Validation of impedance cardiography measurements of cardiac output during limited exercise in heart transplant recipients

Twenty-one patients were studied at rest and during exercise after heart transplantation to compare cardiac output measured by thermodilution and impedance cardiography. Exercise was performed on a bicycle ergometer over a limited range of work load (25 and 50 watt) whilst metabolic gas exchange was recorded. One patient was studied at rest whilst his circulation was maintained by a Jarvik-7 artificial heart. The values of cardiac output measured by impedance cardiography corresponded closely with the flow rate from the artificial heart. There was also close agreement between the impedance and thermodilution measurements of cardiac output at rest and during exercise. Both measurements followed the changes in heart rate and oxygen consumption. Both thermodilution and impedance cardiography methods elicited good reproducibility of cardiac output measurements at rest and during exercise. These observations suggest that the noninvasive and continuous record of cardiac output obtained by impedance cardiography can be used for the postoperative monitoring of heart transplant recipients.     

Lung fluid and impedance cardiography

Serial thermodilution and impedance cardiac output measurements in two critically ill patients are presented. Impedance cardiography failed to detect changes in cardiac output and provided values lower than those derived from thermodilution. Chest X-ray examination and changes in thoracic impedance suggested that these two patients had significantly increased extravascular lung water. This failure of impedance cardiography is attributed to aberrant electrical conduction though the lungs as a result of increased lung fluid that alters the impedance waveform. Although reliable when used in normal subjects, impedance cardiography appears not to provide accurate measurements in critically ill patients.     

Cardiac output monitoring by impedance cardiography in cardiac surgery

The cardiac output monitoring by impedance cardiography, NCCOM3, was evaluated in adult patients (n = 12) who were subjected to coronary artery bypass grafting. Values of cardiac output measured by impedance cardiography were compared to those by the thermodilution method. Changes of base impedance level used as an index of thoracic fluid volume were also investigated before and after cardiopulmonary bypass (CPB). Correlation coefficient (r) of the values obtained by thermodilution with impedance cardiography was 0.79 and the mean difference was 1.29 +/- 16.9 (SD)% during induction of anesthesia. During the operation, r was 0.83 and the mean difference was -14.6 +/- 18.7%. The measurement by impedance cardiography could be carried out through the operation except when electro-cautery was used. Base impedance level before CPB was significantly lower as compared with that after CPB. There was a negative correlation between the base impedance level and central venous pressure (CVP). No patients showed any signs suggesting lung edema and all the values of CVP, pulmonary artery pressure and blood gas analysis were within normal ranges. From the result of this study, it was concluded that cardiac output monitoring by impedance cardiography was useful in cardiac surgery, but further detailed examinations will be necessary on the relationship between the numerical values of base impedance and the clinical state of the patients.     

Descending aortic blood flow and cardiac output: a clinical and experimental study of continuous oesophageal echo-Doppler flowmetry

BACKGROUND: Several studies have demonstrated that perioperative optimisation of oxygen delivery and haemodynamics can reduce mortality and morbidity for high-risk surgical patients. To optimise cardiac output, reliable, continuous and "less invasive" methods for measuring cardiac output are urgently needed. METHODS: Eight landrace pigs were studied during experimental repeated cardiac tamponade and 14 patients during liver transplantation. Aortic blood flow was measured by using transoesophageal echo-Doppler technique. A total of 91 paired measurements of aortic blood flow and cardiac output with different techniques were performed in the pigs and 124 paired measurements in the patients. RESULTS: Transoesophageal echo-Doppler did provide continuous real-time monitoring of the rapid and dramatic haemodynamic changes occurring during cardiac tamponade and during liver transplantation, while only intermittent information was obtained from the bolus thermodilution technique. Changes in haemodynamics were more difficult to detect with the "continual" cardiac output thermodilution technique. Changes in aortic blood flow closely followed changes in cardiac output determined by the bolus thermodilution technique both in pigs (r= 0.89) and in patients (r=0.80). In patients, aortic blood flow constituted about 70% of cardiac output determined by the bolus thermodilution technique. CONCLUSIONS: A combined echo-Doppler technique can be valuable for continuous monitoring of haemodynamic changes in the perioperative setting, and changes in aortic blood flow agree well with corresponding changes in cardiac output intermittently obtained by thermodilution cardiac output measurements. With the combined echo-Doppler technique a proper position of the Doppler beam is greatly facilitated by the M-mode echo visualisation of the aortic wall and aortic cross-sectional area is continuously measured.     

Automated impedance cardiography for detecting ischemic left ventricular dysfunction during exercise testing

Automated impedance cardiography (ICG) is an attractive method for noninvasive hemodynamic evaluation. The objective of our study was to evaluate the feasibility and diagnostic value automated ICG in patients with suspected coronary artery disease (CAD). We measured stroke index (SI) and cardiac index (CI) in 65 patients with suspected CAD at rest and during bicycle exercise testing. All patients underwent subsequent cardiac catheterization including coronary angiography (CA). Depending on the results of CA, patients were divided into three groups, patients without significant CAD (group 0), single vessel disease (group 1) or multivessel disease (group 2-3). In a subset of 20 patients, automated ICG was compared to measurements of CI by the thermodilution (TD) method. Results: There were no significant differences in SI and CI at baseline between the three groups. At 75- and 100-watt exercise, patients in group 2-3 showed significantly lower mean values of SI and CI as compared to patients of group 0 and group 1 (all p < 0.05), indicating exercise-induced ischaemic left ventricular (LV) dysfunction. Three patients had to be excluded because of inappropriate quality of the ICG signals during exercise. Comparison of automated ICG with TD measurements of CI showed good correlations between both methods at rest (r = 0.73) and during exercise (r = 0.89-0.91). Conclusions: We conclude that hemodynamic monitoring by automated ICG is both feasible and practical during exercise testing. Automated ICG can provide reliable and valuable additional diagnostic information on LV function during exercise which is helpful for selecting those patients for angiography who are likely to benefit from coronary interventions.     

Measurement of Rapid Changes in Cardiac Stroke Volume. An Evaluation of the Impedance Cardiography Method

One hundred and twenty-nine simultaneous measurements were carried out on seven patients, 67 at rest and 62 during the Valsalva manoeuvre, in order to compare impedance cardiography with the thermodilution method, during rapid changes in cardiac stroke volume and pulmonary blood volume.
A coefficient of correlation of 0.86 was found following linear regression analysis of the whole material. Analysis of the individual patient showed that the mean coefficient of correlation was 0.94 (range 0.91–0.97), and that the slope of the regression lines was 0.41 to 1.82, and further that the mean intercept was 0.2 ml.
It is concluded that impedance cardiography can be employed for measuring the relative intraindividual changes in cardiac stroke volume during the Valsalva manoeuvre, and it is suggested that it may be of use in other situations, such as during haemorrhage or continuous positive pressure ventilation.     

Thermodilution and rheography: the coincidence and reproducibility of results

A comparison of the pulmonary artery thermodilution and tetrapolar impedance cardiography of cardiac output measurements based on 312 dual determinations has shown a good general coincidence, but better reproducibility of the impedance cardiography data and more evident character of the errors and artifacts. The authors' analysis of the mechanisms and causes of such phenomena is based both on their own conclusions and on a wide spectrum of the data of previous investigations.     

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 (CO^EIC) in 37 mechanically ventilated patients after cardiac surgery. Both methods were used simultaneously.
Results: CO^EIC values were weakly correlated with CO^TD in the total group when the equation of Sramek-Bernstein was employed to calculate CO^EIC (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 (CO^EIC) 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.     

Measurement of cardiac output by transpulmonary arterial thermodilution using a long radial artery catheter. A comparison with intermittent pulmonary artery thermodilution

Cardiac output can be measured accurately by transpulmonary arterial thermodilution using the PiCCO (Pulsion Medical Systems, Munich, Germany) system with a femoral artery catheter. We have investigated the accuracy of a new 50 cm 4 French gauge radial artery catheter and the ability to use the system with a shorter radial catheter. We studied 18 patients who had undergone coronary artery surgery and made three simultaneous measurements of cardiac output by arterial thermodilution and with a pulmonary artery catheter. The radial catheter was withdrawn in 5 cm increments and the measurements were repeated. We found close agreement between arterial thermodilution and pulmonary artery thermodilution with a mean (SD) bias of 0.38 (0.77) l x min(-1). Arterial thermodilution became unreliable once the catheter had been withdrawn by more than 5 cm. We conclude that cardiac output measurement with arterial thermodilution with a radial catheter is interchangeable with that derived from a pulmonary artery catheter, and that a centrally sited arterial catheter is required for accurate determination of cardiac output by transpulmonary arterial thermodilution.     

Clinical evaluation of a new tracheal impedance cardiography method

Non-invasive cardiac output measurement by means of impedance cardiography has been evaluated before, and agreement with other methods has been variable. We decided to study a newly developed tracheal impedance device, that is claimed to be more accurate and reliable. This incorporates new software and mathematical formulae, that are designed to reduce signal noise from diathermy, leading to improved accuracy. In 25 cardiothoracic surgery patients, simultaneous measurements were performed using both pulmonary artery thermodilution and the tracheal impedance device, at five peri-operative time points: before skin incision; after weaning from cardiopulmonary bypass; after sternal closure; and 30 min and 2 h after arrival in the intensive care unit. Mean cardiac output, bias and 95% limits of agreement were 5.3, 0.03 and -2.8 to 2.8 l.min(-1) , respectively. Tracheal impedance showed good correlation with measurement trends using thermodilution in 88% of measurements, with a mean (95% limit of agreement) angular bias of -9.0° (-83.3 to 65.3°). However, the wide limits of agreement and high percentage error of 53% that were apparent in this study mean that, in its present guise, tracheal impedance is not an acceptable alternative to thermodilution in cardiac surgical patients.     

Noninvasive assessment of hemodynamic effects of mitral valve commissurotomy during rest and exercise in patients with mitral stenosis.

Noninvasive radionuclide angiocardiography (RNA) provides simple and accurate assessment of parameters of cardiac function during rest and during maximal exercise. Left ventricular function was assessed by RNA in nine patients with isolated mitral stenosis before and approximately 6 months after mitral commissurotomy. Before operation, the mean mitral valve gradient was 14.0 +/- 2.8 mm Hg, and the mean mitral valve area was 1.20 +/- 0.3 cm2. Each patient was evaluated at rest and during maximal exercise on an isokinetic bicycle ergometer before and after commissurotomy. Heart rate, ejection fraction, end-diastolic volume, stroke volume, pulmonary transit time, cardiac output, and diastolic ventricular filling rate were determined by the radionuclide technique. Before operation, patients with mitral stenosis had characteristic changes from rest to exercise which supported restriction to diastolic ventricular filling as the primary limitation in generating a cardiac output during exercise. The stroke volume was unchanged from rest to exercise. Thus the cardiac output during exercise was heart rate dependent. However, after commissurotomy the stroke volume increased from rest to exercise. Therefore, cardiac output during exercise was achieved by heart rate and an augmented stroke volume. Moreover, the pulmonary transit time was reduced during rest and exercise after operation. The maximum ventricular ejection and filling rates were markedly increased during rest and exercise after commissurotomy. These differences in hemodynamic parameters at rest and during exercise document the mechanics of increased exercise tolerance in patients with mitral stenosis after mitral commissurotomy.     

Haemodynamic changes associated with thermodilution cardiac output determination during metabolic acidosis or hypoxic hypoxia in dogs

Haemodynamic alterations elicited by iced injectate during thermodilution cardiac output measurements were evaluated in the presence of metabolic acidosis or hypoxic hypoxia in 14 instrumented anaesthetized dogs. The alterations in some haemodynamic variables during slowing of the heart rate following injection of 3 ml iced injectate were slightly greater in metabolic acidosis and hypoxic hypoxia as compared to animals without metabolic acidosis or hypoxic hypoxia (P < 0.05), but the changes were clinically insignificant. No serious haemodynamic changes were found during any cardiac output measurement by thermodilution in the presence of metabolic acidosis or hypoxic hypoxia. The values of cardiac output measured by thermodilution correlated closely with those of pulmonary blood flow measured by an electromagnetic flowmeter in the metabolic acidosis and hypoxic hypoxia groups (r > 0.9). It is concluded that thermodilution using iced injectate will estimate right ventricular output accurately in conditions of metabolic acidosis and hypoxic hypoxia.     

Haemodynamic monitoring and liver function evaluation by pulsion cold system Z-201 (PCS) during orthotopic liver transplantation

Pulsion cold system (PCS, COLD) is a haemodynamic monitoring system that allows measurement of cardiac output (CO), partial blood volumes, lung water, and liver function. The aim of the study was to evaluate this monitoring system during human orthotopic liver transplantation (OLT) for the following: (a) to determine agreement between CO measurements via pulmonary artery thermodilution (CO TDpa), and aortic transpulmonary thermodilution (CO TDa); (b) to compare the preload dates obtained with the COLD with central venous pressure (CVP) and pulmonary capillary wedge (PCWP); and (c) to assess the use of the plasma disappearance rate (PDR) of indocyanine green (ICG) as a measure of graft function. Fifteen consecutive patients undergoing OLT were studied. Each patient received a pulmonary artery catheter and a 5F aortic catheter with an integrated thermistor. The thermistor of the aortic catheter were connected to one computer system (COLD-Z201, Pulsion Medical Systems, Munich, Germany). Haemodynamic data were registered an all the phases of OLT. PDR was measured during surgery in 12 patients. Correlations between PDR and the other markers of graft function (transaminases, protrombine time, and bile production) were sought. The correlation coefficient between CO TDa (COLD) and CO TDpa was r = 0.766 (p < 0.001), and an additional analysis according to Bland-Altman was also performed. There was a better correlation between the cardiac index (determined by two monitoring systems) and the volume measurements than the correlation observed with pressure preload parameters. The best correlations were found between the cardiac index in the femoral artery and intrathoracic blood volume index (ITBVI) and pulmonary blood volume index (PBVI) (r = 0.79 and r = 0.72, respectively; p < 0.01). PDR measured in the group patients with bad early graft function were lower (13.6 +/- 2.7) than those in the group with a good graft function (21.6 +/- 9) (p < 0.05). The degree of discrepancy between femoral and pulmonary thermodilution cardiac output measures is very wide during OLT so as to make the techniques using the COLD machine clinically useless. On the other hand, the volumes measured by COLD, specially ITBVI and PBVI, are more useful to asses the pre-load than pressure measurements. In OLT, the PDR measured within the first few hours after liver reperfusion may become a useful tool for early diagnosis of primary graft dysfunction (PDF).     

Impedance cardiographyThe impact of new technology

The 1990s have witnessed major advances in impedance cardiography technology. Problems existed with the methods used to calculate cardiac output. Excessive lung fluid, as often found in critically ill patients, may also invalidate measurements. The signal processing and measurement techniques used in older systems were deficient. The newer systems, of which there are at least six, incorporate novel and improved signal processing techniques. They also offer analog visual displays, personal computer interfacing, sophisticated analytical software and haemodynamic patient management systems. Evaluation of these systems is difficult because no true 'gold standard' method of cardiac output measurement exists. When compared with thermodilution techniques, limits of agreement of ± 20–30% seem acceptable. These limits can be achieved in normal subjects but not in critically ill patients. Validation data are available for only half of the new systems. Until recently, the main application for impedance cardiography has been research but improved accuracy should lead to increased clinical usage.     

Haemodynamic changes associated with thermodilution cardiac output determination during myocardial ischaemia or pulmonary oedema in dogs

Since the technique of thermodilution (TD) cardiac output measurements per se causes haemodynamic alterations, the authors examined whether the alterations elicited by iced injectate are augmented in the presence of myocardial ischaemia (MI) or pulmonary oedema (PE), compromised conditions frequently associated with critically ill patients. MI (N = 7) or PE (N = 7) was induced by clamping the anterior descending coronary artery or by a slow infusion of oleic acid into the right atrium, respectively, in anaesthetized dogs. Injection of iced injectate, 3 ml, caused similar changes in heart rate, mean systemic and pulmonary arterial pressures, pulmonary blood flow, right ventricular dP/dt, and right atrial pressure in dogs with and without MI or PE. Cardiac output estimated by TD correlated closely with pulmonary blood flow measured by electromagnetic flowmeter in both MI and PE (r > 0.9). No profound alterations in haemodynamics were observed at any injection during TD cardiac output measurements under MI or PE. These results indicate that TD cardiac output determination does not cause serious haemodynamic alterations, and can estimate right ventricular output accurately under MI and PE.     

Compromised cardiac function in exercising teenagers with pectus excavatum

Patients with pectus excavatum complain about fatigue, tachypnea, discomfort and dyspnea, but the existence of an equivalent underlying pathophysiology has been questioned. We investigated 75 teenagers (49 pectus excavatum patients and 26 age matched controls) at rest and during bicycle exercise at submaximal exercise levels. At rest cardiac function was determined using echocardiography. During rest and exercise, cardiac output, heart rate and aerobic exercise capacity were measured using photo-acoustic gas-rebreathing technique for non-invasive determination of the cardiopulmonary function. At rest, no cardiac differences were found between control subjects and patients with pectus excavatum. During submaximal exercise, cardiac index was lower 6.6(6.3-7.0)?l/min/m(2) among the pectus patients as compared to the control subjects 8.0(7.3-8.8)?l/min/m(2), P=0.0001. The lower cardiac output among the pectus patients was due to a lower stroke index 42(39-45)?ml/beat/m(2) as compared to controls 54(44-64)?ml/beat/m(2), P=0.0022, whereas heart rate was unchanged. Cardiac function is significantly impaired at submaximal exercise level compared to healthy age matched controls.     

Cardiovascular response during severe acute asthma and its treatment in children.

Heart rate, blood pressure, pulsus paradoxus, and cardiac output measured by means of transthoracic electrical impedance cardiography have been recorded in 29 children mean age 10 years +/- 2 SD during status asthmaticus. Changes were recorded over the first two hours of treatment during which all patients received oxygen, intravenous fluid, and hydrocortisone, and were randomly assigned to receive aminophylline, salbutamol, or both. Admission values showed significant correlation of pulsus paradoxus with PaCO2 (r = 0.66). Pulsus paradoxus was greater than 20 mmHg for all patients with PaCO2 above 5.5 kPa. Mean stroke volume and cardiac output were 89% and 131% of the resting convalescent values in the same children. Stroke volume cardiac output and heart rate did not correlate with peak expiratory flow rate or blood gas measurements. Aminophylline and salbutamol together were associated with significantly greater increase in PEF than aminophylline alone (P less than 0.05). Nebulised salbutamol was just as effective as intravenous salbutamol. Heart rate and systolic blood pressure declined significantly after nebulised salbutamol and aminophylline, but not after intravenous salbutamol and aminophylline. Stroke volume and cardiac output did not change significantly in any treatment group.     

Changes in cardiac output and intrathoracic blood volume: a mathematical coupling of data?

BACKGROUND: Measurements of intrathoracic blood volume (ITBV) provide volumetric information about cardiac preload and are used to investigate the cause of alterations in cardiac output (CO). On the other hand, CO is required to calculate ITBV. Thus, concerns have been raised with respect to a mathematical coupling of data. The aim of this prospective, clinical study was to investigate whether a variation in CO induced by high-dose beta-blockade influences thermodilution measurements of ITBV in the absence of changes in intravascular volume in patients undergoing minimally invasive coronary artery bypass grafting. METHODS: Sixteen patients undergoing elective minimally invasive direct coronay artery bypass (MIDCAB) surgery were studied. Transpulmonary thermodilution measurements of ITBV and CO were simultaneously performed before bypass grafting, during beta-blockade induced by high-dose esmolol and at the end of surgery. RESULTS: During esmolol administration, CO significantly decreased by 33%, whereas ITBV remained unchanged compared to control values (876+/-46 ml m-2 during control versus 860+/-61 ml m-2 during esmolol administration). After the end of esmolol administration, CO significantly increased by 79%. Again, ITBV remained virtually unchanged (860+/-61 ml m-2 during esmolol administration versus 911+/-38 ml m-2 after esmolol administration). CONCLUSIONS: The results of the present study demonstrate that substantial alterations in CO as a consequence of high-dose esmolol infusion are not associated with changes in ITBV. Because haemodynamic changes were induced by factors other than variation of preload, these findings suggest that changes in cardiac output do not influence thermodilution measurements of ITBV in this setting.     

Noninvasive determination of exercise cardiac function following Fontan operation

The purpose of this investigation was to define mechanisms which increase cardiac output during exercise in patients without a functioning right ventricle. Radionuclide studies were performed at rest and during upright bicycle exercise on 16 patients following Fontan procedures and compared to studies in 10 normal children. The mean cardiac index of patients following Fontan procedures was 5.2 +/- 2.0 L/min/m2 (standard deviation) at rest and increased to 9.4 +/- 3.5 L/min/m2 during exercise (p less than 0.0001). Exercise values were similar to those observed in normal children. The mechanisms utilized to achieve high cardiac output following Fontan procedures included an increase in heart rate. Mean systemic ventricular ejection fractions were significantly less than in normal children. The resting ventricular end-diastolic volume and stroke volume indices were above normal and remained constant during exercise to high heart rates. These data clearly document that a large exercise cardiac output can be achieved by patients without a functioning right ventricle.     

A comparison of transoesophageal echocardiographic Doppler across the aortic valve and the thermodilution technique for estimating cardiac output

This study was undertaken in order to elucidate the differences between various planes of measurement and Doppler techniques (pulsed- vs. continuous-wave Doppler) across the aortic valve to estimate cardiac output. In 45 coronary artery bypass patients, cardiac output was measured each time using four different Doppler techniques (transverse and longitudinal plane, pulsed- and continuous-wave Doppler) and compared with the thermodilution technique. Measurements were performed after induction of anaesthesia and shortly after arrival in the intensive care unit. Optimal imaging was obtained in 91% of the patients, in whom a total of 82 measurements of cardiac output were performed. The respective mean (SD) areas of the aortic valve were 3.77 (0.71) cm2 in the transverse plane and 3.86 (0.89) cm2 in the longitudinal plane. A correlation of 0.87 was found between pulsed-wave Doppler cardiac output and the thermodilution technique in either transverse or longitudinal plane. Correlation coefficients of 0.82 and 0.84 were found between thermodilution cardiac output and transverse and longitudinal continuous-wave Doppler cardiac output, respectively. Although thermodilution cardiac output is a widely accepted clinical standard, transoesophageal Doppler echocardiography across the aortic valve offers adequate estimations of cardiac output. In particular, pulsed-wave Doppler cardiac output in both the transverse and longitudinal plane provides useful data.