Comparison of right ventricular impedance, pulse pressure and maximal dP/dt for determination of hemodynamic stability of ventricular arrhythmias associated with coronary artery disease

Determination of left ventricular volume changes by intracardiac conductance using a biventricular electrode configuration

AIMS

The feasibility of determining left ventricular (LV) volume changes by LV conductance measurements with an implantable device was investigated in an animal model.

METHODS AND RESULTS

The haemodynamic state of six mongrel dogs was altered by overpacing with rates up to 140 bpm and by isoprenaline infusion with dosages up to 0.2 microg/kg/min. The LV conductance, aortic blood flow, and LV and aortic pressure were recorded. Conductance measurements were carried out using the two electrodes of a bipolar right ventricular pacing lead for current injection and two epicardial leads screwed into the mid-lateral LV wall for measuring the resulting voltage. Stroke conductance (SY) was correlated with the LV stroke volume (LVSV), which was computed from the aortic flow. The LVSV rose to 188+/-14% with increasing isoprenaline dosage. A strong correlation between the LV conductance SY and the LVSV was found (mean r=0.97). The LVSV decreased to 68+/-8% with an increasing pacing rate. Again, a strong correlation between SY and LVSV was found (mean r=0.89).

CONCLUSION

This animal study confirms the feasibility of assessing changes in LVSV by determining the LV intracardiac conductance. This creates the possibility of continuous haemodynamic monitoring with implantable devices.

TGF-beta1 mediates the hypertrophic cardiomyocyte growth induced by angiotensin II

Angiotensin II (Ang II), a potent hypertrophic stimulus, causes significant increases in TGFb1 gene expression. However, it is not known whether there is a causal relationship between increased levels of TGF-beta1 and cardiac hypertrophy. Echocardiographic analysis revealed that TGF-beta1-deficient mice subjected to chronic subpressor doses of Ang II had no significant change in left ventricular (LV) mass and percent fractional shortening during Ang II treatment. In contrast, Ang II-treated wild-type mice showed a >20% increase in LV mass and impaired cardiac function. Cardiomyocyte cross-sectional area was also markedly increased in Ang II-treated wild-type mice but unchanged in Ang II-treated TGF-beta1-deficient mice. No significant levels of fibrosis, mitotic growth, or cytokine infiltration were detected in Ang II-treated mice. Atrial natriuretic factor expression was approximately 6-fold elevated in Ang II-treated wild-type, but not TGF-beta1-deficient mice. However, the alpha- to beta-myosin heavy chain switch did not occur in Ang II-treated mice, indicating that isoform switching is not obligatorily coupled with hypertrophy or TGF-beta1. The Ang II effect on hypertrophy was shown not to result from stimulation of the endogenous renin-angiotensis system. These results indicate that TGF-beta1 is an important mediator of the hypertrophic growth response of the heart to Ang II.

The measurement of impedance to assess myocardial contractility and rhythm stability

By introducing an alternating current through the ventricular cavity via a multipolar catheter the potential difference measured between adjacent electrodes can be used to assess ventricular volume. This is possible by the measurement of intracardiac impedance. The data obtained can be used to construct pressure-volume loops allowing the continuous real-time assessment of myocardial contractility. Clinical applications of this research tool are now becoming apparent. Further derivations of impedance measurement have been incorporated into commercially available permanent pacing systems, allowing effective heart rate modulation.

Evaluation of antiarrhythmic drug efficacy in patients with an ICD: unlimited potential or replete with complexity and problems?

There are a number of novel ways in which implantable cardioverter defibrillator (ICD) endpoints can be used in clinical trials to evaluate antiarrhythmic drugs. The advances in ICD technology (storage, retrieval, and accurate interpretation of ICD electrograms) expand the potential to include the use of an ICD endpoint as a clinical surrogate for sudden death. The ICD also provides the necessary safety net to test new drugs. The frequent need for antiarrhythmic drugs in patients already fitted with an ICD (e.g., for atrial fibrillation) necessitates knowledge of the drugs' effect on defibrillator threshold. There are interpretative problems and challenges associated with all types of ICD trials. A particular difficult issue is the degree to which the results of data on antiarrhythmic drug efficacy and safety acquired in the context of an ICD endpoint trial might be extrapolated to patient populations in which the device is not used. These and other challenging issues are discussed, with the goal of enhancing the design and interpretation of clinical trials featuring ICD endpoints.

Short-term reproducibility over time of right ventricular pulse pressure as a potential hemodynamic sensor for ventricular tachyarrhythmias

The implantable cardioverter defibrillator (ICD) has been shown to effectively terminate episodes of ventricular tachyarrhythmias. Multiple investigators have suggested that the incorporation of hemodynamic sensors may allow ICDs to differentiate between hemodynamically unstable and stable ventricular tachyarrhythmias (VT), as well as differentiate ventricular from supraventricular tachycardias. Right ventricular (RV) pulse pressure has been shown to possess acceptable characteristics as a sensor for incorporation in ICDs. We sought to determine the short-term reproducibility of RV pulse pressure measurements by comparing RV pulse pressure measured during two separate episodes of VT in each of ten study patients. The mean VT cycle length for VT episode 1 was 293 +/- 15 msec, and was 298 +/- 15 msec for VT episode 2 (P = NS). The decrease in mean arterial pressure was 40 +/- 7 mmHg in episode 1 and 37 +/- 7 mmHg in episode 2 (P = NS). The decrease in RV pulse pressure during episode 1 was -13 +/- 2 mmHg, and -12 +/- 2 during episode 2 (P = NS). The decrease in RV pulse pressure during episodes of VT at two different times during a single electrophysiology study is highly reproducible, suggesting that RV pulse pressure may be a reliable sensor over time.