Impedance Controlled Pacing Rate Limits in
Cardiac Pacemakers -
Experimental Validation on Isolated Heart

A Kink^a, I Rätsep^b, T Parve^a

^aInstitute of Electronics, Tallinn Technical University, Tallinn, Estonia
^bNorth Estonian Regional Hospital, Tallinn, Estonia

Correspondence: T Parve, Institute of Electronics, Tallinn Technical University, 19086 Tallinn, Estonia.
E-mail: parveto@edu.ttu.ee, phone +372 620 2150, fax +372 620 2151

1.  Introduction

Rate adaptive pacemakers are designed to follow the energy consumption of patient 's body W_body using information from sensors reacting to activity, acceleration, minute volume of ventilation, etc. (Fig.1), of the patient [Webster, 1995; Min et al., 2000]. But needs of the body which ought to be covered by the cardiac output CO, can exceed the ability of a deficient heart. Therefore, the maximal pacing rate limit is usually fixed (programmed) in the pacemaker to avoid overpacing of the heart. Several exercise tests and algorithms have been derived to determine the limit value of pacing rate for every patient individually [Parve et al., 2002].

2.  Conditions of Myocardium Energy Balance

The ability of heart to work at higher rates for coping with heavy workloads is found to be in correlation with a value of coronary reserve (CR). The pacing rate is only a parameter that is used for making necessary energetic regulations. The key problem is how to detect an early evidence of myocardium 's energy imbalance.

Figure 2. Ventricular Pressure-Volume loop.

Energy consumption (demand) or work W of the myocardium is characterized by the area S_dem of pressure-volume diagram (P-V loop), which describes relationship between ventricular volume and ventricular pressures (Fig.2). The energy supply E is proportional to pressure difference ΔP between arterial and ventricular pressures and a diastole period t_diast, which means that E is proportional to the area S_sup= ΔPΔt_diast. Thus the myocardium 's energy balance can be characterized as S_dem= S_sup. The area S_dem representing the energy W consumed by the myocardium, increases, but the area S_sup decreases with the increasing of the heart rate. Thus from the certain value of the heart rate the energy balance can not be maintained.

For estimation of the energy balance, the stroke volume SV and duration of diastole period t_diast, can be found from the measurement of the ventricular bioimpedance Z_V between a pacing tip electrode in apex, and an additional ring electrode in ventricle. The coronary reserve CR characterizes the ability of coronary arteries to widen during work. The CR can be defined through coronary resistance R as CR = R_rest/R_min. The current value of the ratio R_rest /R as a coronary resistance ratio can be presented as CRR = (t_diast,rest.AVD_rest.k_O2.SV)/(t_diast.AVD.k_O2,rest.SV_rest). Since k_O2,rest= k_O2, and denoting q = AVD_rest/AVD (q can vary from 1.0 to 0.5 in case of ischemic work of the myocardium) and knowing that AVD is almost constant for the load permitted for pacemaker patients (up to ischemic load limit) due to autonomous regulation of blood circulation inside the myocardium, the CRR can be expressed as CRR = (t_diast,rest/t_diast).(SV/SV_rest).q. So CRR expresses the degree of utilization of the coronary reserve CR: in case of CRR = CR the reserve is used completely. Taking q = 1, there is no overpacing risk and even ischemic operation of the myocardium can be avoided. The inequality describing a criterion for automatic pacing rate limitation is: (t_diast,rest/t_diast).(SV/SV_rest) ≦ CR.

3.  Results

Measurements were performed on 3 isolated human size blood perfused animal hearts. The setup and validation of the model has been described earlier [Parve et al., 2002]. Intraventricular pressure, volume and intracardiac electrical bioimpedance (EBI) were recorded in different energetic situations. Left ventricular P-V area measurements were recorded at pacing rates of 60, 80 and 100 bpm, and intraventricular balloon volume loads of 60, 80, 100, 120 and 140 ml.

Energy demand increases with the increase of SV and generated pressures (Fig.3). Intraventricular volume changes are correlated with the EBI (Fig.4).

Figure 3. P (y-axis) ‑ V (x-axis) loops at rate of 80 bpm.   Figure 4. End diastolic EBI (y-axis) and V correlation.

4.  Discussion and Conclusions

The cardiac energy balance conditions have been formulated through measurement and analysis of relative values ‑ stroke volume SV/SV_rest and diastole period t_diast,rest/t_diast, both available from intracardiac impedance. Furthermore, the described balance conditions have been used for automatic adjusting of the "floating" limits of the pacing rate in rate-adaptive pacemakers. The "floating" limits help pacemaker patients to survive in case of sudden decrease in the resistance of blood vessels or loss of the blood directly, both of them causing insufficiency and delay in the volume of returning blood.

Acknowledgements

This work was supported by Estonian Science Foundation, grant no.4859.

References

Parve T, Kink A, Min M. Using of bioimpedance for estimation of cardiac energy balance. In proceedings of the 1st World Congress on Biomimetics and Artificial Muscles, 2002, 6 p. (in press).

Design of Cardiac Pacemakers. Webster JG, Editor. IEEE Press, N.Y., 1995.

Kink A, Min M, Parve T. A Rate Adaptive Pacemaker. EP1169085 (WO0057954) and EP1169084 (WO0057953), 2000