Abstract
TOE is now commonplace in the cardiac operating theatre and critical care setting. The practising cardiac anaesthetist must be fully trained and proficient at performing TOE, both as a haemodynamic monitor and as a diagnostic tool. This chapter will cover the basic physics of ultrasound and Doppler, the indications and contraindications of TOE and the format of a comprehensive TOE examination.
TOE is now commonplace in the cardiac operating theatre and critical care setting. The practising cardiac anaesthetist must be fully trained and proficient at performing TOE, both as a haemodynamic monitor and as a diagnostic tool. This chapter will cover the basic physics of ultrasound and Doppler, the indications and contraindications of TOE and the format of a comprehensive TOE examination.
Physics of Ultrasound
Sound is a longitudinal, mechanical wave, with alternating regions of high pressure (compressions) and low pressure (rarefactions). Ultrasound has a frequency greater than 20 kHz. TOE probes typically emit ultrasound at a frequency of 4–8 MHz. Higher-frequency probes produce improved image resolution but have lower tissue penetration. The speed of ultrasound in the heart is approximately 1,540 m s–1. Reflections occur at a tissue boundary, where there is a change in acoustic impedance, and the returning signal is processed by the machine to create an image.
Doppler
The frequency of a sound wave reflected by a moving object is different from the frequency emitted: this change of frequency is the Doppler frequency. This principle is used to calculate the velocity of RBCs and myocardial tissue. Once the peak velocity (V) of blood flow is known, the pressure gradient (PG) between two chambers can be estimated using the simplified Bernoulli equation:
Doppler modalities include pulse wave (PWD), continuous wave (CWD) and colour flow Doppler (CFD). CWD measures velocities along the entire ultrasound path and is capable of measuring high velocities. However, it is not possible to determine from where the peak velocity has arisen, known as the range ambiguity artefact. PWD is utilized to measure blood flow in a specific region but is restricted by artefacts at higher velocities. CFD is based on PWD and displays ‘real-time’ blood flow on the 2D image.
Indications
In cardiac surgery, intraoperative TOE is a standard of care for all open-heart procedures and thoracic aortic operations. It is also indicated in selected patients undergoing CABG surgery, although in many centres it has become routine practice in all adult cardiac surgery. In the critical care setting, TOE is invaluable in managing haemodynamic instability where the diagnosis cannot be obtained by TTE. In the cardiac catheter laboratory, TOE is indicated in guiding catheter-based intracardiac procedures, such as septal defect closure.
Absolute contraindications to TOE include oesophagectomy, recent upper GI surgery and significant oesophageal or gastric pathology, such as perforation and active bleeding. In the presence of a relative contraindication, TOE may be performed if the expected benefits outweigh the potential risks.
Complications
Although TOE is generally considered to be safe, it is a semi-invasive procedure and is not without its complications. Injuries may occur anywhere from the oropharynx to the stomach. The most common complications include oral injuries, hoarseness and dysphagia. Major morbidity occurs in approximately 0.2% and the incidence of oesophageal perforation ranges from 0.01–0.1%. The reported mortality is less than 0.01%.
Basic TOE Views
There are four levels for image acquisition (Figure 32.1): upper oesophageal, mid-oesophageal, transgastric and deep transgastric. The TOE probe may be anteflexed or retroflexed, turned left or right, and the plane can be rotated from 0° to 180°.
Figure 32.1 Standard TOE probe levels.
Guidelines recommend performing a comprehensive intraoperative TOE examination, comprising 28 basic 2D views (Figure 32.2) and appropriate use of Doppler modalities. It is prudent to perform the study in a systematic manner and to allow sufficient time, in order to confirm the preoperative diagnosis and to optimize the ability to identify important pathologies that were not detected in the preoperative investigations. Management-changing unexpected intraoperative TOE findings can occur in up to 4% of patients. However, it is important to recognize the effect of anaesthesia on the patient’s pathophysiology and how this influences the TOE assessment.
