Noninvasive Determination of Vortex Formation Time Using Transesophageal Echocardiography During Cardiac Surgery

Summary

We describe a protocol to measure vortex formation time, an index of left ventricular filling efficiency, using standard transesophageal echocardiography techniques in patients undergoing cardiac surgery. We apply this technique to analyze vortex formation time in several groups of patients with differing cardiac pathologies.

Abstract

Trans-mitral blood flow produces a three-dimensional rotational body of fluid, known as a vortex ring, that enhances the efficiency of left ventricular (LV) filling compared with a continuous linear jet. Vortex ring development is most often quantified with vortex formation time (VFT), a dimensionless parameter based on fluid ejection from a rigid tube. Our group is interested in factors that affect LV filling efficiency during cardiac surgery. In this report, we describe how to use standard two-dimensional (2D) and Doppler transesophageal echocardiography (TEE) to noninvasively derive the variables needed to calculate VFT. We calculate atrial filling fraction (β) from velocity-time integrals of trans-mitral early LV filling and atrial systole blood flow velocity waveforms measured in the mid-esophageal four-chamber TEE view. Stroke volume (SV) is calculated as the product of the diameter of the LV outflow track measured in the mid-esophageal long axis TEE view and the velocity-time integral of blood flow through the outflow track determined in the deep transgastric view using pulse-wave Doppler. Finally, mitral valve diameter (D) is determined as the average of major and minor axis lengths measured in orthogonal mid-esophageal bicommissural and long axis imaging planes, respectively. VFT is then calculated as 4 × (1-β) × SV/(πD³). We have used this technique to analyze VFT in several groups of patients with differing cardiac abnormalities. We discuss our application of this technique and its potential limitations and also review our results to date. Noninvasive measurement of VFT using TEE is straightforward in anesthetized patients undergoing cardiac surgery. The technique may allow cardiac anesthesiologists and surgeons to assess the impact of pathological conditions and surgical interventions on LV filling efficiency in real time.

Introduction

Fluid mechanics is a critical yet often underappreciated determinant of left ventricular (LV) filling. A three-dimensional rotational body of fluid, known as a vortex ring, is generated whenever a fluid traverses an orifice^1,2,3. This vortex ring improves the efficiency of fluid transport compared with a continuous linear jet⁴. Movement of blood through the mitral valve during early LV filling causes a vortex ring to form^5,6,7,8 and facilitates its propagation into the chamber by preserving fluid momentum and kinetic energy⁹. These actions enhance LV filling efficiency^{4,10,11,12,13}. The ring not only inhibits blood flow stasis in the LV apex^14,15,16,17 but also directs flow preferentially beneath the anterior mitral leaflet^7,18, effects that decrease the risk of apical thrombus formation and facilitate filling of the LV outflow track¹⁹, respectively. Contrast echocardiography¹⁷, Doppler vector flow mapping^6,20,21, magnetic resonance imaging⁷, and particle imaging velocimetry^9,22,23,24 have been used to demonstrate the appearance and behavior of trans-mitral vortex rings under normal and pathological conditions. The left atrial-LV pressure gradient, the degree of diastolic mitral annular excursion, the minimum LV pressure achieved during diastole, and the rate and extent of LV relaxation are the four major determinants of the duration, size, flow intensity, and position of the trans-mitral ring^{2,12,25,26,27,28,29}.

Vortex ring development is most often quantified with a dimensionless parameter (vortex formation time; VFT) based on fluid ejection from a rigid tube³, where VFT is defined as the product of the time-averaged fluid velocity and the duration of ejection divided by the orifice diameter. The optimal size of a vortex ring is achieved when VFT is 4 in vitro because trailing jets and energetic limitations prevent it from attaining a larger size^3,4. Mitral valve VFT has been approximated clinically using transthoracic echocardiography^8,30,31. Based on analysis of trans-mitral blood flow velocity and mitral valve diameter (D), it can be easily shown⁸ that VFT = 4 × (1-β) × EF ×α³, where β = atrial filling fraction, EF = LV ejection fraction, and α = EDV^1/3/D, where EDV = end-diastolic volume. Ejection fraction is the ratio of stroke volume (SV) and EDV, allowing this equation to be simplified to VFT = 4 × (1-β) × SV/(πD³). Because VFT is dimensionless (volume/volume), this index allows direct comparison between patients of varying size without adjustment for weight or body surface area⁸. Optimal VFT ranges between 3.3 and 5.5 in healthy subjects⁸, and results are consistent with those obtained in fluid dynamics models^3,32. VFT was shown to be ≤ 2.0 in patients with depressed LV systolic function, findings that are also supported by theoretical predictions⁸. Reductions in VFT independently predicted morbidity and mortality in patients with heart failure³⁰. Elevated LV afterload³³, Alzheimer's disease³⁴, abnormal diastolic function¹⁹, and replacement of the native mitral valve with a prosthesis³⁵ have also been shown to decrease VFT. Measurement of VFT may also be useful to identify blood flow stasis or thrombosis in patients with acute myocardial infarction^36,37.

Our group is interested in factors that affect LV filling efficiency during cardiac surgery^38,39,40,41. We use standard two-dimensional and Doppler transesophageal echocardiography (TEE) to noninvasively derive the variables required to calculate VFT. In this report, we describe this methodology in detail and review our findings to date.

Protocol

The Institutional Review Board of the Clement J. Zablocki Veterans Affairs Medical Center approved the protocols. Written informed consent was waived because invasive cardiac monitoring and TEE are routinely used in all patients undergoing cardiac surgery in our institution. Patients with relative or absolute contraindications for TEE, those undergoing repeat median sternotomy or emergency surgery, and those with atrial or ventricular tachyarrhythmias were excluded from participation.

1. Anesthesia

1. Provide each patient with intravenous midazolam (1 to 3 mg) and fentanyl (50 to 150 µg) for conscious sedation before surgery.
2. Use local anesthesia (subcutaneous 1% lidocaine) for insertion of intravenous and radial artery catheters. Test the quality of the local anesthesia with a pinprick.
3. Ensure that the patient receives supplemental oxygen using a nasal cannula (2 to 4 L/min).
4. Place a central venous or pulmonary artery catheter using local anesthesia (subcutaneous 1% lidocaine) under sterile conditions through the right or left internal jugular vein with ultrasound guidance based on appropriate clinical indications.
5. Induce anesthesia using intravenous fentanyl (5 mcg/kg), propofol (1 to 2 mg/kg), and rocuronium (0.1 mg/kg). Maintain anesthesia using inhaled isoflurane (end-tidal concentration of 1%) in an air-oxygen mixture, fentanyl (1 to 2 µg/kg/h), and rocuronium (0.05 mg/kg) titrated to effect using neuromuscular monitoring.
6. Suction the stomach using an oral-gastric tube.
7. Place ultrasound jelly in the patient’s hypopharynx. Lift the jaw anteriorly and advance a TEE probe into the esophagus with gentle pressure to overcome resistance of the hypopharygeus muscle.

2. Transesophageal Echocardiography

1. Perform a comprehensive TEE examination following American Society of Echocardiography/Society of Cardiovascular Anesthesiologists guidelines⁴² in each patient.
2. Place a pulse-wave Doppler sample volume between the tips of the mitral leaflets to record trans-mitral blood flow velocity in the mid-esophageal four-chamber TEE imaging plane (Figure 1).
3. Identify the early LV filling and atrial systole blood flow waveforms of trans-mitral blood flow velocity, and measure their corresponding peak velocities and velocity-time integrals (VTI_E and VTI_A, respectively) using the echocardiography equipment’s integrated software package (Figure 1).
4. Calculate the atrial filling fraction (β) as the ratio of atrial to total LV filling:
   
5. Measure the maximum diameter of the LV outflow tract immediately below the aortic valve in the mid-esophageal aortic valve long axis TEE view during mid-systole (Figure 2A).
6. Calculate the area of the LV outflow tract assuming circular geometry as the product of π/4 and the square of the diameter (see step 2.5 above).
7. Obtain a deep transgastric long axis TEE view, and place a pulse-wave Doppler sample volume in the distal LV outflow tract to record a blood flow velocity envelope (Figure 2B) at the same level where the diameter was measured (see step 2.5 above); integrate the area of this waveform using the echocardiography equipment’s software package to obtain VTI.
8. Multiply the resulting velocity-time integral (VTI) of the LV outflow track blood flow velocity waveform (Figure 2B) by the area of the outflow track (see step 2.6) to obtain stroke volume (SV).
9. Record video clips of the mid-esophageal bicommissural and LV long axis TEE imaging planes, respectively⁴². Be sure to include several cardiac cycles in each recording.
10. Visually inspect slow-motion images of the video clips (see step 2.9 above) after the ECG T-wave to choose the maximum opening of the mitral valve leaflets.
11. Measure the distance between the mitral leaflets (Figures 3A and 3B) using the echocardiography equipment’s “caliper” function.
12. Calculate the mitral valve diameter (D) as the average of the major (transcommissural anterior-lateral-posterior-medial) and minor (anterior-posterior) lengths.
13. Calculate VFT using the formula:
    
14. Perform all quantitative echocardiographic measurements in triplicate at end-expiration.

3. Experimental Design

1. Determine VFT, indices of LV diastolic function, and hemodynamics during steady-state conditions 30 minutes before and 15, 30, and 60 minutes after cardiopulmonary bypass (CPB) in 10 patients with normal preoperative LV ejection fraction under coronary artery surgery to test the hypothesis that CPB transiently decreases VFT³⁹.
2. Test the hypothesis that LV pressure-overload hypertrophy produced by aortic valve stenosis reduces VFT by examining (in one group of 8 patients undergoing aortic valve replacement) for severe aortic stenosis and comparing observations to another group of 8 patients with normal LV wall thickness undergoing coronary artery surgery⁴⁰. Measure VFT, LV diastolic function, hemodynamics, and end-diastolic posterior wall thickness during steady-state conditions 30 minutes before CPB.
3. Test the hypothesis that abnormal diastolic blood flow entering the LV affects trans-mitral LV filling efficiency in 8 patients with aortic valve stenosis and moderate aortic insufficiency versus 8 patients with aortic stenosis who do not have regurgitant valves³⁸. Measure VFT and other parameters as described above (step 3.2).
4. Test the hypothesis that advanced age is associated with a reduction in LV filling efficiency quantified using VFT in 7 octogenarians (82 ± 2 years) compared to 7 younger patients (55 ± 6 years)⁴¹ undergoing coronary artery surgery. Ensure that both groups have normal preoperative LV ejection fraction. Measure VFT and other parameters as described above (step 3.2).

4. Statistics

1. Present the data as mean ± standard deviation.
2. Evaluate data using analysis of variance (ANOVA) followed by Bonferroni’s modification of Student’s t-test.
3. Use linear regression analysis to determine the relationships between VFT and end-diastolic posterior wall thickness and between VFT and age.
4. Reject the null hypothesis when p < 0.05.

Results

The current technique allowed us to reliably measure VFT during cardiac surgery under a variety of clinical conditions by obtaining each determinant from blood flow and dimensional recordings in standard TEE imaging planes. A pulse-wave Doppler sample volume was placed at the tips of the mitral leaflets in the mid-esophageal four-chamber view to obtain the trans-mitral blood flow velocity profile necessary to calculate atrial filling fraction (β; Figure 1). S...

Discussion

The current results illustrate that VFT can be reliably measured during cardiac surgery using the TEE techniques described here. Previous descriptions of VFT used transthoracic echocardiography in conscious subjects, but this approach cannot be utilized when the chest is open. We used intraoperative TEE to determine VFT in the anesthetized patients undergoing cardiac surgery during which changes in LV filling dynamics are often encountered as a result of ischemia-reperfusion injury or surgical interventions. Our findings...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Echocardiography Machine	Philips Ultrasound, Bothall, WA	iE33
Transesophageal Echocardiography Probe	Philips Ultrasound, Bothall, WA	X7-2t
Statistical Software	AnalystSoft, Walnut, CA	StatPlus:mac Pro