Evaluation of impedance cardiography druing anesthesia in extremely obese patients

Improving the accuracy of impedance cardiac output in the intensive care unit: comparison with thermodilution cardiac output

This study examined the effect of impedance algorithm adjustment to reflect abnormalities found in cardiac output estimation in the intensive care unit. Impedance (Kubicek and Sramek equations) and thermodilution were measured concurrently in 61 patients. The mean difference between Kubicek and thermodilution (n=40) was 1.47 L/min (95% confidence interval [CI], 0.47-2.47) and between Sramek and thermodilution (n=54) was 2.68 L/min (95% CI, 1.93-3.44). Exclusion of patients with valve regurgitation improved agreement between Kubicek and thermodilution (n=32), with a mean difference of 2.02 L/min (95% CI, 1.10-2.94). Multiple regression determined the role of skinfold thickness, pH, hematocrit, sodium, chloride, albumin, protein, and urea within impedance. Kubicek was recalculated using the new algorithm and recompared with thermodilution. The mean difference was -0.38 L/min (95% CI, -1.92 to 1.16). This study found poor agreement between impedance and thermodilution in critically ill patients, but exclusion of those with valve regurgitation and adjustment for hematocrit and skinfold thickness improved agreement.

A meta-analysis of three decades of validating thoracic impedance cardiography

OBJECTIVE

To provide a meta-analysis of current literature concerning the validation of thoracic impedance cardiography (TIC) and to explain the variations in the reported results from the differences in the studies.

DATA SOURCES

A computer-assisted search of English-language, German, and Dutch literature was performed for the period January 1966 to April 1997. Moreover, references from review articles were obtained.

STUDY SELECTION

A total of 154 studies comparing measurements of cardiac output or related variables obtained from TIC and a reference method were analyzed.

DATA EXTRACTION

Articles were classified by differences in TIC methodology, reference method, and subject characteristics. Fisher's Zf transformed correlation coefficients were used to compare results. Data were pooled using the random-effects method.

DATA SYNTHESIS

An overall pooled r2 value of .67 (95% confidence interval, 0.64-0.71) was found. However, the correlation was higher in repeated-measurement designs than in single-measurement designs (r2 = .53; 95% confidence interval, 0.43-0.62). Further research using analysis of variance revealed a significant influence of the reference method and the subject characteristics on the correlation coefficient. The correlation was significantly better in animals than in cardiac patients. Subgroup analysis revealed that TIC correlated significantly better to the indirect Fick method than to echocardiography in healthy subjects. No significant influence of the applied TIC methodology was found.

DISCUSSION

The overall r2 value of .67 indicates that TIC might be useful for trend analysis of different groups of patients. However, for diagnostic interpretation, a r2 value of .53 might not meet the required accuracy of the study. Great care should be taken when TIC is applied to the cardiac patient. However, because the applied reference method was of significant influence, differences between TIC and the reference method are incorrectly attributed to errors in TIC alone.

Impedance cardiography in cardiac surgery patients: abnormal body weight gives unreliable cardiac output measurements

BACKGROUND

To study the accuracy of cardiac output measurement by means of Electrical Impedance Cardiography (EIC) in post-cardiac surgery patients.

METHODS

In a prospective study, we compared cardiac output measurements by means of thermodilution (COTD) with impedance cardiographic-derived values (COEIC) in 37 mechanically ventilated patients after cardiac surgery. Both methods were used simultaneously.

RESULTS

COEIC values were weakly correlated with COTD in the total group when the equation of Sramek-Bernstein was employed to calculate COEIC (r = 0.60, P < 0.001, mean difference and standard deviation: -0.06 +/- 1.25 l.min-1). After exclusion of the 12 patients whose body weight differed > 15% from their ideal body weight, no significant difference was found between the mean values (5.40 +/- 1.80 l.min-1 (COEIC) vs 5.31 +/- 1.69 l.min-1, n = 25) while the correlation coefficient increased substantially (r = 0.85, P < 0.001, mean difference and standard deviation: 0.09 +/- 0.96 l.min-1).

CONCLUSIONS

The results of this study indicate that weight is a very important factor in unreliable measurement of CO by impedance cardiography in cardiac surgery patients. The calculation equation as proposed by Sramek and Bernstein is not accurate enough in patients with more than 15% of weight deviation. Therefore, the use of impedance cardiography in these patients is of limited value until an accurate correction factor has been developed.

The influence of weight on stroke volume determination by means of impedance cardiography in cardiac surgery patients

OBJECTIVES

Obesity is thought to be one of the conditions in which the impedance cardiographic method is less reliable for estimating stroke volume (SV). This led to the introduction of a weight correction factor, sigma, into the equation according to Sramek and Bernstein. However, no scientific evidence has been published to support the use of this factor. The objectives of the present study are to evaluate the influence of body weight on the accuracy of impedance cardiography and to validate Bernstein's weight correction factor by comparison with thermodilution in patients after coronary bypass surgery.

DESIGN

Prospective clinical study.

SETTING

A surgical intensive care unit in a university hospital.

PATIENTS

37 consecutive patients 24-36 h after coronary bypass surgery, sub-divided into a normal-weight group (n = 24), patients whose weight deviated less than 15% from their ideal weight, and an obese group (n = 13), patients whose weight deviated more than 15% from their ideal weight.

MEASUREMENTS

Kubicek's impedance cardiographic method and Sramek and Bernstein's method to assess SV are applied and compared to thermodilution. In order to study the validity of sigma, the results are compared between 24 patients with normal weight and 13 obese patients.

RESULTS

A significant correlation between miscalculation of SV by impedance cardiography and the degree of obesity for Sramek and Bernstein's method is found when sigma is not included in the equation (r = -0.55, p < 0.05). This relation, however, remained significant when sigma was included in the equation (r = -0.40, p < 0.05). Kubicek's method shows no significant correlation for this relation (r = -0.30). Besides this, Sramek and Bernstein's method underestimates SV significantly in the obese group, independent of the use of sigma in the equation. These results are explained as being intrinsic to the equation, according to Sramek and Bernstein. In the whole group the impedance-derived SV did not significantly differ from SV as measured by means of thermodilution, independent of the method used to calculate SV. However, a considerably better correlation and agreement (mean difference +/- 2 standard deviations is found when Kubicek's method is applied (r = 0.90, 0.5 +/- 17.1 ml vs 0.64, -4.9 +/- 31.8 ml for Sramek and Bernstein's method).

CONCLUSIONS

Weight significantly influences Sramek and Bernstein's method of impedance cardiography, whereas Kubicek's method is not biased by this factor.

Impedance cardiography for cardiac output estimation in pacemaker patients: review of the literature

Impedance cardiography permits noninvasive beat-to-beat determination of cardiac output, the product of the amplitude of the first derivative of thoracic impedance signal (dZ/dt), the ventricular ejection time, and heart rate corrected by the distance between the measuring electrodes. Its use is based on: (1) the dZ/dt signal that originates from the upper thorax; (2) the ventricular ejection period measured by the dZ/dt curve that occurs between the opening and closing of the aortic valve; (3) the dZ/dt curve is similar in morphology and timing to the aortic flow curve measured by an electromagnetic flowmeter with a significant linear correlation (r = 0.9) between dZ/dt and peak aortic flow; (4) similarity of the linear correlation between stroke volume, determined by the flowmeter and the impedance signal; and (5) significant reduction of the dZ/dt signal by 90% follows simultaneous occlusion of the aorta and the pulmonary artery. The rapid systolic portion of the impedance signal occurs only when blood is ejected into the aorta and is independent of right ventricular ejection. Most studies comparing impedance cardiography results with standard cardiac output determination have shown a correlation of 0.7-0.9. While the accuracy of impedance cardiography remains controversial and can be affected by the inherent limitations of the technique and by low cardiac output, intracardiac shunts, and valvular regurgitation, the high reproducibility of the method is established and may be comparable or superior to other commonly used techniques. When accurate determination of cardiac output is crucial, impedance cardiography may be used in conjunction with a standard technique to establish a baseline reference, thereby permitting further analysis. If only the trend need be followed, the high reproducibility of impedance cardiography measurements allows small changes in cardiac output to be detected on a frequent and ongoing basis. The ease and precision of this technique warrants its more widespread use in the assessment of pacemaker patients. Further use of this promising technique will allow a better definition of its role in the assessment of a wide range of cardiac patients.

Measurement of cardiac output by electrical impedance plethysmography

There are many potential applications for cardiac output measurement in clinical and experimental medicine. The most commonly used techniques are invasive procedures, requiring cardiac or arterial catheterization, a disadvantage that has restricted their wider application. Impedance plethysmography has been developed as a non-invasive, beat-by-beat method of cardiac output measurement, which provides an estimate of stroke volume from changes in the electrical impedance of the thorax during cardiac systole. The values for cardiac output obtained by this technique have been extensively compared with values obtained by other methods, both in experimental animals and in the human subject. In the majority of studies high correlation coefficients have been obtained, although impedance plethysmography has tended to give higher absolute values than most other methods. Values for cardiac output obtained by impedance plethysmography are best assessed by comparison with a series of normal values obtained by this technique, rather than with values obtained by other methods. We discuss the results of an investigation of normal cardiac output by impedance plethysmography; theoretical objections to impedance cardiography are considered, and various methods of determining the specific resistivity of blood are reviewed.

Measurement of cardiac stroke volume during cesarean section: a comparison between impedance cardiography and the dye dilution technique

Simultaneous determination of cardiac stroke volume by impedance cardiography and the dye dilution technique was compared in ten women undergoing elective cesarean section performed under general or epidural anesthesia. The influence of delivery and the anesthetic procedures used on stroke volume determination by the two methods was evaluated and compared. The correlation coefficients for measurements performed before and during anesthesia showed little variation and were largely unchanged after delivery of the child (r = 0.90-0.97). Mean stroke volume determined by impedance cardiography was significantly (P less than 0.001) lower than mean stroke volume calculated from the dye dilution technique. However, there was no significant difference between the mean change in stroke volume determined by the two techniques during serial measurements. Impedance cardiography was found to be a safe, reliable, non-invasive technique for the measurement of changes in stroke volume during cesarean section. The ability of the impedance method to determine changes in stroke volume was unaffected by the anesthetic procedures employed or by delivery of the child.

Measurement of stroke volume with impedance cardiography

Simultaneous determination of stroke volume with impedance cardiography and the dye dilution technique was compared in 11 healthy men before and after exercise. The correlation coefficient for all measurements was 0.82. Mean stroke volume determined by impedance cardiography was significantly (P less than 0.001) lower than mean stroke volume calculated by the dye dilution technique. However, there was no significant difference in the mean change in stroke volume determined by the two techniques during serial measurements. The reproducibility of single impedance-determined stroke volumes (6.9 ml) was not significantly different from single values obtained by dye dilution (7.3 ml). Impedance cardiography was found to be a safe, reliable, non-invasive method for the measurement of changes in stroke volume in healthy individuals before and after exercise. At present, direct estimation of absolute values of stroke volume is not recommended using impedance cardiography. Calibration of the impedance technique against other established techniques in a given application is necessary.

An experimental evaluation of impedance cardiographic and electromagnetic measurements of stroke volumes

The reliability of impedance cardiography for absolute and relative measurements of stroke volumes is assessed by comparing data with electromagnetic flowmeter measurements in 14 anaesthetized mongrel dogs. Cardiac output was altered in each dog (as much as possible) by a variety of physical and pharmacological interventions. Impedance cardiography does not provide absolute values for stroke volumes, although relative changes may be calculated with reasonable reliability if a correction factor ('CIMP') is employed. 'CIMP' is around 0 . 8 in subjects breathing spontaneously or when alternative pressure ventilation is applied. However, this factor can only be used in an intermediate range of stroke volumes (10-30 ml in the dog). At lower and higher cardiac output levels other correction factors must be observed accordingly. Intermittent positive pressure ventilation increases the sensitivity of relative stroke volume measurements by transthoracic impedance, leading to an average 'CIMP' of 1 . 5. For calculation of relative stroke volume changes by impedance cardiography, the simple formula: (formula; see text) is satisfactory instead of the more complicated Kubicek formula.

Considerations on impedance cardiography

Impedance cardiography is an electronic plethysmographic technique which provides information on cardiac stroke index, myocardial performance, thoracic fluid content and peripheral circulation. The method has gained popularity in recent years because it is not invasive. While less precise in absolute terms than invasive methods the results are reproducible, and the technique accurately assesses variations in measurements. It may be used in most anaesthetized patients without the possibility of any of the complications which sometime accompany the use of more precise invasive methods. This paper describes impedance cardiography and methods for its use.