© 1998 by European Society of Cardiology
Copyright © 1998, European Society of Cardiology
The effect of changing excitation frequency on parallel conductance in different sized hearts
aDepartment of Paediatric Cardiology, Royal Brompton Hospital, Sydney Street, London SW3 6NP, UK
bDepartment of Adult Cardiology, Royal Brompton Hospital, Sydney Street, London SW3 6NP, UK
cInstitute of Experimental Clinical Research, Aarhus University Hospital, Aarhus, Denmark
dDepartment of Cardiology, Aarhus University Hospital, Aarhus, Denmark
* Corresponding author. Tel: +44 (171) 351 8546; Fax: +44 (171) 351 8545; E-mail: reding@ibm.net
Received 10 September 1997; accepted 3 February 1998
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Abstract |
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 Top Abstract 1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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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 (
) 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 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.
KEYWORDS Pig; Neonatal; Adult; Left ventricle; Conductance catheter; Saline wash-in; Parallel conductance; Dual frequency excitation
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1 Introduction |
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TopAbstract1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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The conductance catheter technique has successfully been used to study ventricular pressure volume relationships in vivo in animals [1–3], and humans [4, 5], in both the left and right ventricles [6–8]. The calculation of ventricular contractility from these volume measurements has become increasingly popular in humans both perioperatively and in the cardiac catheterisation laboratories, but a potential drawback of this technique is the complexity of absolute volume determination. The volume signal obtained includes a proportion of the actual volume (reflected by the dimensionless gain constant,
, and an element of additional volume measured beyond the ventricular blood pool (parallel conductance). Alpha is measured by calibration against a known ventricular volume, e.g. thermodilution [4, 5, 8], flow probe stroke volume [4, 8], or angiographic assessments [4]. Parallel conductance needs to be measured directly. The conductance volumetric principle is based on the assumption that the electric field is homogenous and parallel to the longitudinal axis of the ventricle and that all current is contained within the ventricular cavity. However, parallel conductance (Vc) occurs because cardiac muscle is not a perfect insulator and a component of the volume signal arises from the conductance of structures extrinsic to the ventricular blood pool. Several investigators have chosen not to quantify the parallel conductance [3, 9–11]and have instead presented relative volumes. While still useful for some applications calibration to absolute volume markedly strengthens the utility of measurements made with the catheter system.
Parallel conductance (Vc(saline dilution)) is normally estimated by injection of hypertonic saline, which transiently changes the conductivity of the blood in the ventricle [4]. This technique has the disadvantage of inappropriate saline loading with multiple measurements and cannot be performed during transient changes in the loading conditions of the ventricle, such as snaring of the inferior vena cava.
To overcome these weaknesses, Gawne proposed a method that exploits the difference in conductivity in muscle and blood at different frequencies [12]. The standard saline dilution technique is usually performed at an excitation frequency of 20 kHz [1, 2, 4, 6]. However, blood has essentially a constant conductivity over the range of frequencies from 2 to 100 kHz [13]. Muscle, is more conductive at frequencies higher than 12 kHz [14]. Thus it might be possible to eliminate or reduce the myocardial component by altering the stimulating frequency. Indeed, Gawne [12]using a swine model, suggested that a dual frequency method could reliably substitute the saline dilution technique in potentially determining species specific parallel conductance, assuming that parallel conductance is mainly capacitive and hence negligible at low frequencies.
This study further investigates and develops the use of a multiple frequency excitation technique over the range of 5–40 kHz in neonatal and adult pigs. We examined the changes in Vc with increasing frequency and determined whether a dual frequency technique could be used as an alternative to the saline dilution technique in different sized hearts.
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2 Conductance catheter |
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TopAbstract1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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The principles and techniques of the conductance catheter to estimate left ventricular volume are described in detail elsewhere [4, 15]. Briefly, the conductance catheter method is based on the measurement of the electric conductivity of blood in a ventricular cavity. A custom built signal conditioning and processing unit (Cardiodynamics, The Netherlands) was constructed in order to allow for a variable excitation frequency while at the same time maintaining a constant electrical current of 30 µA. An alternating current of 5–40 kHz and 30 µA is applied between the two outermost electrodes (electrodes 1 and 8) to generate an intracavity electric field. The remaining electrodes are used to measure the potential difference and therefore derive the time varying ventricular conductance (Gt) as the sum of five segments. Total left ventricular conductance is calculated as the sum of the segmental conductances. The relationship between time varying volume (Vt) and time varying conductance (Gt) is given by the simple formula:
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| (1) |
Where is the slope of the relation between conductance derived volume and true volume, 
is the blood resistivity, L is the interelectrode distance and Gp is parallel conductance. The offset volume Vc, caused by the parallel conductance, Gp is equal to:
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| (2) |
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3 Method |
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TopAbstract1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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3.1 In vitro: the effect of excitation frequency on ventricular volume measurements
Preliminary in vitro measurements were obtained with the modified conductance catheter system to check the accuracy and linearity in glass cylinders containing 0.9% NaCl solution (Abbott Laboratories). The resistivity of this solution was measured using the four electrode cuvette integral to the Sigma 5 signal conditioning and processing unit. The mean of three estimates was used as the resistivity of the 0.9% NaCl for that experiment and was entered into the custom software along with the interelectrode distance of the catheter.
The relationship between conductance volume, known volume and frequency was initially investigated using three glass cylinders of differing diameter. A 7 French 8 electrode conductance catheter with a total interelectrode distance of 7 cm (Webster) was positioned in the central long axis of the cylinder. The cylinder was initially filled until all electrodes were just covered by fluid with a known volume of 7 ml, 22.5 ml and 62 ml. Measurements of conductance volume were recorded after changing the catheter excitation frequency by 5 kHz increments between 5 and 40 kHz.
3.2 In vivo study
The investigation conformed with the Guide for the Care and Use of Laboratory Animals published by the US National Institute of Health [16].
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4 Preparation |
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TopAbstract1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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Studies were performed in seven 5 kg (group 1) and six 50 kg (group 2) Danish Landrace pigs. The neonatal pigs (group 1) were sedated and paralysed with Midazolam (0.1 mg/kg i.v), Fentanyl (5 µg/kg/h i.v) and Pancuronium (0.1 mg/kg i.v). The adult pigs (group 2) were paralysed with Pancuronium (0.04 mg/kg/h i.v), while anaesthesia was maintained with Propofol (8–10 mg/kg/h i.v) and Fentanyl (0.3 mg/h i.v). All animals were intubated and mechanically ventilated with positive pressure ventilation using a Siemens Servo 900 C (Siemens Elema, Solna, Sweden). Serial blood gas measurements were performed to maintain a physiological level of oxygenation and ventilation. The right and left carotid arteries and the internal jugular vein (in group 2) were cannulated. Access to the heart was obtained through a standard median sternotomy, the pericardium was opened, and the heart was suspended in a pericardial cradle.
4.1 Group 1: 5 kg neonatal pigs
A 2.5 French micromanometer (Millar Instruments, Houston, TX) was inserted through the left ventricular apex. A 5 French custom built 8 electrode conductance catheter (Numed, Hopkinton, NY) with a total interelectrode distance of 3 or 4 cm to match the longitudinal axis of the ventricle was advanced through the right carotid artery sheath into the left ventricle using fluoroscopic guidance. Confirmation of correct positioning was obtained from typical waveforms. A thermodilution catheter (5.5 French) was placed into the pulmonary artery via the right internal jugular vein, again guided by fluoroscopy.
4.2 Group 2: 50 kg adult pigs
The preparation of these larger animals was similar to group 1 except that a 6 French 8 electrode conductance catheter (Webster) with a total interelectrode distance of 6 cm was placed in the left ventricle, and a 24 mm transit time flow probe (Transonic, Ithaca, NY) was placed around the main pulmonary artery, just distal to the pulmonary valve, in order to measure pulmonary flow.
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5 Protocol |
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TopAbstract1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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The conductance catheter was connected to our custom built Sigma 5 DF signal conditioning and processing unit (Cardiodynamics, The Netherlands). Analogue signals representing the five segmental conductances, left ventricular pressure, and ECG were digitised (12 bit, 250 Hz), using a DT 2821 A–D convertor (Data Translation, Marlboro, MA), monitored on line and stored on a microcomputer for later analysis using custom software. Having obtained an optimum conductance catheter position, a 4 ml blood sample was taken to measure blood resistivity (
) and entered into the custom software. This software allows the analysis of multiple indices of ventricular performance (end systolic pressure volume relationship, end diastolic pressure volume relationship, preload recruitable stroke work, etc.), but in this study we only used end systolic volume (ESV), end diastolic volume (EDV) and stroke volume (SV) in order to calculate Vc(saline dilution), Vc(dual frequency) and (see below). Measurements of were repeated after each injection of saline.
Cardiac output was measured in the neonatal group (group 1) by thermodilution. The mean of four measurements was taken as the cardiac output for that specific time point. In group 2, cardiac output was determined on a beat by beat basis from the flow probe around the pulmonary artery. The conductance catheter gain factor () was calculated as the ratio of conductance derived cardiac output to that measured by thermodilution in group 1, and by flowmeter in group 2. Two measurements were made at steady state at each excitation frequency over the range of 5–40 kHz during suspended ventilation at end expiration. To calculate Vc(saline dilution), a small volume 0.7 ml of 10% hypertonic saline (group 1) and 7 ml of 10% (group 2) was then injected into the pulmonary artery and the superior vena cava respectively during continuous data acquisition. If changes in heart rate or ventricular pressure were apparent during the saline injection, the run was repeated with a smaller injectate volume. End diastolic volume (EDV) was plotted against the subsequent end systolic volume (ESV) of each beat during the ascending limb of the saline wash-in. The points are linearly regressed by the least squares method. When this regression line is extrapolated to the line of identity (EDV=ESV) the blood conductivity is zero and all the current goes through the ventricular wall. The volume at this point is due to parallel conductance (Vc(saline dilution)). Two measurements were made per animal at each excitation frequency in both group 1 and group 2 from which the mean was obtained to determine Vc(saline dilution). If ectopy occurred during the saline dilution data collection, we performed another injection in order to obtain at least two acceptable recordings. Each dataset was acquired for approximately 20 s during suspended ventilation.
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6 Gawnes dual frequency technique to determine Vc |
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TopAbstract1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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This method uses a dual frequency technique [12]to measure Vc(dual frequency). Vc(saline dilution) was first measured at an excitation frequency of 5 kHz. This value was expressed as a percentage of end systolic volume measured at 5 kHz. These values were plotted against the percentage rise in end systolic volume as the excitation frequency was increased from 5 to 40 kHz. A linear regression analysis was performed on the data with the y intercept set at or close to zero, which is similar to that used by Gawne reflecting the original theoretical basis on which the technique was proposed. The reciprocal of this regression slope is used to determine a constant which when multiplied by the absolute rise in ESV which occurred between the measurements at 5 and 40 kHz would estimate Vc(dual frequency). A standard least squares regression analysis was also performed on the data for further comparison.
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7 Data analysis/statistics |
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TopAbstract1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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Data are expressed as mean (± 1SD). A Bland Altman methods comparison was used to compare Vc(saline dilution) and Vc(dual frequency). The variability of stroke volume, end systolic volume, end diastolic volume, parallel conductance volume (Vc(saline dilution)) and conductance gain factor (
) were assessed over the excitation range of 5–40 kHz by using an analysis of variance. The relationship between Vc(saline dilution) as a percentage of end systolic volume with the percent rise in end systolic volume with frequency was assessed by a linear regression using the least squares method with the y intercept set at or close to zero. A standard least squares regression analysis was also performed on the data for further comparison. The null hypothesis was rejected when p<0.05. |
8 Results |
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TopAbstract1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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8.1 In vitro: the effect of excitation frequency on volume measurements
There was no volume variation with frequency. The mean gradient of the regression lines for all frequencies was 0.83±0.007 suggesting little frequency dependence. The y-intercept ranged from –0.3 ml at 5 kHz to –0.69 ml at 40 kHz. The mean y intercept for all frequencies was –0.61 ml±0.16 ml. The mean value of R2 for each frequency was 1.00.
8.2 In vivo study
8.2.1 Group 1: 5 kg neonatal pigs
Group data for parallel conductance (Vc(saline dilution)) and ventricular volumes are presented in Table 1. Parallel conductance volume (Vc(saline dilution)) varied significantly with frequency (5 kHz=8.63±2.74 ml; 20 kHz=9.43±3.25 ml; 40 kHz=11.51±2.65 ml) (p=0.01) (Fig. 1). There was no detectable difference in stroke volume (SV) over the range of excitation frequencies; The mean SV at 5 kHz=4.13±1.45 ml demonstrated a non-significant rise to 4.75±1.52 ml at 40 kHz (p=0.19). Although the end systolic volume rose with frequency (5 kHz=12.55±2.96 ml; 20 kHz=13.70±3.83 ml; 40 kHz=14.72±4.43 ml), the difference was not significant when the ESV at 5 kHz and 40 kHz were compared (p=0.16). Pressure volume loops demonstrating this change in end systolic volume between the different excitation frequencies are shown in Fig. 2. Similarly the end-diastolic volume demonstrated a non-significant rise (5 kHz=16.69±3.55 ml; 20 kHz=18.29±4.49 ml; 40 kHz=19.47±5.40 ml) (p=0.12). Nonetheless an analysis of variance for repeated measures throughout the group, at all frequencies does show a highly significant dependence of volumes on frequency. In this group as was measured by thermodilution we were unable to assess the variability of over the excitation frequency range.
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8.3 Gawnes dual frequency technique to determine Vc
Due to technical difficulties it was not possible to estimate Vc(saline dilution) in one pig. However, results for the remaining six pigs are reported accordingly. From a graph of parallel conductance (Vc(saline dilution)) as a percentage of end systolic volume (ESV) plotted against the rise in end systolic volume as the excitation frequency was increased from 5–40 kHz a constant was derived. The derivation of this constant (1/(
G –1)), found to be 4.55 for the neonatal group, when multiplied by the percent change in end systolic volume from 5–40 kHz would yield an estimate of parallel conductance volume (Vc(dual frequency)). However, the relationship of Vc(saline dilution) and Vc(dual frequency) produce a poor correlation (R2=0.69). A standard regression analysis using the least squares method was performed on the graph of parallel conductance (Vc(saline dilution)) as a percentage of end systolic volume (ESV) plotted against the percent rise in esv as the frequency was increased from 5–40 kHz for comparative purposes. A negative slope (Y=24.56–0.14 X) and a poor correlation coefficient (R2=0.064) were obtained.
8.4 Group 2: 50 kg adult pigs
Parallel conductance (Vc(saline dilution)) increased with frequency by 31.42% from 69.43±27.76 ml at 5 kHz to 101.24±15.21 ml at 40 kHz (p<0.001). Unlike in the neonatal group (group 1), there was a significant difference in stroke volume (SV) over the range of excitation frequencies (SV at 5 kHz=30.82±3.70 ml; 40 kHz=38.44±5.24 ml) (p=0.003). The end systolic and diastolic volumes also increased significantly with frequency (ESV5 kHz=140.17±13.36 ml; ESV40 kHz=170.53±21.44 ml (p=0.002); EDV5 kHz=170.98±14.04 ml; EDV40 kHz=208.84±23.07 ml) (p<0.001) (see Table 2). The slope or gain factor was determined in this group by comparing conductance derived cardiac output to that obtained on a beat to beat basis by a transit time flow probe. The mean for the group at 5 kHz (0.62±0.14) increased to 0.75±0.16 at 40 kHz (p=0.18). Correcting EDV and ESV for both Vc and produced no change in stroke volume with frequency (corr SV5 kHz=52.28±17.98 ml; corr SV40 kHz=53.81±19.90 ml (p=0.89)).
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8.5 Gawnes dual frequency technique to determine Vc
Similar results were obtained in the adult group. The parallel conductance volume (Vc(dual frequency)) as a percent of end systolic volume for the adult group was found to be 3.45 times percent rise in ESV with changing frequency (Fig. 3). There was no significant relationship between Vc(saline dilution) and Vc(dual frequency) (R2=0.22). The standard least squares regression analysis was performed as in group 1. Again a poor correlation coefficient was obtained (Y=14.18+0.05 x R2=0.067).
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9 Comparison of methods |
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TopAbstract1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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It is possible to compare Vc(saline dilution) to that obtained by a dual frequency excitation technique (Vc(dual frequency)) using a Bland Altman comparison [17]. Using this, Vc(saline dilution) and Vc(dual frequency) did fall within the 95% limits of agreement, for both the neonatal group (group 1) or the adult group (group 2). However, there is no obvious relation between the difference and the mean. Under these circumstances we can summarise the lack of agreement by calculating the bias, estimated by the mean difference and the standard deviation of the differences. Thus for the neonatal pigs (group 1) Vc(dual frequency) maybe –9.88 ml below or 16.56 ml above Vc(saline dilution). Similarly in the adult pigs (group 2) Vc(dual frequency) maybe –63.66 ml below or 94.50 ml above that estimated by Vc(saline dilution). Thus, the dual frequency estimation of Vc cannot be used to reliably substitute the saline dilution technique.
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10 Discussion |
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TopAbstract1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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The conductance technique has widely been applied to the study of left ventricular pressure volume relationships. However, a drawback of the conductance catheter method is its inability to determine absolute volume without calibration factors. Calculation of absolute left ventricular volume from conductance measurements requires the estimation of parallel conductance volume (Vc) and a slope factor (
). Parallel conductance volume accounts for a substantial portion of the total conductance signal, ranging from approximately 50% to as much as 70% of the total signal [18]. Thus Vc is not small and should not be ignored. Parallel conductance volume is normally determined by injecting a small amount of hypertonic saline into the pulmonary artery which transiently increases the conductivity of blood in the ventricle [4]. The disadvantage of this technique is that measurements have to be performed during steady state, and it is not possible to determine whether parallel conductance changes during a loading manoeuvre such as transient snaring of the inferior vena cava. In addition it is subject to the variability inherent, at best 10%, to indicator dilution methods. As the parallel conductance volume can be as much as twice the actual ventricular volume, small errors in parallel conductance volume will have a larger impact on subsequent estimation of absolute left ventricular volume [18]. This study investigated the effect of changing excitation frequency over the range of 5–40 kHz on left ventricular volume measurement according to the technique described by Gawne [12], who described a species specific substitute to the saline dilution method in the estimation of parallel conductance volume.
In vitro there was no significant relationship between frequency and volume, confirming the linearity and accuracy of our custom-built hardware. However, in both the 5 kg neonatal (group 1) and 50 kg adult pigs (group 2) we observed a significant change in Vc(saline dilution) with frequency. In group 1 Vc(saline dilution) increased by 25.02% between 5 kHz and 40 kHz. Similarly in group 2 a difference of 31.42% was observed. These findings suggest that the lower the excitation frequency, the lower the dissipation of current beyond the blood pool. These data indicate that the resistive component of Vc is substantial. For instance, estimated Vc at 0 Hz in group 1 is 8.01 ml compared with 11.51 ml at 40 kHz.
At frequencies of 20 kHz or greater, the resistivity of the myocardium is less than 400 
·cm which is only 2.5 times greater than that of blood (150 ·cm) [10]. However, by reducing the excitation frequency to 5 kHz the resistivity of the myocardial tissue is considerably greater and thus a much smaller percentage of the measuring current extends beyond the blood pool. For this excitation frequency, cardiac tissue and blood are predominantly resistive [19–21]. Similar results were observed by Steendijk who, using a novel approach to measure anisotropic myocardial resistivity, showed a reduction in both longitudinal and transverse fibre measurement with increasing frequency. These results show that over the range of 5–60 kHz both transverse and longitudinal resistivity decreased by approximately 25% [22], which was similar to that obtained by Sperelakis [23]. A discrepancy can be seen between our data and those obtained by Duck [19], Rush [20]and Schwan [21]which maybe explained by (i) differences in preparation, e.g. the difference in resistivity between blood and Tyrode solution; (ii) the condition of the preparation e.g. ischaemia; and (iii) the excitation frequency at which the resistivity measurement has been performed.
Most studies have used an excitation frequency of 20 kHz, 30 µA, but a few investigators have used lower excitation frequencies to reduce the effect of parallel conductance [5, 10]. At a frequency of 1.3 kHz the resistivity of the myocardial tissue is greater than 1000 ·cm and a much smaller percentage of the measuring current dissipates beyond the ventricle [10]. McKay concluded that the decrease in current leakage that occurs at a reduced excitation frequency may be responsible for both the improved accuracy of volume measurements reported in his study as well as a tendency slightly to overestimate stroke volumes.
The dual frequency technique described by Gawne et al. [12]uses the change in conductance with frequency to estimate parallel conductance and it has been argued that this technique could obviate the need for the saline dilution technique altogether. When parallel conductance (Vc(saline dilution)), is expressed as a percentage of end-systolic volume at 3.3 kHz and plotted against the percent rise in end-systolic volume with frequency he obtained a highly linear relationship. From this he calculated a species specific calibration factor [12]. Gawne therefore assumes that the parallel conductance is a simple linear function with frequency. This implies that the coupling between parallel conductance and left ventricular volume is wholly capacitive. However, our results are not consistent with a capacitively coupled parallel conductance with a break frequency of 12 kHz. Our results have shown that parallel conductance has a significant resistive component which cannot be estimated from the frequency dependence measured with the conductance catheter.
A linear regression analysis was performed on the data, for both the neonatal (group 1) and adult (group 2) pigs, with the y intercept set at or close to zero, which is similar to that used by Gawne reflecting the original theoretical basis on which the technique was proposed. In Gawne's paper [12]the regression analysis (we presume with an independent y intercept) produced a remarkably linear result with a y intercept essentially at zero. Our data clearly does not support this. Indeed a standard least squares analysis in group 1 gives a negative slope, further undermining this technique.
In our study, using an excitation frequency of 5 and 40 kHz, there was the expected increase in parallel conductance at the higher frequency with no significant change in measured stroke volume in the neonatal pigs (group 1). However, in adult pigs (group 2) both parallel conductance volume and stroke volume increased significantly with frequency, which probably reflected a change in with frequency. Comparison of SV, corrected for and Vc, at 5 kHz showed no significant difference to that obtained at 40 kHz (corr SV5 kHz=52.28±17.98 ml; corr SV40 kHz=53.81±19.90 ml) (p=0.89). However, by comparing the total corrected end-systolic volume (ESV-Vc) for both groups an increase was observed when frequency was varied from 5–40 kHz. The results obtained in group 2 can theoretically still be used to show the failure of the dual frequency technique to substitute for the saline dilution method.
Thus we have shown why the results in this study contradict those obtained by Gawne [12]. The fundamental difference between our study and those of previous investigators is that both the neonatal pigs (group 1) and the adult pigs (group 2) were studied with an open chest and open pericardium. Furthermore, we have shown a direct relationship between excitation frequency and alpha, in adult animals, and inferentially also in group 1 (Fig. 2). This unexpected finding probably relates to changes with field density patterns and uniformity in these larger hearts. Alpha was not measured in Gawnes study (using mid-sized pigs) and this finding alone undermines the theoretical basis of his multiple frequency technique.
In conclusion, while parallel conductance is frequency dependent it has a substantial resistive component. At a lower excitation frequency of 5 kHz the resistivity of the myocardium is considerably greater than that of blood. Therefore a smaller percentage of measuring current extends beyond the blood pool (reduced Vc). The dual frequency technique 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 reliably be used to substitute the saline dilution technique in different sized hearts.
Time for primary review 27 days.
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Acknowledgements |
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This work was supported by the Scott Rhodes Research Fund, Garfield Weston Research Fund, Clinical Research Committee (Royal Brompton Hospital NHS Trust), The Institute of Experimental Clinical Research and the Danish Heart Foundation. Thanks must also be given to Dr R Szwarc (BioMetrics, Las Vegas) for provision of the analysis software used in this paper.
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TopAbstract1 Introduction2 Conductance catheter3 Method4 Preparation5 Protocol6 Gawnes dual frequency...7 Data analysis/statistics8 Results9 Comparison of methods10 DiscussionReferences |
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