF in the normal heart. The management of VF occurring in patients receiving prior treatment with these therapeutic agents should not require any specific modification in protocol.

Acknowledgements Funding: Dr. Harbinson held a Royal Victoria Hospital Research Fellowship. We are indebted to the staff of the Medical Research Unit, Queen’s University Belfast for valuable technical assistance with the conduct of these studies, and to Dr. T.C. Hamilton (SmithKline Beecham) for the generous gift of the levcromakalim.

References [1] Keung EC, Qian L. Lactate activates ATP-sensitive potassium channels in guinea pig ventricular myocytes. J Clin Invest 1991;88:1772–7. [2] De Weille JR. Modulation of ATP sensitive potassium channels. Cardiovasc Res 1992;26:1017–20.

[3] Patel DJ, Purcell HJ, Fox KM et al. Cardioprotection by opening of the KATP channel in unstable angina. Eur Heart J 1999;20:51–7. [4] Black SC, Lucchesi BR. Potassium channel openers are likely to be proarrhythmic in the diseased human heart. Cardiovasc Res 1994;28:923–4. [5] The University Group Diabetes Program. A study of the effects of hypoglycaemic agents on vascular complications in patients with adult-onset diabetes. Diabetes 1970;19(Suppl 2):785–830. [6] Billman GE, Avendano CE, Halliwill JR, Burroughs JM. The effects of the ATP-dependent potassium channel antagonist, glyburide, on coronary blood flow and susceptibility to ventricular fibrillation in unanaesthetised dogs. J Cardiovasc Pharmacol 1993;21:197–204. [7] D’Alonzo AJ, Darbenzio RB, Hess TA, Sewter JC, Sleph PG, Grover GJ. Effect of potassium on the action of the K-ATP modulators cromakalim, pinacidil, or glibenclamide on arrhythmias in isolated perfused rat heart subjected to regional ischaemia. Cardiovasc Res 1994;28:881–7. [8] Carlisle EJF, Allen JD, Kernohan WG, Anderson J, Adgey AAJ. Fourier analysis of ventricular fibrillation of varied aetiology. Eur Heart J 1990;11:173–81. [9] Carlisle EJF, Allen JD, Kernohan WG, Leahey W, Adgey AAJ. Pharmacological analysis of established ventricular fibrillation. Br J Pharmacol 1990;100:530–4. [10] Stewart AJ, Allen JD, Devine AB, Adgey AAJ. Effects of blockade of fast and slow inward current channels on ventricular fibrillation in the pig heart. Heart 1996;76:513–9. [11] Stewart AJ, Allen JD, Adgey AAJ. Frequency analysis of ventricular fibrillation and resuscitation success. Q J Med 1992;85:761–9. [12] Strohmenger HU, Linder KH, Brown CG. Analysis of the ventricular fibrillation ECG signal amplitude and frequency parameters as predictors of countershock success in humans. Chest 1997;3:584–9. [13] Babbs CF. Alteration of defibrillation threshold by antiarrhythmic drugs: a theoretical framework. Crit Care Med 1981;9:362–3. [14] McLaughlin NB, Campbell RWF, Murray A. Comparison of automatic QT measurement techniques in the normal 12 lead electrocardiogram. Br Heart J 1995;74:84–9. [15] Mizumura T, Nithipatikom K, Gross GJ. Effects of nicorandil and glyceryl trinitrate on infarct size, adenosine release, and neutrophil infiltration in the dog. Cardiovasc Res 1995;29:482–9. [16] D’Alonzo AJ, Darbenzio RB, Parham CS, Grover GJ. Effects of intracoronary cromakalim on postischaemic contractile function and action potential duration. Cardiovasc Res 1992;26:1046–53. [17] Mei DA, Gross GJ. Evidence for the involvement of the ATPsensitive potassium channel in a novel model of hypoxic preconditioning in dogs. Cardiovasc Res 1995;30:222–30. [18] Jiang C, Mochizuki S, Poole-Wilson PA, Harding SE, MacLeod KT. Effect of levakalim on action potentials, intracellular calcium and contraction in guinea pig and human cardiac myocytes. Cardiovasc Res 1994;28:851–7. [19] Guo AC, Dianco J, Feuvray D. Comparison of effects of aprakalim and of hypoxic and ischaemic preconditioning on extracellular potassium accumulation, metabolism and functional recovery of the globally ischaemic rat heart. Cardiovasc Res 1994;28:864–71. [20] Russell DC, Oliver MF. Ventricular refractoriness during acute myocardial ischaemia and its relationship to ventricular fibrillation. Cardiovasc Res 1978;12:221–7. [21] Brown GB, Dzwonczyk R, Werman HA, Hamlin RL. Estimating the duration of ventricular fibrillation. Ann Emerg Med 1989;18:1181– 5. [22] Brown CG, Griffith RF, Van Ligten P, Hoekstra J, Nejman G, Mitchell L, Dzwonczyk R. Median Frequency: a new parameter for predicting defibrillation success rate. Ann Emerg Med 1991;20:787– 9.

M.T. Harbinson et al. / International Journal of Cardiology 76 (2000) 187 – 197 [23] Brown GB, Dzwonczyk R, Martin DR. Physiologic measurement of the ventricular fibrillation ECG signal: estimating the duration of ventricular fibrillation. Ann Emerg Med 1993;22:70–4. [24] Martin DR, Brown CG, Dzwonczjk R. Frequency analysis of the human and swine electrocardiogram during ventricular fibrillation. Resuscitation 1991;22:85–91. [25] Janse MJ, van Capelle FJL, Morsink H, Kleber AG, Wilms-Schopman F, Cardinal R et al. Flow of ‘injury’ current and patterns of excitation during early ventricular arrhythmias in acute regional myocardial ischemia in isolated porcine and canine hearts. Circ Res 1980;47:151–65. [26] Kern KB, Garewal HS, Sanders AB, Janas W, Nelson J, Sloan D, Tacker WA, Ewy GA. Depletion of myocardial adenosine triphosphate during prolonged untreated ventricular fibrillation: effect on defibrillation success. Resuscitation 1990;20:221–9. [27] Findlay I. Sulphonylurea drugs no longer inhibit ATP-sensitive K1 channels during metabolic stress in cardiac muscle. J Pharmacol Exp Ther 1993;266:456–67.

197

[28] Tacker WA, Niebauer MJ, Babbs CF, Combs WJ, Hahn BM, Barker MA, Seipel JF, Bourland JD, Geddes LA. The effect of newer antiarrhythmic drugs on defibrillation threshold. Crit Care Med 1980;8:177–80. [29] Kerber RE, Pandian NG, Jensen SR, Constantin L, Kieso RA, Melton J, Hunt M. Effect of lidocaine and bretylium on energy requirements for transthoracic defibrillation: Experimental studies. J Am Coll Cardiol 1986;7:397–405. [30] Koo CC, Allen JD, Pantridge JF. Lack of effect of bretylium tosylate on electrical ventricular defibrillation in a controlled study. Cardiovasc Res 1984;18:762–7. [31] Kerber RE, Pandian NG, Hoyt R, Jensen SR, Koyanagi S, Grayzel J, Kieso R. Effect of ischemia, hypertrophy, hypoxia, acidosis and alkalosis on canine defibrillation. Am J Physiol 1983;244:H825–31.