Determination of cardiac output during mechanical ventilation by electrical bioimpedance or thermodilution in patients with acute lung injury: effects of positive end-expiratory pressure

Comparative study of cardiac output measurement by regional impedance cardiography and thermodilution method in patients undergoing off pump coronary artery bypass graft surgery

Background:

An ideal CO monitor should be noninvasive, cost effective, reproducible, reliable during various physiological states. Limited literature is available regarding the noninvasive CO monitoring in open chest surgeries.

Aim:

The aim of this study was to compare the CO measurement by Regional Impedance Cardiography (RIC) and Thermodilution (TD) method in patients undergoing off pump coronary artery bypass graft surgery (OPCAB).

Settings and Design:

We conducted a prospective observational comparative study of CO measurement by the noninvasive RIC method using the NICaS Hemodynamic Navigator system and the gold standard TD method using pulmonary artery catheter in patients undergoing OPCAB. A total of 150 data pair from the two CO monitoring techniques were taken from 15 patients between 40-70 years at various predefined time intervals of the surgery.

Patients and Methods:

We have tried to find out the accuracy, precision and cost effectiveness of the newer RIC technique. Mean CO, bias and precision were compared for each pair i.e.TD-CO and RIC-CO as recommended by Bland and Altman. The Sensitivity and specificity of cutoff value to predict change in TD-CO was used to create a Receiver operating characteristic or ROC curve.

Results:

Mean TD-CO values were around 4.52 ± 1.09 L/min, while mean RIC- CO values were around 4.77± 1.84 L/min. The difference in CO change was found to be statistically not significant (p value 0.667). The bias was small (-0.25). The Bland Altman plot revealed a mean difference of -0.25 litres. The RIC method had a sensitivity of 55.56 % and specificity of 33.33 % in predicting 15% change in CO of TD method and the total diagnostic accuracy was 46.67%.

Conclusion:

A fair correlation was found between the two techniques. The RIC method may be considered as a promising noninvasive, potentially low cost alternative to the TD technique of hemodynamic measurement.

Accuracy of impedance cardiography for evaluating trends in cardiac output: a comparison with oesophageal Doppler

BACKGROUND

Impedance cardiography (ICG) enables continuous, beat-by-beat, non-invasive, operator-independent, and inexpensive cardiac output (CO) monitoring. We compared CO values and variations obtained by ICG (Niccomo™, Medis) and oesophageal Doppler monitoring (ODM) (CardioQ™, Deltex Medical) in surgical patients.

METHODS

This prospective, observational, single-centre study included 32 subjects undergoing surgery with general anaesthesia. CO was measured simultaneously with ICG and ODM before and after events likely to modify CO (vasopressor administration and volume expansion). One hundred and twenty pairs of CO measurements and 94 pairs of CO variation measurements were recorded.

RESULTS

The CO variations measured by ICG correlated with those measured by ODM [r=0.88 (0.82-0.94), P<0.001]. Trending ability was good for a four-quadrant plot analysis with exclusion of the central zone (<10%) [95% confidence interval (CI) for concordance (0.86; 1.00)]. Moderate to good trending ability was observed with a polar plot analysis (angular bias: -7.2°; 95% CI -12.3°; -2.5°; with radial limits of agreement -38°; 24°). After excluding subjects with chronic obstructive pulmonary disease, a Bland-Altman plot showed a mean bias of 0.47 litre min(-1), limits of agreements between -1.24 and 2.11 litre min(-1), and a percentage error of 35%.

CONCLUSION

ICG appears to be a reliable method for the non-invasive monitoring of CO in patients undergoing general surgery.

Physiologic and Clinical Principles behind Noninvasive Resuscitation Techniques and Cardiac Output Monitoring

Clinical assessment and vital signs are poor predictors of the overall hemodynamic state. Optimal measurement of the response to fluid resuscitation and hemodynamics has previously required invasive measurement with radial and pulmonary artery catheterization. Newer noninvasive resuscitation technology offers the hope of more accurately and safely monitoring a broader range of critically ill patients while using fewer resources. Fluid responsiveness, the cardiac response to volume loading, represents a dynamic method of improving upon the assessment of preload when compared to static measures like central venous pressure. Multiple new hemodynamic monitors now exist that can noninvasively report cardiac output and oxygen delivery in a continuous manner. Proper assessment of the potential future role of these techniques in resuscitation requires understanding the underlying physiologic and clinical principles, reviewing the most recent literature examining their clinical validity, and evaluating their respective advantages and limitations.

Cardiac output measurements with electrical velocimetry in patients undergoing CABG surgery

BACKGROUND AND OBJECTIVE

The purpose was to study the agreement between cardiac output measurements with electrical velocimetry vs. intermittent thermodilution before and after coronary artery bypass graft surgery.

METHODS

Cardiac output was measured simultaneously with electrical velocimetry and intermittent thermodilution before and immediately after coronary artery bypass graft surgery, and in the intensive care unit. Measurements were performed in three different body positions. The results were analysed according to Bland and Altman.

RESULTS

The mean bias of all 150 paired measurements in 16 patients was 0.21 +/- 0.78 L min(-1), and the mean error was 40%. Before skin incision the mean bias was 0.04 +/- 0.41 L min(-1), and the mean error was 25%. After skin closure the mean bias was 0.57 +/- 0.92 L min(-1), and the mean error was 42%. In the intensive care unit the mean bias was 0.26 +/- 0.68 L min(-1), and the mean error was 32%.

CONCLUSIONS

The agreement between cardiac output measurements with electrical velocimetry and intermittent thermodilution was clinically acceptable only before skin incision in coronary artery bypass graft surgery. The mean error was unacceptably high immediately after skin closure and was at a borderline level in the intensive care unit. Thus, the overall accuracy of cardiac output measurements with the electrical velocimetry technique during coronary artery bypass graft surgery is not clinically unacceptable.

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.

Stroke volume equation for impedance cardiography

The study's goal was to determine if cardiac output (CO), obtained by impedance cardiography (ICG), would be improved by a new equation N, implementing a square root transformation for dZ/dtmax/Z0, and a variable magnitude, mass-based volume conductor Vc. Pulmonary artery catheterisation was performed on 106 cardiac surgery patients pre-operatively. Post-operatively, thermodilution cardiac output (TDCO) was simultaneously compared with ICG CO. dZ/dtmax/Z0 and Z0 were obtained from a proprietary bioimpedance device. The impedance variables, in addition to left ventricular ejection time TLVE and patient height and weight, were input using four stroke volume (SV) equations: Kubicek (K), Sramek (S), Sramek-Bernstein (SB), and a new equation N. CO was calculated as SV x heart rate. Data are presented as mean +/- SD. One way repeated measures of ANOVA followed by the Tukey test were used for inter-group comparisons. Bland-Altman methods were used to assess bias, precision and limits of agreement. P< 0.05 was considered statistically significant. CO implementing N (6.06 +/- 1.48 l min(-1)) was not different from TDCO (5.97 +/- 1.41 l min(-1)). By contrast, CO calculated using K (3.70 +/- 1.53 l min(-1)), S (4.16 +/- 1.83 l min(-1)) and SB (4.37 +/- 1.82 l min(-1)) was significantly less than TDCO. Bland-Altman analysis showed poor agreement between TDCO and K, S and SB, but not between TDCO and N. Compared with TDCO, equation N, using a square-root transformation for dZ/dtmax/Z0, and a mass-based Vc, was superior to existing transthoracic impedance techniques for SV and CO determination.

Transthoracic electrical bioimpedance: a means of filling the void?

OBJECTIVES

To examine the published evidence regarding the use of transthoracic electric bioimpedance (TEB) for the non-invasive monitoring of cardiac output in the ED.

METHOD

Databases of the medical literature, relevant textbooks and the Internet were searched for articles regarding TEB. Criteria for inclusion were drawn up prior to examination of the articles and included adherence to guidelines for comparing methods of clinical measurement.

RESULTS

Results are discussed under the following headings: technological capability, diagnostic accuracy, limitations, range of possible uses, therapeutic impact, impact on health care providers, patient outcome and future directions.

CONCLUSION

TEB is a technique for the non-invasive monitoring of cardiac output whose ease of use, continuous data acquisition and versatility suggest it may have a role to play in the care of patients in our EDs.

Equipment review: the success of early goal-directed therapy for septic shock prompts evaluation of current approaches for monitoring the adequacy of resuscitation

A recent trial utilizing central venous oxygen saturation (SCVO₂) as a resuscitation marker in patients with sepsis has resulted in its inclusion in the Surviving Sepsis Campaign guidelines. We review the evidence behind SCVO₂ and its relationship to previous trials of goal-directed therapy. We compare SCVO₂ to other tools for assessing the adequacy of resuscitation including physical examination, biochemical markers, pulmonary artery catheterization, esophageal Doppler, pulse contour analysis, echocardiography, pulse pressure variation, and tissue capnometry. It is unlikely that any single technology can improve outcome if isolated from an organized pattern of early recognition, algorithmic resuscitation, and frequent reassessment. This article includes a response to the journal's Health Technology Assessment questionnaire by the manufacturer of the SCVO₂ catheter.

Comparison of initial distribution volume of glucose and plasma volume in thoracic fluid-accumulated patients

OBJECTIVE

We have reported that initial distribution volume of glucose indicates the central extracellular fluid volume in the presence of fluid gain or loss. The purpose of this study was to describe changes in initial distribution volume of glucose, plasma volume determined by the indocyanine green dilution method (PV-ICG), and thoracic fluid content by thoracic electrical bioimpedance in patients with or without apparent thoracic fluid accumulation in the absence of pleural effusion. We also sought to test whether initial distribution volume of glucose rather than PV-ICG mirrors thoracic fluid content.

DESIGN

Prospective, clinical study.

SETTING

General intensive care unit.

PATIENTS

Eleven consecutive patients with apparent thoracic fluid accumulation as judged by thoracic fluid content >0.05/ohm and underlying pathology and 20 consecutive acute myocardial infarction patients within 24 hrs after its onset were selected for study. None of the acute myocardial infarction patients had thoracic fluid content >0.05/ohm.

INTERVENTIONS

Five grams of glucose and 25 mg of indocyanine green were administered simultaneously to calculate initial distribution volume of glucose and PV-ICG daily for the fluid-accumulated patients, and the same dosages were administered to the acute myocardial infarction patients immediately after their admission to the intensive care unit after percutaneous coronary angioplasty. Only the data on the day of the maximal and minimal thoracic fluid content in the fluid-accumulated patients were used for the study. The relationship between these two fluid volumes and thoracic fluid content was evaluated in the two patient groups.

MEASUREMENTS AND MAIN RESULTS

Initial distribution volume of glucose and thoracic fluid content rather than PV-ICG and thoracic fluid content moved together in the same direction in each fluid-accumulated patient. Neither pulmonary artery occlusion pressure, central venous pressure, nor PV-ICG produced a better correlation with cardiac index when compared with initial distribution volume of glucose in patients with or without thoracic fluid accumulation.

CONCLUSIONS

We suggest that initial distribution volume of glucose rather than PV-ICG is a better indicator of the intrathoracic blood volume status, even although intravenously administered glucose cannot stay in the intravascular compartment.

Use of impedance cardiography to monitor haemodynamic changes during laparoscopy in children

METHODS

Haemodynamic changes were measured noninvasively using impedance cardiography (ICG) in 30 ASA I children during laparoscopic varicocelectomy under general anaesthesia. After induction and intubation, mechanical ventilation was started, then pneumoperitoneum (PP) was created. During the course of anaesthesia, values of endtidal CO2 pressue (PECO2), peak inspiratory airway pressure (PIP), heart rate (HR), mean arterial blood pressure (MABP), stroke volume index (SVI), cardiac index (CI) and systemic vascular resistance index (SVRI) were recorded at 1 min intervals. We analysed four periods: T1, before induction; T2, after induction; T3, during PP; T4, after desufflation of PP until awake.

RESULTS

After induction of anaesthesia a significant reduction of HR, MABP and CI was recorded. Creating PP together with the use of a 15 degrees head down tilt resulted in a further drop in CI, mainly caused by the reduction of SVI, and an elevation of MABP and SVRI. We measured a 25% total decrease of CI.

CONCLUSION

Our patients tolerated this significant reduction of cardiac output well. We have demonstrated that ICG can be used to track the haemodynamic changes caused by PP in children, and suggest that this type of monitoring is useful in this group of age during laparoscopy.