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The effect of changing excitation frequency on parallel conductance in different sized hearts
P A White1, C I Brookes, H B Ravn
1Department of Paediatric Cardiology, Royal Brompton Hospital, London, UK.
Insights
The dual frequency technique for estimating parallel conductance (Vc) in the heart is unreliable because parallel conductance has a significant resistive component, unlike the method
Area of Science:
- Cardiovascular physiology
- Biomedical engineering
- Electrophysiology
Background:
- Ventricular volume measurement using conductance catheters is affected by parallel conductance (Vc).
- Vc arises from the electric field extending beyond the ventricular blood pool.
- The saline dilution method is the standard for estimating Vc, but a simpler dual frequency technique has been proposed.
Purpose of the Study:
- To evaluate if the dual frequency technique can replace the saline dilution method for Vc estimation.
- To assess Vc estimation in different-sized animal hearts.
Main Methods:
- A custom-built conductance catheter system was validated in vitro.
- Conductance catheters and micromanometers were implanted in the left ventricles of pigs (5 kg and 50 kg).
- Parallel conductance was estimated using both saline dilution and the dual frequency technique across a range of frequencies (5-40 kHz).
Main Results:
- Parallel conductance (Vc) varied significantly with frequency in both pig groups using the saline dilution method.
- The dual frequency technique showed poor correlation with the saline dilution method (R² = 0.69 in group 1, R² = 0.22 in group 2).
- A substantial resistive component of parallel conductance was identified, contradicting the dual frequency method's assumption.
Conclusions:
- Parallel conductance is frequency-dependent and possesses a significant resistive component.
- The assumption that parallel conductance is negligible at low frequencies is incorrect.
- The dual frequency technique is not a suitable substitute for the saline dilution method in estimating parallel conductance.
Objective:
An important component of the ventricular volume measured using the conductance catheter technique is due to parallel conductance (Vc), which results from the extension of the electric field beyond the ventricular blood pool. Parallel conductance volume is normally estimated using the saline dilution method (Vc(saline dilution)), in which the conductivity of blood in the ventricle is transiently increased by injection of hypertonic saline. A simpler alternative has been reported by Gawne et al. [12]. Vc(dual frequency) is estimated from the difference in total conductance measured at two exciting frequencies and the method is based on the assumption that parallel conductance is mainly capacitive and hence is negligible at low frequency. The objective of this study was to determine whether the dual frequency technique could be used to substitute the saline dilution method to estimate Vc in different sized hearts.
Methods:
The accuracy and linearity of a custom-built conductance catheter (CC) system was initially assessed in vitro. Subsequently, a CC and micromanometer were inserted into the left ventricle of seven 5 kg pigs (group 1) and six 50 kg pigs (group 2). Cardiac output was determined using thermodilution (group 1) and an ultrasonic flow probe (group 2) from which the slope coefficient (alpha) was determined. Steady state measurements and Vc estimated using saline dilution were performed at frequencies in the range of 5-40 kHz. All measurements were made at end-expiration. Finally, Vc was estimated from the change in end-systolic conductance between 5 kHz and 40 kHz using the dual frequency technique of Gawne et al. [12].
Results:
There was no change in measured volume of a simple insulated cylindrical model when the stimulating frequency was varied from 5-40 kHz. Vc(saline dilution) varied significantly with frequency in group 1 (8.63 +/- 2.74 ml at 5 kHz; 11.51 +/- 2.65 ml at 40 kHz) (p = 0.01). Similar results were obtained in group 2 (69.43 +/- 27.76 ml at 5 kHz; 101.24 +/- 15.21 ml at 40 kHz) (p < 0.001). However, the data indicate that the resistive component of the parallel conductance is substantial (Vc at 0 Hz estimated as 8.01 ml in group 1 and 62.3 ml in group 2). There was an increase in alpha with frequency in both groups but this did not reach significance. The correspondence between Vc(dual frequency) and Vc(saline dilution) methods was poor (group 1 R2 = 0.69; group 2 R2 = 0.22).
Conclusion:
At a lower excitation frequency of 5 kHz a smaller percentage of the electric current extends beyond the blood pool so parallel conductance is reduced. While parallel conductance is frequency dependent, it has a substantial resistive component. The dual frequency method is based on the assumption that parallel conductance is negligible at low frequencies and this is clearly not the case. The results of this study confirm that the dual frequency technique cannot be used to substitute the saline dilution technique.