In the perioperative period, assessment of left ventricular (LV) volumes and ejection fraction (EF) is frequently needed for early detection of cardiac dysfunction and for guidance of therapeutic interventions. For the hemodynamically unstable patient, assessment of cardiac volume and function needs to be performed rapidly, easily, and accurately. In this issue, Meris et al.1 report the intraoperative measurement of LV volume and EF with 3-dimensional transesophageal echocardiography (3D TEE) in patients undergoing elective cardiac surgery. The authors found that LV volumes were consistently larger when measured with 3D than 2D TEE.
Anesthesiologists and intensivists are increasingly relying on echocardiographic imaging for perioperative management of their patients. With TEE and increasingly, with transthoracic echocardiography (TTE), qualitative techniques such as “eye-balling,” the change of LV area in the midtransgastric short-axis and midesophageal views are used for estimation of left ventricular ejection fraction (LVEF) and LV volumes. Less frequently, the end-diastolic (ED) and end-systolic (ES) LV volumes are measured with the method of disks (MOD) by tracing the endocardial border in the midesophageal views, and the stroke volume and EF are subsequently calculated.2
Notwithstanding the constraints of the perioperative environment, which make the quantitative evaluation of LV function impractical, additional technical factors may introduce measurement errors. Qualitative evaluation is subjective, less accurate in the hands of an inexperienced echocardiographer, and has limited reproducibility even among experienced echocardiographers. If the true LV major axis, that is, the distance between the mitral plane and apex, is underappreciated, the LV cavity will appear smaller. Appreciation of the true LV major long-axis is important for the MOD technique to minimize error, as is the accurate tracing of the endocardial border for the correct calculation of LV volumes. Last but not least, the LV cavity is assumed to have the shape of a truncated ellipsoid, which is not the case for patients with dilated or aneurysmal ventricles. In such patients, volume measurements performed in only 2 orthogonal midesophageal views may be erroneous. It is therefore not surprising that LV volumes measured by 2D techniques underreport those derived by cardiac magnetic resonance imaging (CMRI), which is a widely accepted reference method for LV volume determination.3 In a meta-analysis by Dorosz et al.,4 which included 23 studies with 1638 echocardiograms, 3D echocardiography-derived LV volumes highly correlated with those derived from CMRI, the reference method. In a subanalysis of 9 studies comparing 2D TTE versus CMRI, the pooled biases were −48.2 ± 55.9 mL for left ventricular end-diastolic volume LVEDV), −27.7 ± 45.7 mL for left ventricular end-systolic volume (LVESV), and 0.1% ± 13.9% for LVEF.4 Others have pointed out that this underestimation of LV volumes with 2D echocardiography is more significant with reduced LV function (LVEF <50%), a group where accurate diagnosis and proper treatment is most important.5
Three-dimensional echocardiography has mitigated many of these limitations faced by the 2D technique for assessment of LV volume and function. The entire LV is sampled by means of 3 to 8 pyramidal datasets (“slices”). Each dataset is acquired over a single beat in real time, using the electrocardiograph signal as the trigger. These slices are then “stitched” together to generate a 3D image of the entire LV. Semiautomated analysis software available in current 3D echocardiographic platforms (under proprietary names but performing essentially similar functions) directs the echocardiographer to “point-and-click” 4 to 5 reference points or axes in the mitral annulus and cardiac apex in ED and ES frames. From these references, the software traces the endocardial border, producing a cast of the LV endocardial surface in LV ED and ES, thus measuring 3D volumes and EF. The operator has the ability to adjust the traced LV endocardial border to improve accuracy of measurements. However, this subjective technique may introduce errors. Despite its simplicity, the technique is dependent on impeccable echocardiographic imaging and is therefore liable to ultrasound artifacts (shadowing from the mitral annulus as may occur with TEE imaging) and irregular R-R intervals, such as nonsinus rhythms and other arrhythmias or respiration-induced artifacts, that may not allow for perfect stitching of the datasets. Furthermore, temporal resolution is lower with 3D than 2D echocardiography, introducing some limitations at faster heart rates.
Similar to 2D echocardiography, the more experienced the operator, the more robust the results. In a multicenter study, the bias was inversely related to the level of experience.6
The study by Meris et al.1 is a valuable attempt to assess the feasibility of performing 3D TEE in the perioperative period and its comparative value to 2D TEE. The 2D LV volumes were measured with the MOD technique from midesophageal images. Full volume 3D datasets were collected and analyzed with semiautomated software in real time, to derive LV volumes. The authors report that the 3D-derived LV volumes were larger than those measured with 2D TEE (95% limits of agreement [LOA] −9.4 to 14.1 mL/m2 for indexed LVEDV and −5.2 to 10.1 mL/m2 for LVESV), but the EF was essentially similar (95% LOA −8.6% to 8.8%). These findings reflect what has been shown in ambulatory patients with TTE. Despite the larger 3D LV volumes, 93% of these patients were categorized correctly based on either measured 2D or 3D volumes. Other important findings are that the acquisition time, image quality (the reader should notice that greater than 2 LV segments were not imaged in 15% and 12% of patients with 3D and 2D TEE, respectively), and inter- and intraobserver variability were similar for 2D and 3D techniques. However, time to analyze the 3D volume data was significantly longer for 3D TEE by an average of nearly 2 minutes, reflecting the need to manually adjust the 3D endocardial border tracing in 59% of the patients. This contrasts favorably with an earlier study by Culp et al.,7 wherein intraoperative cardiac output was measured with 3D TEE using a software that mandated the manual tracing of the endocardial border in 4 orthogonal planes. Apart from the wide LOA (−1.64 to 2.17 L/min, approximately ± 35%) between 3D and thermodilution, the 3D LV volume measurements took 7 minutes to calculate, and the authors appropriately raised the issue of applicability of the 3D TEE technique, particularly in less stable hemodynamic situations.7 This study demonstrates that the anesthesiologist may be able to acquire and analyze the LV dimensions and calculate stroke volume and EF in a relatively short period of time using 3D TEE. One may point out that the LV function was correctly categorized in 145 of the 156 patients, irrespective of the 2D or 3D TEE technique, thus challenging the benefit of the 3D volumetric technique. However, the additional 3 or more minutes required for the postprocessing of the 3D volume datasets are a small delay when considering the imaging errors (apical foreshortening with 2D TEE and demand for endocardial tracing during the MOD technique) and significant underestimation of LV volumes that are associated with 2D techniques.
Recent improvements in 3D echocardiography technology, while encouraging, need further enhancement. Current software tools allow nearly automated measurement of LV volumes; however, the operator’s knowledgeable participation is still essential as previously described. Thus, the 3D calculation of LV volume may not be completely “hands-free,” but, as proven here by Meris et al.1 for TEE, 3D echocardiographic techniques consistently provide larger LV volumes than 2D techniques. Based on reports showing similar LV dimensions between 2D TTE and 2D TEE,8–10 the error in LV volume calculations with 2D TEE may have to do with the geometric assumptions of the 2D techniques.
Whether 3D TEE measured volumes are similar to CMRI-based volumes remains an unanswered question; designing such a study would likely pose significant challenges from technical and cost perspectives. But there are other important avenues to explore in advancing the role of 3D TEE in the perioperative period. In the Meris et al.1 study, and in all previous TTE studies, only “stable” patients undergoing elective procedures were enrolled, those with other than normal sinus rhythm were excluded, and most importantly, measurements were done once only, in the period previous to cardiopulmonary bypass. It remains to be proven whether similar results, including inter- and intraobserver, as well as, the acquisition and analysis time requirements will be found when 3D TEE is used more than once intraoperatively, particularly during unstable hemodynamic states. Equally important, 3D TEE LV function should be correlated with other standard techniques, such as the still widely used thermodilution cardiac output or esophageal Doppler flow measurements.
The study by Meris et al.1 will hopefully be the harbinger of other reports exploring the utility of 3D TEE for the perioperative assessment of LV volume and function.
Name: Nikolaos J. Skubas, MD, FASE, DSc.
Contribution: This author helped design the study and prepare the manuscript.
Attestation: Nikolaos J. Skubas approved the final manuscript.
Name: Aman Mahajan, MD, PhD.
Contribution: This author helped design the study and prepare the manuscript.
Attestation: Aman Mahajan approved the final manuscript.
This manuscript was handled by: Martin J. London, MD.
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