Transesophageal Echocardiography

Introduction

The ability to visualize the chambers of the heart during neurosurgical procedures provides the anesthesiologist with data regarding cardiac function, valvular abnormalities, and presence or absence of intracardiac shunts; helps to visualize intracardiac extraneous material such as clots and air; and helps facilitate certain bedside hemodynamic and neurosurgical procedures.

Via a detailed literature search, this chapter reviews the utilization of transesophageal echocardiography (TEE) in the management of patients during neuroanesthesia. Upon reading the chapter, the reader will be able to understand the common indications, procedural characteristics, and practical applications of this technique.

Basics of Transesophageal Echocardiography

Unlike surface cardiography, TEE allows real-time visualization of various cardiac chambers. It is facilitated via placement of a TEE probe into the esophagus aimed anteriorly at the cardiac chambers. Due to the proximity to the heart, TEE examination generally provides images with a high degree of spatial resolution. With the probe in place, cardiac walls, valves, interatrial and interventricular septum, pericardial structures, and left and right outflow tract structures can be easily visualized. A traditional complete TEE examination covers such views.

Common Indications for Utilization of Transesophageal Echocardiography in Neuroanesthesia

1.
Screening for vascular air embolism during neurosurgical procedures
2.
Screening, risk stratification, and preparation of patients at high risk for vascular air embolism prior to proposed neurosurgical procedures
3.
Verification of multiorifice catheter placement to assist in air aspiration in high-risk neurosurgical procedures
4.
Verification of distal end of a ventriculoatrial (VA) shunt and assessment of VA shunt patency
5.
Cardiac monitoring in patients with neurogenic stunned myocardium for neurosurgical and nonneurosurgical procedures

Screening for Venous Air Embolism During Neurosurgical Procedures

Although CO ₂ was demonstrated to be a useful agent aiding the detection of venous air embolism (VAE) by Bethune et al. in 1968, it was not until 1980 that detection of intracardiac air by esophageal echocardiography was first described by Duff et al. , who reported detection of entrapped intracardiac air by use of M-mode echocardiography. Since then many investigators have reported utilization of TEE in preoperative screening of patent foramen ovale (PFO) and intraoperative detection of VAE. Pertinent historical landmarks related to VAE and PFO detection are highlighted in Table 15.1 .

Table 15.1

Historical Landmarks in Detection of Venous Air Embolism

Year	Authors	Screening Method
1968	Bethune et al.	Carbon dioxide used to detect VAE
1972	Michenfelder et al.	Precordial Doppler to diagnose VAE
1975	Seward et al.	Contrast ECHO studies (first findings)
1975	Munson et al.	Swan–Ganz catheters (first use)
1976	Frazin et al.	First description of esophageal echocardiography
1980	Duff et al.	Intraoperative surface echocardiography to detect intracardiac air
1980	Marshall et al.	VAE detection by Swan–Ganz catheter
1981	Bedford et al.	VAE detection by precordial doppler, ETCO ₂ and pulmonary artery catheter
1983	Furuya et al.	VAE detection by TEE
1984	Cucchiara et al.	VAE detection by TEE
1985	Cucchiara et al.	PFO detection by TEE using positive airway pressure
1990	Black et al.	Pre- and intraoperative ECHO to detect right to left shunt in neurosurgical patients
1991	Nemec et al.	Comparison of TCD and TEE to detect intracardiac shunts
1994	Papadopoulos et al.	Compared intraoperative TEE and preoperative TTE to detect PFO in neurosurgical patients
2000	Stendel et al. ¹¹	TCD in preoperative setting to screen for PFO
2001	Kubo et al.	VAE due to PFO detected by use of harmonic contrast ECHO

ECHO , echocardiography; ETCO ₂ , end-tidal carbon dioxide; PFO , patent foramen ovale; TCD , transcranial Doppler; TEE , transesophageal echocardiography; TTE , transthoracic echocardiography; VAE , vascular air embolism.

VAE has been described in the setting of sitting position craniotomies, where the noncollapsible dural venous sinuses by the virtue of being at higher level than the right atrium are exposed to a negative pressure gradient, facilitating air entrainment into the right atrium, with subsequent passage to right ventricle and pulmonary circulation with sometimes fatal effect. If there is an intracardiac shunt, then it poses an even increased risk for paradoxical air embolism into the cerebral circulation.

TEE remains the most invasive, yet the most sensitive of all available methods to detect air embolism. As little as 0.25 mL/kg of air can be detected by TEE in comparison of 0.5mL/kg as detected by precordial Doppler.

Preparation of Patient for Venous Air Embolism Detection by Transesophageal Echocardiography

The decision to insert a TEE probe for high-risk neurosurgical procedures is left to the attending anesthesiologist. The patient is first screened for any contraindications for placement of TEE probe such as esophageal pathology, gastrointestinal bleeding, or recent gastrointestinal surgery.

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