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American Journal of Critical Care. 2003;12: 336-342
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Physicians’ Estimates of Cardiac Index and Intravascular Volume Based on Clinical Assessment Versus Transesophageal Doppler Measurements Obtained by Critical Care Nurses

By Manuel G. Iregui, MD, Donna Prentice, MSN, Glenda Sherman, BSN, Lynn Schallom, MSN, Carrie Sona, MSN and Marin H. Kollef, MD. From Pulmonary and Critical Care Division, Department of Internal Medicine, Washington University School of Medicine (MGI, MHK), and Department of Nursing, Barnes-Jewish Hospital (DP, GS, LS, CS), St. Louis, Mo.


   Abstract
Top
 Abstract
Materials and Methods
 Results
 Discussion
 References
 
• Objectives To compare physicians’ estimates of cardiac index and intravascular volume with transesophageal Doppler measurements obtained by critical care nurses, to assess the overall safety of transesophageal Doppler imaging by critical care nurses, and to compare hemodynamic measurements obtained via transesophageal Doppler imaging with those obtained via pulmonary artery catheterization.

• Methods Data were collected prospectively on 106 patients receiving mechanical ventilation. Physicians estimated cardiac index and intravascular volume status by using bedside clinical assessment; critical care nurses, by using transesophageal Doppler imaging. In 24 patients, Doppler measurements were obtained within 6 hours of placement of a pulmonary artery catheter and recording of cardiac output and pulmonary artery occlusion pressure.

• Results With Doppler measurements as the reference, physicians correctly estimated cardiac index in 46 (43.8%) of 105 patients, underestimated it in 24 (22.9%), and overestimated it in 35 (33.3%). They correctly estimated volume status in 31 patients (29.5%), underestimated it in 16 (15.2%), and overestimated it in 58 (55.2%). Doppler measurements of cardiac output correlated with those obtained via pulmonary artery catheterization (r = 0.778; P < .001). Two patients had minor complications: dislodgement of a nasogastric tube and inability to obtain a Doppler signal.

• Conclusion Physicians’ assessment of cardiac index and intravascular volume in patients receiving mechanical ventilation is correct less than half of the time. Transesophageal Doppler imaging by critical care nurses appears to be a safe method for measuring cardiac index and estimating intravascular volume. Measurements obtained via Doppler imaging correlate well with those obtained via pulmonary artery catheterization.

Using pulmonary artery catheterization to assess hemodynamic status in critically ill patients is a common practice. However, several reports1–4 suggest that important complications are often associated with pulmonary artery catheterization, including venous thrombosis, arrhythmias, pneumothorax, and catheter-associated bloodstream infections. Connors et al5 found that 30-day mortality, lengths of stay in the intensive care unit (ICU) and hospital, and use of resources were significantly increased among patients with pulmonary artery catheters. Subgroup analysis did not show any group for which right-sided heart catheterization was associated with improved outcomes. However, confounding variables such as differences in patients’ severity of illness or comorbid conditions might account for the greater mortality observed among patients with pulmonary artery catheters.

A consensus panel convened by the National Heart, Blood, and Lung Institute and the Food and Drug Administration evaluated pulmonary artery catheterization as a medical technology.6 The panel developed 4 specific recommendations for the future evaluation and use of pulmonary artery catheters:

  1. standardize, monitor, and measure physicians’ and nurses’ education to improve the quality, safety, and use of clinical information obtained by using pulmonary artery catheters;
  2. conduct clinical trials to determine the safety and effectiveness of using pulmonary artery catheters in persistent/refractory congestive heart failure (New York Heart Association class IV) and severe sepsis/septic shock;
  3. systematically evaluate new device technology in critical care medicine to improve patients’ outcomes; and 4. encourage international collaboration.

These recommendations were aimed at developing a reference standard for testing new devices and treatments in the critically ill.

Because of the controversy over the usefulness and safety of pulmonary artery catheterization, other less invasive technologies have been sought to provide similar clinical data at the bedside. We performed a study in which we used a commercially available transesophageal Doppler device to address the third recommendation of the panel. Our study had 3 main goals:

  1. to compare physicians’ estimates of cardiac index and intravascular volume based on clinical assessment with measurements obtained by using transesophageal Doppler imaging,
  2. to assess the overall safety of transesophageal Doppler imaging as applied by critical care nurses, and
  3. to compare hemodynamic measurements obtained via transesophageal Doppler imaging with those obtained via pulmonary artery catheterization.


Significant complications accompany the use of pulmonary artery catheters. The development of standards to enhance their safety and efficacy is recommended.

 


   Materials and Methods
Top
 Abstract
 Materials and Methods
Results
 Discussion
 References
 
Study Location and Sample
The study was done at Barnes-Jewish Hospital, a university-affiliated urban teaching hospital of 1000 beds in St. Louis, Mo. During a 1-year period (February 2001 through February 2002), all critically ill patients receiving mechanical ventilation in the medical ICU (19 beds) and the surgical ICU (18 beds) were eligible for evaluation. Patients were included in the study if their ICU physician team requested that a transesophageal Doppler probe be inserted and the Doppler data collected and interpreted by critical care nurses. The main indications for use of transesophageal Doppler imaging were assessment of cardiac function and intravascular volume. The main alternative for obtaining this information in these ICUs was placement of a pulmonary artery catheter. Patients were not eligible for transesophageal Doppler imaging if they had a history of esophageal varices, were pregnant, or were not tracheally intubated. The requirement for tracheal intubation and mechanical ventilation was included to minimize the potential for aspiration or cardiopulmonary failure. The medical and surgical ICUs are closed units; all care is provided by multidisciplinary teams directed by board-certified critical care physicians. The study was approved by the appropriate human studies committee.

Study Design and Data Collection
A prospective, observational cohort study design was used. The physician teams caring for patients in the medical and surgical ICUs determined the need for hemodynamic monitoring. Each physician team consisted of a board-certified critical care physician, a critical care fellow, and medical and surgical house officers. For each patient, before placement of the transesophageal Doppler probe, the patient’s physician team was asked to provide an estimate of the patient’s cardiac index and intravascular volume. Each team was instructed to describe their combined team estimates as high, normal, or low for both cardiac index and intravascular volume. The physician teams estimated cardiac index and intravascular volume status on the basis of physical examination, vital signs, laboratory values, measurements of central venous pressure if available from an appropriately placed central venous catheter, and chest radiographs. Within 1 hour after these clinical estimates were obtained, the trans-esophageal Doppler probe was placed and the resultant hemodynamic data were compared with the physician team’s clinical estimates. Changes in medical treatment that occurred within 2 hours after trans-esophageal Doppler measurements were also recorded.

For all patients, the following characteristics were prospectively recorded by one of the investigators: age, ethnicity, weight, height, body surface area, indication for mechanical ventilation, ratio of PaO2 to fraction of inspired oxygen, and severity of illness as indicated by scores on the Acute Physiology and Chronic Health Evaluation (APACHE) II.7 One of the investigators made daily rounds in the ICUs to identify eligible patients. Patients entered into the study were followed up until they were successfully weaned from mechanical ventilation, discharged from the hospital, died, or transferred to a long-term care facility. This follow-up was specifically done to monitor patients for delayed consequences of transesophageal Doppler imaging (eg, esophageal perforation with delayed manifestations). The safety of the imaging procedure was assessed by determining the occurrence of specific complications that occurred during or within 6 hours of transesophageal Doppler measurements. The complications assessed included oxygen saturation less than 92% as determined by oximetry, atrial or ventricular arrhythmias, gastrointestinal bleeding, and unintentional tracheal extubation.


This study compared cardiac index and intravascular volume status measurements using physician clinical assessment, transesophageal Doppler, and pulmonary artery catheterization.

 

Measurements of Cardiac Output
For the purpose of analysis, the first set of trans-esophageal Doppler measurements was used for comparison with the physician teams’ estimates of cardiac index and intravascular volume. Additional monitoring was performed at the recommendation of the physician teams treating the patients. The ICU nurses who placed the transesophageal Doppler probes and recorded hemodynamic data underwent 2 hours of instruction about the devices. Each nurse was also supervised by one of the investigators (M.G.I., L.S., or D.P.) until he or she demonstrated proficiency in the use of the probe. The nurses who obtained the hemodynamic data had no knowledge of the physician teams’ clinical estimates of cardiac index and intravascular volume. Subsequent need for a pulmonary artery catheter was determined by each patient’s physician team.

  Transesophageal Doppler Monitoring.   The trans-esophageal Doppler device consists of a continuous-wave Doppler transducer (4 MHz) mounted at 45° at the tip of a transesophageal probe (diameter, 8 mm). The probe is connected to a monitor (DOPTEK-ODM1, Deltex Medical Ltd, Chichester, England) that displays the blood-flow velocity profile after spectral analysis of the reflected Doppler-shift signal (fast Fourier transformation).8 After oral introduction, the probe is advanced gently until its tip is located in the midesophagus, approximately 35 cm from the incisors, and a characteristic aortic blood flow signal is obtained. Insertion of the probe is optimized to record peak velocity by using slow rotation of the probe in the long axis and alteration of the depth of insertion to generate a clear signal. Gain setting is adjusted to obtain the best outline of the aortic velocity waveform, and a 300-Hz high-pass filter is used to eliminate the noise related to low-frequency motion of the vessel wall. Before each measurement, the position of the probe was verified to ensure optimal acquisition of the maximal velocity signal.

Stroke volume (in milliliters) was calculated as follows: stroke volume = CSAAo x K x T{int}0 VAo(t)dt, where VAo(t) represents instantaneous maximum aortic velocity, T is the cardiac ejection time (the integral of instantaneous maximum velocity during cardiac ejection, representing the stroke distance), CSAAo is the cross-sectional area of the descending thoracic aorta (in centimeters squared), and K is a correcting factor (1.43) whose purpose is to transform the blood flow measured in the descending thoracic aorta into a global measure of cardiac output, based on the assumption that a constant fraction (70%) of the total blood flow passes through the descending aorta. CSAAo is estimated from a nomogram on the basis of the patient’s age, height, and weight. The monitor was preset to calculate cardiac output (liters per minute) by averaging stroke volume over 12 beats and multiplying the value obtained by the heart rate. The left ventricular flow time (ie, ventricular ejection time) corrected for heart rate (ie, corrected flow time) provides an index of left ventricular end-diastolic volume or preload.8 Previous investigators9–13 have validated the accuracy of values obtained via transesophageal Doppler imaging for both cardiac output and the corrected flow time compared with measurements obtained via pulmonary artery catheterization.


The transesophageal Doppler, placed near the thoracic aorta, is used to calculate stroke volume, based primarily on aortic velocity and cardiac ejection time, which is then used to calculate cardiac index.

 

For purposes of analysis, values of cardiac index (calculated as cardiac output in liters per minute divided by body surface area in square meters) obtained with transesophageal Doppler imaging that were less than 2.5 were considered low, values greater than 4.5 were considered high, and all other values were considered normal. Corrected flow time measurements less than 330 ms were considered low, values greater than 360 ms were considered high, and all other values were considered normal.

  Thermodilution Method.   For thermodilution measurements of cardiac output, an 8F balloon-tipped pulmonary artery catheter connected to a cardiac output monitor (model 58-1386-R3; Abbott Laboratories, Mountain View, Calif) was used. A total of 10 mL of a 5% solution of dextrose was injected at end-expiration, and cardiac output was obtained by averaging the results of at least 3 measurements. Variability of measurements was assumed to be a result of positive-pressure ventilation and was not a criterion for rejection as long as the cardiac output measured by thermodilution was satisfactory as judged by examination of the waveform obtained. Measurements of pulmonary artery occlusion pressure were obtained in a standard manner as an index of left ventricular end-diastolic volume or preload.13

Statistical Analysis
All comparisons between the transesophageal Doppler measurements and physicians’ clinical estimates were paired, and all tests of significance were 2-tailed. Continuous variables were compared by using the t test. The {chi}2 test was used to compare categorical variables. Correlation between measurements obtained via transesophageal Doppler imaging and those obtained via a pulmonary artery catheter were determined by using simple linear regression analysis.


   Results
Top
 Abstract
 Materials and Methods
 Results
Discussion
 References
 
Sample
A total of 106 patients receiving mechanical ventilation underwent hemodynamic monitoring via a trans-esophageal Doppler probe as applied by 7 ICU nurses (mean of 15 procedures per nurse). The patients’ characteristics and indications for mechanical ventilation are provided in the TableGo.


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  		Characteristics of the sample (N = 106)*

 

Physician clinical assessments were similar to transesophageal Doppler measurements less than half of the time for cardiac index and less than one third of the time for volume status measurements.

 

Usefulness of Transesophageal Doppler
Transesophageal Doppler imaging was successful in all but 1 patient. This patient died within 6 hours of admission to the medical ICU without an adequate Doppler flow velocity waveform being obtained. Doppler flow velocity waveforms were adequate in 105 patients (99.1%). In 24 patients (22.9%), trans-esophageal Doppler measurements were obtained within 6 hours of placement of a pulmonary artery catheter and recording of cardiac output and pulmonary artery occlusion pressure.

For cardiac index, the physician teams’ estimates and the transesophageal Doppler measurements were similar in 46 (43.8%) of the 105 patients. The teams underestimated the index in 24 patients (22.9%) and overestimated it in 35 (33.3%). For intravascular volume, the teams’ estimates agreed with the transesophageal Doppler measurements of the corrected flow time in 31 patients (29.5%). The teams underestimated the intravascular volume in 16 patients (15.3%) and overestimated it in 58 (55.2%).

A total of 57 patients (54.3%) had a change in their treatment that occurred within 2 hours after the transesophageal Doppler measurements were obtained. These therapeutic changes were administration of fluid boluses (27 patients), intravenous diuretics (12 patients), dobutamine infusion (8 patients), fluid boluses with vasopressor infusion (8 patients), and vasodilator infusions (2 patients).

Safety of Transesophageal Doppler Imaging
In addition to the 1 patient in whom a Doppler flow velocity waveform could not be obtained, 1 patient had unintentional removal of an orogastric tube during removal of the transesophageal Doppler probe. Additional sedation was required on the basis of nursing assessments in 40 (37.7%) of 106 patients during placement of the transesophageal Doppler probe, primarily in the form of intravenous boluses of midazolam. We did not observe any accidental tracheal extubation, gastrointestinal bleeding, esophageal perforation, arrhythmias, or oxygen desaturation during or within 6 hours of placement of the transesophageal Doppler probe.

Comparison of Measurements Obtained via Transesophageal Doppler Imaging With Measurements Obtained via Pulmonary Artery Catheterization
Transesophageal Doppler measurements of cardiac output were compared with thermodilution measurements of cardiac output in 24 patients (Figure 1Go). The correlation between the 2 types of measurement was significant (r = 0.778; P < .001). A significant correlation was also found between the corrected flow time obtained with transesophageal Doppler imaging and measurements of pulmonary artery occlusion pressure (r = 0.541; P = .01; Figure 2Go).



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  		Figure 1 Scatter plot of measurements of cardiac output obtained with transesophageal Doppler imaging (TED) compared with values obtained with the thermodilution method (TH).

 


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  		Figure 2 Scatter plot of corrected flow time measurements obtained with transesophageal Doppler imaging compared with pulmonary artery occlusion pressures obtained with a pulmonary artery catheter.

 

   Discussion
Top
 Abstract
 Materials and Methods
 Results
 Discussion
References
 
Transesophageal Doppler imaging is a minimally invasive method for the continuous monitoring of cardiac output in critically ill patients. In our study, ICU nurses were able to safely place the Doppler probe in the esophagus and obtain adequate aortic flow velocity waveforms in more than 90% of the patients. The cardiac output values from transesophageal Doppler imaging correlated well with the measurements of cardiac output obtained by using thermodilution. We also found a significant correlation between measurements of pulmonary artery occlusion pressure and corrected flow time measurements as estimates of intravascular volume. Most important, physician teams’ estimates of cardiac index differed from measurements obtained via transesophageal Doppler imaging in more than 50% of the patients. On the basis of this experience, we conclude that transesophageal Doppler imaging can provide potentially useful estimates of cardiac index and intravascular volume in critically ill patients who require mechanical ventilation.

Clinical assessment of cardiac output at the bedside by physicians can be problematic, especially in patients with underlying cardiac disease.14–18 Previous studies14–18 indicated that physicians’ estimates of cardiac output are correct only about 50% of the time. In one study,15 performed almost 2 decades earlier at Barnes-Jewish Hospital, pulmonary artery occlusion pressure was correctly predicted 30% of the time, and cardiac output, systemic vascular resistance, and right atrial pressure were correctly predicted approximately 50% of the time. In that study, after the results of pulmonary artery catheterization were known, medical therapy was altered in 58% of the patients. This value is remarkably similar to the 54.3% of patients who had alterations in their medical therapy after transesophageal Doppler imaging in our study. However, no study to date has indicated significant improvement in patients’ outcomes associated with the use of hemodynamic data derived from either pulmonary artery catheterization or transesophageal Doppler imaging.

The limitations of the currently available techniques for measuring cardiac output suggest a need for transesophageal Doppler imaging and possibly other less invasive yet accurate monitoring methods. Thermodilution is affected by a variety of external factors and patient-related conditions (eg, tricuspid regurgitation, intracardiac shunts, and low cardiac output states).19–21 Similarly, estimations of cardiac output obtained by using the Fick method can be adversely affected by the lack of hemodynamic stability often encountered in patients receiving mechanical ventilation.22,23 More important, patients’ outcomes may be adversely affected by techniques used to assess hemodynamic status.


Bedside assessments of cardiac index and volume status are often inaccurate. Transesophageal Doppler performed by critical care nurses may provide a useful, safe alternative to pulmonary artery catheterization.

 

The investigators in the Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatments (SUPPORT) examined more than 5000 patients and found, by case-matching analysis, that patients with right-sided heart catheterization had an increased 30-day mortality (odds ratio, 1.24; 95% CI, 1.03–1.49).5 These investigators also found that the mean cost per hospital stay was $49 300 for patients who had right-sided heart catheterization and $35 700 for patients who did not. Patients with a higher baseline probability of surviving 2 months had the highest relative risk of death after right-sided heart catheterization. The investigators concluded that additional well-designed prospective studies were needed to identify patients who would most benefit from pulmonary artery catheterization. This view was supported by a recent consensus panel.6

Obtaining accurate readings with various hemodynamic monitoring techniques is another important limitation of the thermodilution method and transesophageal Doppler imaging. The accuracy of hemodynamic measurements can be high when experts use well-defined protocols to obtain and interpret the information.15,24 However, the need for multiple steps in obtaining hemodynamic data, including catheter placement, calibration of transducers, and interpretation of waveforms, increases the likelihood for errors in interpretation.25,26 Transesophageal Doppler imaging can also provide inaccurate data if the operator is unskilled in the technique and unaware of the pitfalls. Lefrant et al27 reported that a period of training is required to ensure that reliable measurements of cardiac output are being obtained. However, an important advantage of transesophageal Doppler imaging is the ability to provide continuous real-time monitoring of cardiac output.13

Our study has several important limitations. First, it was performed at a single hospital. Therefore, the results may not be applicable to other medical centers. Second, we did not dictate any treatment parameters in the patients in the sample. This characteristic limits our ability to determine whether the changes in therapy after transesophageal Doppler imaging were directly related to the hemodynamic measurements obtained. Additionally, the study was not designed to determine whether the use of transesophageal Doppler imaging could influence patients’ outcomes. Third, our sample was limited to patients receiving mechanical ventilation. Future studies in patients who are not receiving ventilatory support and in other patients who require hemodynamic assessment are warranted. Finally, we did not attempt to correlate the findings of transesophageal Doppler imaging with findings obtained by using other methods available for evaluation of cardiac function and hemodynamic parameters (eg, transthoracic or transesophageal echocardiography). Such a comparison was not practical because of the limitations of our study design and sample size.

In conclusion, our results confirm that bedside assessments of cardiac index and intravascular volume are often inaccurate. The data also suggest that transesophageal Doppler imaging may be an alternative to pulmonary artery catheterization for obtaining hemodynamic data in some patients. Future outcome studies should be performed in patients with specific medical conditions (eg, severe sepsis or septic shock, acute respiratory distress syndrome) in which use of transesophageal Doppler imaging is compared with use of pulmonary artery catheterization.


   ACKNOWLEDGMENTS
 
This work was supported in part by the Barnes-Jewish Hospital Foundation.

To purchase reprints, contact The InnoVision Group, 101 Columbia, Aliso Viejo, CA 92656. Phone, (800) 809-2273 or (949) 362-2050 (ext 532); fax, (949) 362-2049; e-mail, reprints{at}aacn.org.

Commentary by Mary Jo Grap (see shaded boxes).


   REFERENCES
Top
 Abstract
 Materials and Methods
 Results
 Discussion
 References
 

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