Effect Of Ivabradine-induced Heart Rate Reduction On Postresuscitation Myocardial Function And Its Mechanism

Background:About 90%of patients with out-of-hospital cardiac arrest die due to delayed treatment,while less than 10%of the remaining receive CPR.Of these,only 29.7%achieve restoration of spontaneous circulation.However,these survivors also have to experience a"second hit",that is,post-cardiac arrest syndrome.PRMD is a multiple organ dysfunction syndrome secondary to cardiac arrest and resuscitation.In the process of cardiac arrest and resuscitation,cardiac arrest leads to comprehensive ischemia,hypoxia,the release of inflammatory factors,and the production of various metabolic products.After resuscitation,reperfusion then induces obvious oxidative stress and other reactions and causes serious multiple organ dysfunction or disorder.myocardial dysfunction is a key component of post-cardiac arrest syndrome and an important cause of early death after resuscitation.It has also become an important target for intervention to reduce death after resuscitation.Under the situation of postresuscitation myocardial dysfunction,a rapid heart rate appears,which in a short time maintains sufficient cardiac output,while if of long duration,it further worsens the insulted myocardial dysfunction.Ivabradine is a simple heart rate inhibitor.Clinical research has illustrated the effectiveness of Ivabradine in treating chronic myocardial dysfunction.Additionally,several animal studies have also shown the beneficial effects of heart rate reduction induced by Ivabradine.Therefore,we speculate that Ivabradine-induced heart rate reduction can also improve myocardial function post-resuscitation and prolong the survival duration.Objective:In this study,we first set up a CPR model and test the appropriate dose of Ivabradine.Then we observe the follow-on effect of Ivabradine-induced heart rate reduction on myocardial function after cardiac arrest and resuscitation with this dose.Lastly,we explore the underlying mechanism for the effect.Method:（1）Part Ⅰ:Preliminary observation of heart rate and the intervention effect of Ivabradine after cardiac arrest and resuscitation.Sprague-Dawley rats were left untreated for 6minutes after alternating current-induced ventricular fibrillation（VF）,and then received8 minutes（8 minutes）of chest compression and mechanical ventilation（CPR）（6 min-VF+8 min-CPR）for 4 J DC defibrillation.The animals successfully resuscitated were randomly divided into 4 groups:normal saline group and Ivabradine high,medium,and low groups,with 2 in each group.One hour after resuscitation,normal saline,and Ivabradine hydrochloride were given intravenously at 2.0 mg/kg,1.0 mg/kg,and 0.5mg/kg,respectively.Heart rate and survival duration in each group were observed.At the same time,the right atrial pacing of rats was tested.（2）Part Ⅱ:Effect of Ivabradine-induced heart rate reduction on myocardial function after cardiac arrest and resuscitation.Sprague-Dawley rats were given 6 min-VF+8 min-CPR.The animals successfully resuscitated were randomly divided into the normal saline group and the Ivabradine group,and each group was subdivided into non-survival and survival subgroups（n=8 for each group）.0.5 mg/kg of normal saline or Ivabradine was given 1hour after resuscitation.Analyses of the heart rate（HR）,mean arterial pressure（MAP）,end-expiratory carbon dioxide（ETCO₂）,RR interval（RRI）,left ventricular filling time（LVFT）,and left ventricular end-diastolic volume（LVEDV）,Stroke volume（SV）,cardiac output（CO）,ejection fraction（EF）,myocardial performance index（MPI）and systemic vascular resistance（SVR）were done at baseline and every hour from 1 hour to 5 hours after resuscitation.The perfusion vessel density（PVD）and microvascular flow index（MFI）were measured at baseline and every hour from 1 hour to 5 hours after resuscitation.Analysis of the heart rate variability（HRV）was performed at baseline and 1,3,and 5hours after resuscitation;The levels of serum epinephrine and cardiac troponin I（cTnI）were measured at baseline and 1,3,and 5 hours after resuscitation in the non-surviving subgroup.The survival duration of the survival subgroup was observed.RRI and HR were calculated on a stable ECG lasting for 5 minutes;MAP is calculated from the formula mean arterial pressure=diastolic arterial pressure+1/3×（systolic arterial pressure-diastolic arterial pressure）;ETCO₂ reading is directly got from the detector;LVFT,LVEDV,SV,CO,EF,MPI were measured by echocardiography,in which LVFT was calculated using the derivative method of Tei et al.,LVEDV was calculated using the area length method,and LVOT-VTI was measured;EF（SV/LVEDV）,CO（HR×SV）,MPI（（a-b）/b,a=interval between mitral valve closure and opening,b=left ventricular ejection time）and SVR（（MAP-RAP）/CO）were calculated by formula method frequency domain analysis was performed using software HRVanalysis 1.0;PVD and MFI were detected by side flow dark-field imaging equipment.The levels of serum cTnI and epinephrine were measured with enzyme-linked immunosorbent assay kits.The survival duration was observed and recorded.（3）Part Ⅲ:Exploratory study on the mechanism underlying the follow-on effect of Ivabradine-induced heart rate reduction on postresuscitation myocardial function.Sprague-Dawley rats were randomly divided into three groups:SHAM（surgery only,not VF+CPR）,4 min-VF+8 min-CPR,and 6 min-VF+8 min-CPR.The left ventricular anterior papillary muscle of rats in the CPR group was taken at 1 hour after resuscitation（SHAM group at parallel time point）,and the parameters of passive tension,active tension,frequency-dependent,beta adrenergic-dependent,Ca²⁺-dependent contraction were measured in the tissue bath using the mechanical testing system.Observation of the effect of Ivabradine-induced heart rate reduction on the active and passive tension of papillary muscles was also done at the same time.Results:（1）In Part Ⅰ,a total of 11 animals was used,3 were excluded and 8 survived.The success rate of model-making is 73%.The baseline heart rate of rats is about 390 beats per minute（bpm）.After resuscitation,the heart rate of the rats was about 405 bpm,and the survival time was 500 minutes.One hour after resuscitation,the heart rate reduction corresponding to the administration of 2.0 mg/kg,1.0 mg/kg,and 0.5 mg/kg Ivabradine was 65 bpm（16%）,50 bpm（12.3%）,30 bpm（7.4%）,and the survival time was 60 minutes,90 minutes,and 300 minutes respectively.The right atrial pacing of rats was not achieved.（2）In Part Ⅱ,a total of 43 rats were used with the exclusion of 11 animals（6 with poor baseline data,5 without resuscitation）and 32 were successfully resuscitated.The baseline heart rate is about 390 bpm.The average increase in heart rate after resuscitation isΔ≈+15 bpm（relative ratio≈+3.8%）,making the heart rate 405 bpm and lasting more than 5hours.Compared with the control group,the Ivabradine group achievedΔ≈-30 bpm heart rate reduction（relative ratio≈-7.4%）.It has higher MAP and lower ETCO₂.RRI,LVFT,and LVED increased while SV decreased in the Ivabradine group.CO,EF,and MPI were significantly worse Ivabradine group.The change in HRV and the increase in serum epinephrine level were indicative of hyper-sympathetic activity.The microcirculation derangement got worse in the Ivabradine group.Additionally,the Ivabradine group showed deteriorated myocardium insult and shortened survival duration.（3）Part Ⅲ:compared with the SHAM group,the mechanical properties of the papillary muscles including passive tension,active tension,frequency-dependent,beta-adrenergic-dependent,and Ca²⁺-dependent contraction in the CPR groups were significantly inhibited and those mechanics in the CPR-2 group were significantly weaker than those in the CPR-1 group.Passive tension and active tension were not affected by Ivabradine in both SHAM and CPR groups.Conclusion:The phenomenon that"heart rate increases and lasts for a long time after cardiac arrest and resuscitation"has been repeatedly verified.Ivabradine has no direct negative inotropic effect.However,the slight heart rate reduction induced by Ivabradine significantly increases the severity of PRMD and significantly shortens the survival time.The underlying mechanism is related to the reduction of myocardial tension and contractility after cardiac arrest and resuscitation.The Frank-Starling mechanism cannot be effectively used to convert the slowing heart rate into increased myocardial contractility and SV.