White P A, Brookes C I, Ravn H B, Stenbøg E E, Christensen T D, Chaturvedi R R, Sorensen K, Hjortdal V E, Redington A N
Department of Paediatric Cardiology, Royal Brompton Hospital, London, UK.
Cardiovasc Res. 1998 Jun;38(3):668-75. doi: 10.1016/s0008-6363(98)00052-2.
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.
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].
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).
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.
使用电导导管技术测量心室容积的一个重要组成部分是由于平行电导（Vc），它是由电场延伸到心室血池之外导致的。平行电导容积通常使用盐水稀释法（Vc（盐水稀释））进行估算，在该方法中，通过注射高渗盐水使心室内血液的电导率暂时增加。Gawne等人[12]报道了一种更简单的替代方法。Vc（双频）是根据在两个激励频率下测得的总电导之差估算得出的，该方法基于这样的假设，即平行电导主要是电容性的，因此在低频时可忽略不计。本研究的目的是确定双频技术是否可用于替代盐水稀释法来估算不同大小心脏中的Vc。
首先在体外评估定制的电导导管（CC）系统的准确性和线性度。随后，将一根CC和微压计插入7只5千克猪（第1组）和6只50千克猪（第2组）的左心室。使用热稀释法（第1组）和超声流量探头（第2组）测定心输出量，并由此确定斜率系数（α）。在5 - 40千赫兹的频率范围内进行稳态测量，并使用盐水稀释法估算Vc。所有测量均在呼气末进行。最后，使用Gawne等人[12]的双频技术根据5千赫兹和40千赫兹之间收缩末期电导的变化估算Vc。
当刺激频率从5 - 40千赫兹变化时，简单绝缘圆柱形模型的测量容积没有变化。第1组中Vc（盐水稀释）随频率显著变化（5千赫兹时为8.63±2.74毫升；40千赫兹时为11.51±2.65毫升）（p = 0.01）。第2组也得到了类似结果（5千赫兹时为69.43±27.76毫升；40千赫兹时为101.24±15.21毫升）（p < 0.001）。然而，数据表明平行电导的电阻性成分很大（第1组0赫兹时估算的Vc为8.01毫升，第2组为62.3毫升）。两组中α均随频率增加，但未达到显著水平。Vc（双频）方法与Vc（盐水稀释）方法之间的对应性较差（第1组R2 = 0.69；第2组R2 = 0.22）。
在5千赫兹的较低激励频率下，延伸到血池之外的电流百分比更小，因此平行电导降低。虽然平行电导与频率有关，但它有很大的电阻性成分。双频方法基于平行电导在低频时可忽略不计的假设，而实际情况显然并非如此。本研究结果证实双频技术不能用于替代盐水稀释技术。
