Image quality using a micromultiplane transesophageal echocardiographic probe in older children during cardiac surgery

Editor, Transoesophageal echocardiography (TOE) is a standard procedure for the intraoperative evaluation and monitoring of adult and paediatric patients undergoing cardiac surgery [1–5]. Cardiac performance can be assessed and residual lesions are immediately corrected [3], thus avoiding reoperations and reducing morbidity, mortality and costs [1]. We recently showed that the micromultiplane TOE probe (8.2–7 mm diameter, 24 mm length tip and with a 5.2 mm diameter shaft; Oldelft, Delft, The Netherlands) allows diagnostic intraoperative TOE assessment of children and neonates as small as 2.5 kg without major complications [3]. However, in two heavier children (>20 kg), we noticed poor quality images. TOE is a semi-invasive procedure and, although complications are rare (local trauma to the oropharynx and the oesophagus), the use of the smallest probe available with acceptable image quality should be preferred [4]. The purpose of the current study is to define the upper weight of patients in which the micromultiplane TOE probe provides diagnostic images. All patients weighing less than 50 kg and above 15 kg undergoing cardiac surgery at the Thoraxcentre of the Erasmus Medical Centre, Rotterdam, between January 2006 and December 2007 were included. All patients at our centre routinely undergo TOE examination during cardiac surgery. Institutional review approval/consent was waived. Before giving anaesthesia, all patients were monitored with a five-lead, two-channel ECG, noninvasive blood pressure measurement and pulse oximetry. After the insertion of a peripheral venous line, general anaesthesia was induced with midazolam 0.2 mg kg−1, sufentanil 2 μg kg−1 and pancuronium 0.15 mg kg−1. Patients were tracheally intubated and pressure-controlled ventilated (PCV) using a Siemens 900C ventilator (Siemens, Lund, Sweden). Anaesthesia was maintained with midazolam 0.1 mg kg−1 h−1 and sufentanil 1 μg kg−1 h−1. Invasive monitoring via a radial arterial line and an internal jugular central venous catheter was performed, and a Foley bladder catheter with an integrated temperature probe was inserted. Using a latex-free micropaediatric ultrasound transducer probe cover (Palmedic, Lichtenvoorde, The Netherlands) the micromultiplane TOE probe was inserted blindly with or without a jaw thrust of the mandible or under direct laryngoscopic view. All TOE examinations were performed using a Philips iE 33 ultrasound system (Philips, Andover, Massachusetts, USA) equipped with two-dimensional, pulsed, continuous and colour Doppler capabilities. All TOE examinations were conducted and digitally recorded by the same cardiac anaesthesiologist in the presence of a second anaesthesiologist who was responsible for the care of the patient. Afterwards, an experienced echocardiographer and cardiac anaesthesiologist reviewed and scored all images separately. Images were divided into mid-oesophageal (MOE) and transgastric views. All images were evaluated as follows: 1, excellent; 2, good; 3, poor and 4, not interpretable. The micromultiplane TOE probe was developed at the Thoraxcentre to study neonates and small children and consists of a rotational-phased array ultrasound multiplane transducer (centre frequency ∼7.5 MHz, bandwidth > 40%, 48 elements and 0.1 mm pitch), mounted at the tip of a flexible gastroscope. The transducer has an octagonal 48-element array, 5.0 mm elevation, 4.7 mm lateral aperture and is rotatable through 180°. The gastroscope has a thickness of maximum 5.2 mm, a length of 70 cm and a bending neck capable of articulating in the anterior and posterior directions (120° ± 10° upward anterior and 90° ± 10° backward posterior). The bending neck has a diameter of 5.6 mm and a length of 40 mm. The bead on the transition to the shaft has a diameter of 6.2 mm. The tip is 8.2 mm wide, 7.0 mm thick and has a length of 24.0 mm (Fig. 1) [6].Fig. 1The patients' characteristics and, diagnostic and evaluation data are summarized in Table 1. The patients' ages ranged from 3 to 13 years and weight from 15 to 50 kg. The images up to a body weight of 20 kg were evaluated ‘excellent’ in all views. From a body weight of 20 to 25 kg, images were evaluated ‘excellent’ in the MOE views and ‘good’ in the transgastric views; from 25 to 27 kg, images were considered ‘good’ at the MOE level and only ‘fair’ at the transgastric level. In the patients weighing 33 kg and more, all images were assessed ‘poor’ and we had to use an adult TOE probe, providing excellent image quality.Table 1: Patients' characteristics and evaluationTOE is a semi-invasive procedure for cardiac imaging, and, although complications are rare, the smallest probe providing diagnostic images should be used. It is not surprising that inability to insert the TOE probe, oesophageal trauma, airway compromise and aortic compression occur predominantly with thicker TOE probes [7]. There are several reasons why there is an upper weight limit for the micromultiplane TOE probe. First, because the centre frequency of the micromultiplane TOE transducer is 7.5 MHz, which gives an optimal image to a depth of 6–7 cm [8]. This explains why the image quality in the far field becomes ‘poor’ in heavier patients. Second, during TOE examination of heavier patients (>25 kg), it was impossible to acquire deep transgastric long axis and transgastric long axis views. This can be explained by the shorter bending neck (length of only 40 mm) of the micromultiplane TOE probe, as compared with that of an adult TOE probe, which is approximately 80 mm long (Fig. 1). This makes it impossible to reach the apex of the left ventricle. A third explanation for the quality becoming poorer within heavier patients is the fact that the micromultiplane-phased array has fewer elements than the standard adult TOE probe (48 vs. 64). This results in a lower resolution and consequently a lower image quality [4]. Finally, a small probe within heavier patients may have less-than-optimal acoustic coupling with the heart [4]. In conclusion, the micromultiplane TOE probe provides images of ‘excellent’ and ‘good’ quality in patients up to a weight of 25 kg. In patients above 25 kg, an adult TOE probe, providing excellent image quality, can be used.