Prediction of peak oxygen uptake in men using pulmonary and hemodynamic variables during exercise.

PURPOSE: Many attempts have been made to predict peak VO2 from data obtained at rest or submaximal exercise. Predictive submaximal tests using the heart rate (HR) response have limited accuracy. Some tests incorporate submaximal gas exchange data, but a predictive test without gas exchange measurements would be of benefit. Addition of stroke volume and pulmonary function (PF) measurements might increase the predictability of a submaximal exercise test. METHODS: In this study, an incremental exercise test (10 W x min(-1)) was performed in 30 healthy men of various habitual activity levels. Step-wise multiple regression analysis was used to isolate the most important predictor variables of peak VO2 from a set of measurements of PF: lung volumes, diffusion capacity, airway resistance, and maximum inspiratory and expiratory pressures; gas exchange; minute ventilation (V(E)), tidal volume (V(T)), respiratory exchange ratio (RER = carbon dioxide output divided by VO2); and hemodynamics (HR, stroke index (SI) = stroke volume/body surface area, and mean arterial pressure). These measurements were made at rest and during submaximal exercise. RESULTS: Using the set of PF variables (expressed as percentages of predicted), FEV1 explained 30% of the variance of peak VO2. No other PF variables were predictive. After addition of resting hemodynamic data, SI was included in the prediction equation, raising the predictability to 40%. At the 60-W exercise level, 48% of the variance in peak VO2 could be explained by SI and FEV1. At 150 W, the prediction increased to 81%. At this level VCO2/O2 (RER) also entered the prediction equation of peak VO2: 6.44 x FEV1(%) + 13.0 x SI - 1921 x RER + 2380 (SE = 142 mL x min(-1) x m(-2), P < 0.0001). Leaving out the gas exchange variable RER, maximally 64% of the variance in peak VO2 could be explained. CONCLUSION: In conclusion, inclusion of pulmonary function and hemodynamic measurements could improve the prediction accuracy of a submaximal exercise test. The submaximal exercise test should be performed until a level of 150 W is reached. Noninvasive stroke volume measurements by means of EIC have additional value to measurement of HR alone. Finally, measurement of gas exchange significantly improves the predictability of peak VO2.

The haemodynamic response to exercise in chronic obstructive pulmonary disease: assessment by impedance cardiography.

This study aimed to determine the differences in haemodynamic responses to a standard incremental exercise test between outpatients with chronic obstructive pulmonary disease (COPD) and age-matched controls and to discover the relationship between severity of airflow obstruction and exercise haemodynamics in COPD. Twenty-two male patients with COPD (forced expiratory volume in one second (FEV1)/vital capacity (VC))<80% predicted) and 20 age-matched male controls performed an incremental exercise test (10 W x min(-1)) with ventilatory function and changes in stroke volume (deltaSV) and cardiac output (deltaCO) measured by means of electrical impedance cardiography (EIC). Submaximal deltaSV and deltaCO were lower in COPD patients. Peak exercise deltaSV were equal in patients and controls (128+/-33 versus 129+/-29%, p=0.98), whereas peak deltaCO was lower in patients (COPD versus controls: 232+/-71 versus 289+/-54%, p<0.005). In COPD patients, FEV1 (% pred) was significantly correlated to deltaSV at all submaximal exercise intensities, to peak exercise deltaSV and to peak exercise deltaCO. FEV1/VC (% pred) was significantly correlated to deltaSV at 30 and 60 W. In conclusion, in chronic obstructive pulmonary disease an aberrant haemodynamic response to exercise was found, especially in patients with severe airflow obstruction. This aberrant response is related to the degree of airflow obstruction and may limit exercise performance in patients with severe chronic obstructive pulmonary disease.

Non-invasive assessment of cardiac output during exercise in chronic obstructive pulmonary disease: comparison of the CO2-rebreathing method and electrical impedance cardiography.

In exercise testing of patients with chronic obstructive pulmonary disease (COPD), non-invasive assessment of stroke volume (SV) and cardiac output (CO) would be valuable. Electrical impedance cardiography (EIC) has proved to be a valid and reliable instrument in healthy subjects. In this study it is investigated whether this also applies to patients with COPD. In 19 COPD patients simultaneous SV measurements were performed during steady-state exercise using the CO2-rebreathing method and EIC (using a fixed blood resistivity value (rho = 135 or 150 omega cm: EIC-135 and EIC-150) or a haematocrit based rho (EIC-ht)). Although close correlations were found (overall correlation between CO2-rebreathing and EIC-ht: R = 0.92 for CO, R = 0.79 for SV), SV and CO measured by means of EIC were significantly higher at low-intensity exercise and lower at high-intensity exercise. The mean differences between the CO2-rebreathing method and EIC-ht were 0.55 ml for SV and 0.01 l min-1 for CO (overall exercise data). The limits of agreement (2SD of the mean difference) were 24.7 ml for SV and 2.56 l min-1 for CO. These figures are comparable to what is found when healthy subjects are studied. CO was closely correlated to oxygen uptake using the CO2-rebreathing as well as the EIC method; the slope of the regression line was closer to what has been reported in the literature with EIC. Results were better with the EIC-ht than with the EIC-135 and EIC-150 methods. It is concluded that EIC is a reliable and valid method for measurements of SV and CO in COPD during exercise.

Prediction of pulmonary capillary wedge pressure and assessment of stroke volume by noninvasive impedance cardiography.

Early recognition of heart failure is important because early treatment reduces mortality and hospitalization rates. In screening for this disease, there is a need for a simple, safe, and cost-effective method to obtain cardiovascular variables. Therefore we developed a noninvasive impedance cardiographic method to predict the pulmonary capillary wedge pressure (PCWP) from the impedance cardiogram. The impedance cardiographic technique, though, was originally designed for stroke volume (SV) determination. The objectives of this study were to validate both variables by comparison with the paired, invasively obtained equivalents. PCWP, measured with a pulmonary artery catheter, was related to the O/C ratio from the impedance cardiogram. The O/C ratio was calculated as the amplitude of the impedance cardiogram during diastole (O) divided by the maximum height during systole (C). Stroke volume was also calculated from the impedance cardiogram according to the equation of Kubicek (SVIC) and compared with thermodilution (SVTD). Data analysis was performed in 24 stable patients who underwent diagnostic heart catheterization. Linear regression analysis showed that the O/C ratio was strongly related to the invasively measured PCWP over a range of 3 to 30 mm Hg (r = 0.92, standard error of the estimate, 3.2 mm Hg). Between SVIC and SVTD a moderate correlation was established (r = 0.69), but after exclusion of the data from patients with an aortic valve disorder (n = 5), the correlation increased considerably (r = 0.87). No significant differences between SVIC and SVTD were found (mean difference +/- 2 SD = 1.8 +/- 28.8 ml). These preliminary observations suggest that impedance cardiography can predict PCWP and measure SV over a wide range of clinically relevant values. The combined measurement of SV and PCWP by impedance cardiography might be a clinical useful tool in screening for heart failure.

Assessment of the haemodynamic response to exercise by means of electrical impedance cardiography: method, validation and clinical applications.

Over the past three decades, the technique of electrical impedance cardiography (EIC) has developed into a valid and reliable instrument for the assessment of stroke volume. Recent developments have made EIC suitable for routine use during exercise testing, too. However, standardization of electrode positioning, stroke volume calculation, and data processing is lacking. In our opinion the most reliable options are, respectively, a modified semicircular electrode array, the Kubicek equation including a haematocrit-based resistivity value, and computerized signal averaging. Although EIC derived stroke volume calculation is based on several debated assumptions, numerous validation studies have shown good accuracy and reproducibility, also during exercise. Addition of EIC measurements during standard clinical exercise testing might be of benefit in occupational medicine, cardiology and pulmonary medicine. Although in the latter setting no validation studies have been performed, major methodological problems are not expected.

Haemodynamic response to exercise in healthy young and elderly subjects.

Whereas with advancing age, peak heart rate (HR) and cardiac index (CI) are clearly reduced, peak stroke index (SI) may decrease, remain constant or even increase. The aim of this study was to describe the patterns of HR, SI, CI, arteriovenous difference in oxygen concentration (Ca-vO2), mean arterial pressure (MAP), systemic vascular resistance index (SVRI), stroke work index (SWI) and mean systolic ejection rate index (MSERI) in two age groups (A: 20-30 years, n = 20; B: 50-60 years n = 20). After determination of pulmonary function, an incremental bicycle exercise test was performed, with standard, gas-exchange measurements and SI assessment using electrical impedance cardiography. The following age-related changes were found: similar submaximal HR response to exercise in both groups and a higher peak HR in A than in B[185 (SD 9) vs 167 (SD 14) beats.min-1, P < 0.0005]; increase in SI with exercise up to 60-90 W and subsequent stabilization in both groups. As SI decreased towards the end of exercise in B, a higher peak SI was found in A [57.5 (SD 14.0) vs 43.6 (SD 7.7) ml.m-2, P < 0.0005]; similar submaximal CI response-to exercise, higher peak CI in A [10.6 (SD 2.5) vs 7.2 (SD 1.3) 1.min-1.m-2, P < 0.0005]; no differences in Ca-vO2 during exercise; higher MAP at all levels of exercise in B; higher SVRI at all levels of exercise in B; lower SWI in B after recovery; higher MSERI at all levels of exercise in A. The decrease in SI with advancing age would seem to be related to a decrease in myocardial contractility, which can no longer be compensated for by an increase in preload (as during submaximal exercise). Increases in systemic blood pressure may also compromise ventricular function but would seem to be of minor importance.

Standardization of non-invasive impedance cardiography for assessment of stroke volume: comparison with thermodilution.

Since its introduction by Kubicek and colleagues, impedance cardiography has been suggested as a non-invasive, simple, safe and cost-effective method of measuring stroke volume. Several controversial reports on its validity have been published. Pitfalls of this method included the nature of the electrode system and the validity of the equations. Therefore, the purpose of this study was to compare two different spot electrode arrays and the two most frequently used stroke volume equations with each other and with thermodilution. In 37 patients, 24-36 h after cardiac surgery, we performed simultaneous measurements of stroke volume with impedance cardiography (SVIC) and with thermodilution (SVTD). SVIC was obtained using the lateral spot (LS) electrode array, according to Bernstein, and a newly proposed modified semi-circular (MSC) spot electrode array. The equations of Kubicek and Sramek-Bernstein were used to calculate SVIC. The Sramek-Bernstein equation was valid only when the LS array was used; the Kubicek equation determined SVTD correctly only when the MSC array was used. However, a considerably better correlation and agreement (mean difference (2 SD)) was found between SVIC and SVTD for the latter (r = 0.90, 0.5 (17.1) ml vs r = 0.64, -4.9 (31.8) ml for the Sramek-Bernstein equation). We conclude that the most valid measurement of stroke volume using impedance cardiography was obtained when the MSC array was used together with Kubicek's equation.

Impedance cardiography. Importance of the equation and the electrode configuration.

OBJECTIVE: Electrical impedance cardiography (EIC) has been suggested as a non-invasive method to measure cardiac output. In several studies it proved to be a reliable method, although there were some restrictions. In 1966 Kubicek et al. developed an impedance cardiac output system based upon electrodes and a specific stroke volume formula. In 1983 Sramek et al. developed a new electrode configuration, and a new equation to calculate stroke volume, an equation that was adjusted by Bernstein in 1986. Since then these two methods have been used in clinical medicine. The purpose of the present study was to compare both electrode configurations and both stroke volume calculation equations with each other. The cardiac output (CO) values obtained by means of EIC are compared with CO values obtained by means of thermodilution. DESIGN: Prospective study. SETTING: Surgical intensive care unit of a university hospital. PATIENTS: 20 mechanically ventilated patients after cardiac surgery. MEASUREMENTS AND RESULTS: Simultaneous measurement of CO by means of electrical impedance cardiography (COEIC) and thermodilution (COTD) was performed. COEIC was obtained using the lateral spot electrode configuration (LS) and an adjusted circular electrode configuration (SC). The formulas of Sramek (S), Sramek-Bernstein (SB), Kubicek (K) and an adjusted Kubicek formula (aK) were employed. Using the LS electrode configuration, significant differences were found between COEIC and COTD with the S formula (p < 0.005), the K formula (p < 0.001), and the aK formula (p < 0.05). Using the SC electrode configuration, significant differences between COEIC and COTD were found with the K formula (p < 0.005), the S formula (p < 0.01), and the SB formula (p < 0.05). No significant differences was found between EIC and TD using the LS electrode configuration together with the SB formula or using the SC electrode configuration with the aK formula. In both cases a good correlation was found between COEIC and COTD (r = 0.86, p < 0.001 and r = 0.79, p < 0.001, respectively). The mean difference between EIC and TD was 0.15 +/- 0.96 1/min and 0.19 +/- 1.19 1/min, respectively.

Comparison of the respiratory and hemodynamic responses of healthy subjects to exercise in three different protocols.

Although the importance of exercise testing has been well established, standardization of protocols is lacking. In the current study three protocols were compared with respect to respiratory and hemodynamic variables at submaximal and peak exercise. Fifteen healthy young men underwent three maximal exercise tests using the following protocols: (I) an increase of 30 Watt, every three minutes; (II) an increase of 10 Watt, every minute; (III) a continuous load increase of 10 Watt/min. Respiratory measurements were made of oxygen uptake (VO2), carbon dioxide output (VCO2), minute ventilation (VE) and tidal volume (VT). Hemodynamic measurements were made of ECG, heart rate (HR), blood pressure and stroke volume (SV). The latter variable was measured by means of electrical impedence cardiography (EIC). There were no differences in mean maximum load or peak-VO2 between protocols I, II and III. The course of SV was similar in all protocols, i.e. an increase of about 30% until 100 Watt, with a subsequent stabilization until maximum load. All other hemodynamic measurements were similar in both protocols, too. Significant differences were found in submaximal values of VO2 and VCO2. There were no differences in other gas-exchange variables at any moment during exercise. With respect to the VO2max or the hemodynamic response to exercise, any protocol can be used. For the evaluation of submaximal exercise, the protocol that has been used has to be taken into account. Differences at these levels are not related to differences in hemodynamic responses.

The intra- and interobserver variability of impedance cardiography in patients at rest and during exercise.

We studied the intra- and interobserver variability in the calculation of stroke volume by the impedance technique, using the recently proposed refinements in the electrode configuration and signal processing. Three groups of patients were included in this study: ten stable cardiac patients who underwent a diagnostic heart catheterization, ten patients 24-26 h after coronary artery bypass surgery and ten patients with severe chronic obstructive pulmonary disease (COPD). The first two groups were studied at rest and the COPD group during submaximal exercise. The intra-observer variability was 4.2%, 3.9-4.0% and 6.0-6.9% for the catheterized, surgical and COPD groups, respectively. The interobserver variability was 4.3%, 2.6% and 2.4%, respectively. It is concluded that highly reproducible data can be obtained with the newly proposed impedance technique in patients at rest and exercise which may be comparable or superior to other techniques used in clinical settings.

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.

The lowering of stroke volume measured by means of impedance cardiography during endexpiratory breath holding.

Impedance cardiography is a reliable method for estimating stroke volume (SV). Breathing, however, causes artefacts, which can be avoided by measuring during breath holding. This study investigated whether SV determination is accurate during breath holding. Twelve healthy subjects were tested in the supine position at rest and during two levels of exercise: 100 and 200 W. Averaged SV values were monitored by means of impedance cardiography before and after endexpiratory breath holding. During breath holding, SV measurement was on a beat-to-beat basis. An obvious decrease in SV during breath holding was noticed, being significant only during exercise (mean decrease of 38% at 100 W and 58% at 200 W). The rest measurements were repeated with open and closed glottis, which yielded the same results. This indicates that the SV decrease was not caused by a Valsalva-like manoeuvre. The mean SV value calculated by means of impedance cardiography for the total breath hold period was significantly lower than the SV during breathing, both at rest (91.7 +/- 2.4%) and at 100 W (90.5 +/- 7.0%). From this study it can be concluded that averaging of the impedance signal, measured while the subject is breathing, is preferential to measuring during breath holding, because the latter condition systematically underestimates SV.

Bioelectrical impedance analysis: clinical tool in assessing total body water and thoracic fluid.

Bioelectrical impedance analysis forms a non-invasive tool for detection of body fluids. Total body measurement gives total body water (TBW) and, in case of multi-frequency analysis, of intra- and extracellular fluid volume. The thoracic approach measures thoracic fluid (TF). The set-up of both techniques is discussed. An overview is given of the clinical usefulness of the total body technique to monitor fluid changes and the process of refill during hemodialysis and to detect dry weight. The simultaneous measurement of TBW and TF was applied to obtain a more detailed picture of the body fluids. In a group of healthy subjects the age dependency of both variables was shown. During hemodialyss TBW and TF showed a major and comparable decrease. Fluid retention during cardiac surgery led to a slightly more pronounced increase of TF than of TBW. The combination of both impedance techniques offers clinicians a means to monitor alterations in fluid status in patients in more detail.