Global and regional myocardial oxygen consumption and blood flow in severe cardiomyopathy with left bundle branch block

Localization of myocardial FDG uptake for prognostic risk stratification in corticosteroid-naïve cardiac sarcoidosis

BACKGROUND

The localization of myocardial 18F-fluorodeoxyglucose (FDG) uptake affecting long-term clinical outcomes has not been elucidated in patients with corticosteroid-naïve cardiac sarcoidosis (CS).

OBJECTIVES

This study sought to investigate the localization of myocardial FDG uptake on positron emission tomography (PET) and myocardial perfusion abnormality to predict adverse events (AEs) for a long-term follow-up in patients with corticosteroid-naïve CS.

METHODS

Consecutive 90 patients with clinical suspicion of CS who underwent FDG-PET imaging to assess for inflammation were enrolled. AEs were defined as a composite of sustained ventricular tachycardia (VT), heart transplantation, and all-cause death, which were ascertained by medical records, defibrillator interrogation, and telephone interviews.

RESULTS

Of 90 patients, 42 patients (mean age 62.9 ± 12.0 years; 76.2% females) were confirmed active cardiac involvement. Over a median follow-up of 4.9 years, 15 patients with CS experienced AEs including 6 sustained ventricular tachycardias (VT) and 9 deaths. Cox proportional-hazards model after adjustment for left ventricular systolic dysfunction revealed that FDG uptake in the right ventricle (RV) or basal anterolateral area of the left ventricle (LV) with myocardial perfusion abnormality was predictive of AEs.

CONCLUSIONS

FDG uptake in the RV or basal anterolateral area of the LV with myocardial perfusion abnormality provides long-term prognostic risk stratification in patients with corticosteroid-naïve CS.

Pacing-induced cardiomyopathy in chronic right ventricular apical pacing: a midterm follow-up study

Background

Data concerning the effect of chronic right ventricular pacing in patients with normal left ventricular ejection fraction (LVEF%) are contradictory. The aim of this study is to evaluate the prevalence of pacing-induced cardiomyopathy (PICM) at midterm follow-up after permanent pacemaker implantation (PPM).

Methods

A series of 170 patients were submitted to PPM within our facility. Inclusion criteria were the absence of structural heart disease and a preserved LVEF% (> 45%) at the time of PPM. A midterm clinical and echocardiographic follow-up was performed, and data were collected and analyzed retrospectively. PICM was defined as follow-up LVEF ≤ 45%, dyskinesia during RV pacing, and the absence of other known causes of cardiomyopathy.

Results

At a median echocardiographic follow-up of 24.5 months (IQR 10.0–43.0 months), the overall mean LVEF% decreased from a preimplantation value of 66.7% (± 8.6%) to 63.2% (± 10.6%) (p < 0.0001). PICM occurred in 11 patients (6.5%). Patients developing PICM had a significantly lower preimplantation LVEF% (58.4 ± 8.0% vs. 67.3 ± 8.4%; p = 0.005), a trend for higher right ventricular pacing time rate (0.7 ± 0.3 vs. 0.5 ± 0.4; p = 0.1), a significantly lower rate of PPM indication for sick sinus syndrome (SSS) (18.2% vs. 61.0%; p = 0.009), and significantly higher rate of second-grade cardiac conduction block (36.4% vs. 11.3%; p = 0.03). At multivariate logistic regression, only preimplantation LVEF% (OR = 0.88; CI 0.80–0.96; p = 0.006) and the presence of SSS (OR = 0.1; CI 0.03–0.9; p = 0.04) were independently related (inverse relationship) to follow-up PICM.

Conclusions

In this selected PPM patient cohort with preserved LVEF%, the rate of PICM at midterm follow-up is relatively low, but its occurrence seems to be related to baseline LVEF% and PPM indication category.

Intracoronary administration of nicorandil during primary percutaneous coronary intervention: Impact on restoration of regional myocardial perfusion in reperfused myocardium during the subacute phase of myocardial infarction

Background

The impact of nicorandil as adjunctive therapy for percutaneous coronary intervention (PCI) in patients with ST-elevation myocardial infarction (STEMI) is controversial. We performed ¹⁵O-labeled water positron emission tomography (PET) to quantify regional myocardial perfusion in patients with STEMI who received nicorandil or no adjunctive therapy during PCI.

Methods

PCI was performed within 8 h after STEMI onset in 33 patients. 14 patients received intracoronary nicorandil 2 mg immediately after recanalization of the culprit lesion (Nico group). After 3–4 weeks, PET was performed in which myocardial blood flow (MBF) was measured at baseline and during adenosine triphosphate (ATP)-induced hyperemia. Myocardial vascular resistance (MVR) was calculated for all segments. Data were obtained from the reperfused (Rep) and normal segments (Cont) in each patient.

Results

In patients not given nicorandil (No-Nico group), the MBF was significantly lower in Rep than that in Cont at baseline and during hyperemia (Cont vs. Rep: 0.82 ± 0.14 vs. 0.68 ± 0.11, P = 0.001, ATP-Cont vs. ATP-Rep: 2.00 ± 0.72 vs. 1.52 ± 0.61, P = 0.017), which was restored in the Nico group (Cont vs. Rep: 0.79 ± 0.17 vs. 0.78 ± 0.20; ATP-Cont vs. ATP-Rep: 2.02 ± 0.84 vs. 1.84 ± 0.62). MVR was elevated in Rep at baseline and during hyperemia in the No-Nico group. MVR elevation in Rep was prevented in the Nico group.

Conclusions

¹⁵O-labeled water PET was feasible for segmental analysis of MBF during the subacute phase of STEMI. It revealed that intracoronary administration of nicorandil to STEMI patients who underwent PCI prevented MVR elevation and thus restored MBF in the reperfused segments to a level similar to that in the normal segments.

•

¹⁵O-labeled water positron emission tomography (PET) is a tool to compare directly reperfused segments with normal segments simultaneously in each patient's hearts.

•

¹⁵O-labeled water PET permits noninvasive quantitative measurement of myocardial blood flow during the subacute phase of myocardial infarction.

•

¹⁵O-labeled water PET detects restoration of myocardial blood flow after intracoronary administration of nicorandil in STEMI patients.

Cardiac resynchronization therapy is appropriate for all patients requiring chronic right ventricular pacing: the con perspective

There has been compelling evidence for the use of biventricular (BiV) pacing to reduce mortality and heart failure (HF) hospitalization in patients with HF symptoms, depressed ejection fraction (EF), and a broad QRS. Cardiac resynchronization therapy (CRT) provides benefit for patients with depressed EF and minimal HF symptoms. Although data from CRT trials are compelling, clinical evidence to recommend BiV pacing to all patients who require a high degree of right ventricular (RV) pacing are lacking. This article summarizes the relevant studies demonstrating that BiV pacing is not appropriate therapy for all patients who require chronic RV pacing.

Septal and anterior reverse mismatch of myocardial perfusion and metabolism in patients with coronary artery disease and left bundle branch block

The effects of left bundle branch block (LBBB) on left ventricular myocardial metabolism have not been well investigated. This study evaluated these effects in patients with coronary artery disease (CAD).Sixty-five CAD patients with complete LBBB (mean age, 61.8 ± 9.7 years) and 65 without LBBB (mean age, 59.9 ± 8.4 years) underwent single photon emission computed tomography, positron emission tomography, and contrast coronary angiography. The relationship between myocardial perfusion and metabolism and reverse mismatch score, and that between QRS length and reverse mismatch score and wall motion score were evaluated.The incidence of left ventricular septum and anterior wall reverse mismatching between the two groups was significantly different (P < 0.001 and P = 0.002, respectively). The incidences of normal myocardial perfusion and metabolism in the left ventricular lateral and inferior walls were also significantly different between the two groups (P < 0.001 and P < 0.001, respectively). The incidence of septal reverse mismatching in patients with mild to moderate perfusion was significantly higher among those with LBBB than among those without LBBB (P < 0.001). In CAD patients with LBBB, septal reverse mismatching was significantly more common among those with mild to moderate perfusion than among those with severe perfusion defects (P = 0.002). The correlation between the septal reverse mismatch score and QRS length was significant (P = 0.026).In patients with CAD and LBBB, septal and anterior reverse mismatching of myocardial perfusion and metabolism was frequently present; the septal reverse mismatch score negatively correlated with the QRS interval.

Non-invasive assessment of coronary artery disease in patients with left bundle branch block

There is a high prevalence of coronary artery disease (CAD) in patients with left bundle branch block (LBBB); however there are many other causes for this electrocardiographic abnormality. Non-invasive assessment of these patients remains difficult, and all commonly used modalities exhibit several drawbacks. This often leads to these patients undergoing invasive coronary angiography which may not have been necessary. In this review, we examine the uses and limitations of commonly performed non-invasive tests for diagnosis of CAD in patients with LBBB.

Cardiac resynchronization therapy and AV optimization increase myocardial oxygen consumption, but increase cardiac function more than proportionally

BACKGROUND

The mechanoenergetic effects of atrioventricular delay optimization during biventricular pacing ("cardiac resynchronization therapy", CRT) are unknown.

METHODS

Eleven patients with heart failure and left bundle branch block (LBBB) underwent invasive measurements of left ventricular (LV) developed pressure, aortic flow velocity-time-integral (VTI) and myocardial oxygen consumption (MVO2) at 4 pacing states: biventricular pacing (with VV 0 ms) at AVD 40 ms (AV-40), AVD 120 ms (AV-120, a common nominal AV delay), at their pre-identified individualised haemodynamic optimum (AV-Opt); and intrinsic conduction (LBBB).

RESULTS

AV-120, relative to LBBB, increased LV developed pressure by a mean of 11(SEM 2)%, p=0.001, and aortic VTI by 11(SEM 3)%, p=0.002, but also increased MVO2 by 11(SEM 5)%, p=0.04. AV-Opt further increased LV developed pressure by a mean of 2(SEM 1)%, p=0.035 and aortic VTI by 4(SEM 1)%, p=0.017. MVO2 trended further up by 7(SEM 5)%, p=0.22. Mechanoenergetics at AV-40 were no different from LBBB. The 4 states lay on a straight line for Δexternal work (ΔLV developed pressure × Δaortic VTI) against ΔMVO2, with slope 1.80, significantly >1 (p=0.02).

CONCLUSIONS

Biventricular pacing and atrioventricular delay optimization increased external cardiac work done but also myocardial oxygen consumption. Nevertheless, the increase in cardiac work was ~80% greater than the increase in oxygen consumption, signifying an improvement in cardiac mechanoenergetics. Finally, the incremental effect of optimization on external work was approximately one-third beyond that of nominal AV pacing, along the same favourable efficiency trajectory, suggesting that AV delay dominates the biventricular pacing effect - which may therefore not be mainly "resynchronization".

Devices in the management of advanced, chronic heart failure

Heart failure (HF) is a global phenomenon, and the overall incidence and prevalence of the condition are steadily increasing. Medical therapies have proven efficacious, but only a small number of pharmacological options are in development. When patients cease to respond adequately to optimal medical therapy, cardiac resynchronization therapy has been shown to improve symptoms, reduce hospitalizations, promote reverse remodelling, and decrease mortality. However, challenges remain in identifying the ideal recipients for this therapy. The field of mechanical circulatory support has seen immense growth since the early 2000s, and left ventricular assist devices (LVADs) have transitioned over the past decade from large, pulsatile devices to smaller, more-compact, continuous-flow devices. Infections and haematological issues are still important areas that need to be addressed. Whereas LVADs were once approved only for 'bridge to transplantation', these devices are now used as destination therapy for critically ill patients with HF, allowing these individuals to return to the community. A host of novel strategies, including cardiac contractility modulation, implantable haemodynamic-monitoring devices, and phrenic and vagus nerve stimulation, are under investigation and might have an impact on the future care of patients with chronic HF.

Relationship between mechanical and metabolic dyssynchrony with left bundle branch block: evaluation by 18-fluorodeoxyglucose positron emission tomography in patients with non-ischemic heart failure

BACKGROUND

Ventricular dyssynchrony is a common finding in patients with heart failure (HF), especially in the presence of conduction delays. The loss of ventricular synchrony leads to progressive impairment of contractile function, which may be explained in part by segmental abnormalities of myocardial metabolism. However, the association of these metabolic disarrangements with parameters of ventricular dyssynchrony and electrocardiography (ECG) findings has not yet been studied.

METHODS

Our aim was to determine the correlation between the presence of left bundle branch block (LBBB) with left ventricular (LV) mechanical synchrony assessed by multiple-gated acquisition scan (MUGA) and with patterns of 18-fluorodeoxyglucose (18FDG) uptake in patients with non-ischemic heart failure. Twenty-two patients with non-ischemic cardiomyopathy, LV ejection fraction (LVEF) ≤45% and New York Heart Association (NYHA) Functional Class II or III symptoms under standard medical therapy were included, along with 10 healthy controls matched for age and gender. A 12-lead ECG was obtained to measure the length of the QRS. Mechanical LV synchrony was assessed by MUGA using phase analysis. All patients and controls underwent positron emission tomography with 18FDG to determine the distribution of myocardial glucose uptake. The standard deviation of peak (18)FDG uptake was used as an index of metabolic heterogeneity. Student's t-test and Pearson's correlation were used for statistical analysis.

RESULTS

The mean age of the patients with HF was 54 ± 12 years and 72% were male. The length of the QRS was 129 ± 31 milliseconds and LBBB was present in 9 patients. Patients with HF had decreased LV 18FDG uptake compared with controls (7.56 ± 3.36 vs. 11.63 ± 4.55 standard uptake value; p = 0.03). The length of the QRS interval correlated significantly with glucose uptake heterogeneity (r = 0.62; p = 0.002) and mechanical dyssynchrony (r = 0.63; p = 0.006). HF patients with LBBB showed marked glucose uptake heterogeneity compared with HF patients without LBBB (41.4 ± 10 vs 34.7 ± 4.9 ml/100 g/min, respectively; p = 0.01).

CONCLUSIONS

Patients with non-ischemic heart failure exhibit a global decrease in myocardial glucose uptake. Within this group, subjects who also have LBBB exhibit a marked heterogeneity in segmental glucose uptake, which directly correlates with QRS duration.

[11C]Acetate rest-stress protocol to assess myocardial perfusion and oxygen consumption reserve in a model of congestive heart failure in rats

This study describes an [(11)C]acetate rest-stress method to obtain an indirect estimate of myocardial blood flow (MBF) and myocardial oxygen consumption (MVO(2)) in rats. Doxorubicin cardiotoxicity was used to test the usefulness of this approach for the assessment of congestive heart failure.

METHODS

[(11)C]Acetate rest-stress studies have been used in clinical research to assess the capacity of the coronary arteries to respond to stress. In this article, we used this approach to assess the cardiotoxicity of doxorubicin in a rat model. The method was first validated in a group of healthy rats and then used to follow the effect of doxorubicin chemotherapy on cardiac function. The effect of doxorubicin on myocardial perfusion and oxygen consumption reserve was measured at rest and under dobutamine stimulation.

RESULTS

Validation of the protocol showed a good correlation between the MBF and MVO(2) (r(2)=.68). The doxorubicin-treated group showed a significant (P=.04) decrease in cardiovascular perfusion reserve at 1.3±0.2 compared with the control animals at 1.6±0.2. Similar results were obtained for the MVO(2) reserve (treated 1.8±0.4 vs. controls 2.3±0.3; P=.02).

CONCLUSIONS

We describe an [(11)C]acetate PET rest-stress protocol for the assessment of congestive heart failure in rats and its application to the follow-up of cardiotoxicity under doxorubicin chemotherapy. This is a rapid and reliable approach to the measurement of cardiac perfusion and oxygen consumption reserve that could be applied to the development of new strategies to reduce the cardiotoxicity of anthracycline.

Heterogeneous myocardial FDG uptake and the disease activity in cardiac sarcoidosis

OBJECTIVES

This study evaluated the usefulness of fasting (18)F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) in the diagnosis and management of cardiac sarcoidosis (CS) and compared it with FDG uptake in dilated cardiomyopathy (DCM).

BACKGROUND

Cardiac sarcoidosis may clinically present as DCM but is amenable to systemic corticosteroid therapy if disease activity is high. Although alterations of FDG uptake have been reported in CS, limited information is available on the quantitative estimates of FDG uptake.

METHODS

Fasting FDG-PET was performed in 24 systemic sarcoidosis patients and was compared with 8 age-matched DCM patients. FDG-PET was also performed in 15 age-matched healthy control subjects. Twelve of the 24 sarcoidosis patients had cardiac involvement based on criteria established by the Japanese Ministry of Health and Welfare; the remaining 12 of 24 patients revealed no evidence of cardiac involvement. The myocardial FDG uptake was quantified by measuring the standardized uptake value in 17 myocardial segments in each subject. Coefficient of variation (COV), which equals the standard deviation of uptake divided by the average uptake of 17 segments, was calculated as an index of heterogeneity in the heart.

RESULTS

The FDG uptake was distinctly heterogeneous in CS patients. The COV value was significantly greater in CS patients (0.25 ± 0.05) than control subjects (0.14 ± 0.03, p < 0.01), sarcoidosis patients without cardiac involvement (0.14 ± 0.03, p < 0.01), or DCM patients (0.15 ± 0.02, p < 0.01). The COV value in DCM patients was similar to control subjects or sarcoidosis patients without cardiac involvement. The cutoff COV value for the diagnosis of CS was 0.18 (sensitivity: 100%; specificity: 97%). After corticosteroid therapy in CS patients, the COV value was decreased to 0.14 ± 0.06 (p < 0.05) and became essentially similar to the other groups.

CONCLUSIONS

Heterogeneous myocardial FDG uptake may be a useful diagnostic marker of disease activity for CS.

Spontaneous remission of left bundle branch block immediately after denitrogenation with inhalation of 100% oxygen prior to anesthetic induction

Complete left bundle branch blocks have focused our attention, primarily because they are a sign predictive of mortality and a predictor of coexisting cardiovascular disease. Left bundle branch block (LBBB) is usually permanent but may occur transiently or intermittently. Spontaneous remission of LBBB rarely occurs during anesthesia. We present two patients in whom chronic LBBB reverted to normal sinus rhythm shortly following denitrogenation with inhalation of 100% oxygen during induction of anesthesia.

Radionuclide angiographic determination of regional left ventricular systolic function during rest and exercise in patients with nonischemic cardiomyopathy treated with cardiac resynchronization therapy

Cardiac resynchronization therapy (CRT) can improve global left ventricular (LV) function. However, limited data are available on regional LV contractility at rest and during exercise. The aim of the present study was to prospectively investigate the effects of CRT on regional LV ejection fraction (EF), global LVEF, and dyssynchrony, during rest and exercise, using radionuclide angiography. A total of 32 consecutive patients with heart failure and nonischemic cardiomyopathy underwent technetium-99m radionuclide angiography with bicycle exercise immediately after CRT implantation (during spontaneous rhythm and after CRT activation) and 3 months later. The regional EF was assessed in the interventricular septum and the lateral wall (LW). Intraventricular dyssynchrony was evaluated using Fourier phase analysis. During spontaneous rhythm, the EF was severely depressed in the septum compared to in the LW. CRT improved septal EF at rest and during exercise both at baseline (p <0.001) and after 3 months (p <0.05). The basal LW EF decreased during CRT (p <0.05, both at rest and during exercise). LV dyssynchrony decreased both at baseline and during follow-up, and the global LVEF showed improvement only at 3 months (p <0.001). In conclusion, in patients with nonischemic cardiomyopathy, CRT affects regional LV function by increasing the septal EF and reducing LW contractility, both at rest and during exercise. This was associated with an improvement in global LVEF and dyssynchrony.

Comparison between the prognostic value of left ventricular function and myocardial perfusion reserve in patients with ischemic heart disease

The purpose of this study was to compare the prognostic value of left ventricular ejection fraction (LVEF) and myocardial perfusion reserve (MPR) assessed with PET in patients with ischemic heart disease (IHD). Myocardial perfusion is the main determinant of left ventricular function in patients with IHD. The prognostic value of LVEF has been widely established. In addition, MPR determines survival in patients with hypertrophic and dilated cardiomyopathies. In the present study, we evaluated whether MPR also determines survival in patients with IHD.

METHODS

Between 1995 and 2003, 480 consecutive patients with chronic IHD underwent dipyridamole stress and rest 13N-ammonia PET to determine MPR. Additionally, 18F-FDG PET was performed for viability (mismatching defects), infarction (matching defects), and left ventricular function assessment. Patients were followed for all causes of mortality and major cardiovascular events.

RESULTS

In 463 of the 480 patients, valid MPR could be measured (368 men; mean age, 66+/-11 y; LVEF, 35%+/-15%). One hundred nineteen patients underwent a PET-driven revascularization (67 through percutaneous coronary intervention and 52 through coronary artery bypass grafting). The remaining 344 patients were the subject of this study. The overall MPR was 1.71+/-0.50 (intertertile boundaries, 1.49 and 1.84). After adjustment for age and sex, MPR was associated with a hazard ratio for cardiac death of 4.11 (95% confidence interval, 2.98-5.67) per SD decrease, whereas the risk for LVEF was 2.76 (2.00-3.82) per SD decrease.

CONCLUSION

Patients with IHD with a low MPR are at high risk of cardiac death. MPR is a more sensitive predictor for cardiac death than is LVEF.

Effects of cardiac resynchronization therapy on coronary blood flow: evaluation by transthoracic Doppler echocardiography

BACKGROUND

Relatively limited and conflicting data are available on the effects of cardiac resynchronization therapy (CRT) on coronary blood flow (CBF).

AIMS

To investigate changes in the left anterior descending coronary artery (LAD) flow under different CRT pacing modes by means of transthoracic Doppler echocardiography (TTE).

METHODS

Twenty-two responders to CRT (67+/-11 years) with idiopathic dilated cardiomyopathy underwent TTE assessment of LAD flow and Tissue Velocity Imaging during 4 programming modes: intrinsic conduction (IC), right ventricular pacing (RV), simultaneous biventricular pacing (BVP), BVP with left ventricular (LV) pre-activation.

RESULTS

Mean coronary flow velocity (CFV) was increased by simultaneous BVP (p=0.0063 vs. IC) and BVP with LV pre-activation (p<0.0001 vs. IC; p=0.027 vs. simultaneous BVP). Peak CFV and LAD flow velocity/time integral were highest during BVP with LV pre-activation. A reduction in septal-to-lateral delay and an increase in peak systolic velocity in the basal septum were observed during simultaneous BVP and BVP with LV pre-activation.

CONCLUSIONS

In CRT responders with idiopathic dilated cardiomyopathy, an increase in LAD flow, assessed by TTE, was observed during simultaneous BVP and BVP with LV pre-activation. This was associated with an improvement in regional myocardial contraction and a decrease in intraventricular dyssynchrony.

Effects of cardiac resynchronization therapy on disease progression in chronic heart failure

Despite the alleviation of symptoms and longer survival conferred by pharmacological management of chronic congestive heart failure (CHF), this progressive syndrome remains associated with high morbidity and premature death. A new treatment of CHF should ideally alleviate symptoms, improve functional capacity, decrease mortality, and slow or reverse its progression without adding risks for the patient that outweighs the benefits. Growing evidence indicates that devices implanted to resynchronize ventricular contraction are a beneficial adjunct in the treatment of CHF. This review discusses the remodelling process, and its clinical and prognostic significance. We also discuss the impact of CRT, on remodelling and disease progression with a particular focus on patients with asymptomatic or mild heart failure (NYHA Class I-II).

Electromechanics of paced left ventricle simulated by straightforward mathematical model: comparison with experiments

Intraventricular synchrony of cardiac activation is important for efficient pump function. Ventricular pacing restores the beating frequency but induces more asynchronous depolarization and more inhomogeneous contraction than in the normal heart. We investigated whether the increased inhomogeneity in the left ventricle can be described by a relatively simple mathematical model of cardiac electromechanics, containing normal mechanical and impulse conduction properties. Simulations of a normal heartbeat and of pacing at the right ventricular apex (RVA) were performed. All properties in the two simulations were equal, except for the depolarization sequence. Simulation results of RVA pacing on local depolarization time and systolic midwall circumferential strain were compared with those measured in dogs, using an epicardial sock electrode and MRI tagging, respectively. We used the same methods for data processing for simulation and experiment. Model and experiment agreed in the following aspects. 1) Ventricular pacing decreased systolic pressure and ejection fraction relative to natural sinus rhythm. 2) Shortening during ejection and stroke work declined in early depolarized regions and increased in late depolarized regions. 3) The relation between epicardial depolarization time and systolic midwall circumferential strain was linear and similar for the simulation (slope = -3.80 +/- 0.28 s(-1), R2 = 0.87) and the experiments [slopes for 3 animals -2.62 +/- 0.43 s(-1) (R2 = 0.59), -2.97 +/- 0.38 s(-1) (R2 = 0.69), and -4.44 +/- 0.51 s(-1) (R2 = 0.76)]. We conclude that our model of electromechanics is suitable to simulate ventricular pacing and that the apparently complex events observed during pacing are caused by well-known basic physiological processes.