Lowering heart rate with an optimised breathing protocol for prospectively ECG-triggered CT coronary angiography

Objectives

The aim was to prospectively characterise the effect of the level of breath-hold on heart rate in CT coronary angiography (CTCA) with prospective electrocardiogram (ECG) triggering and its impact on coronary artery attenuation.

Methods

260 patients (86 women; mean age 59 ± 11 years) underwent 64-slice CTCA using prospective ECG triggering. Prior to CTCA, heart rates were recorded during 15 s of breath-hold at three different levels of inspiration (normal, intermediate and deep). The inspiration level with the lowest heart rate was chosen for actual CTCA scanning. Coronary artery attenuation was measured, and the presence of backflow of contrast material into the inferior vena cava (as an indicator of increased intrathoracic pressure) was recorded.

Results

The mean heart rate at breath-hold was significantly different for the three inspiration levels (normal, 60 ± 8 bpm; intermediate, 59 ± 8 bpm; deep, 57 ± 7 bpm; p<0.001). The maximum heart rate reduction in each patient at breath-hold averaged 5.3 ± 5.1 bpm, and was observed at a normal inspiration depth in 23 (9%) patients, at an intermediate inspiration depth in 102 (39%) patients and at deep inspiration in 135 (52%) patients. Overall, there was no association between the level of breath-hold and coronary vessel attenuation (p-value was not significant). However, the backflow of contrast material into the inferior vena cava (n = 26) was found predominantly at deep inspiration levels (p<0.001), and, when it occurred, it was associated with reduced coronary attenuation compared with patients with no backflow (p<0.05).

Conclusion

The breath-hold level to best reduce heart rate for CTCA should be individually assessed prior to scanning because a mean heart rate reduction of 5 bpm can be achieved.Low-dose CT coronary angiography (CTCA) with prospective electrocardiogram (ECG) triggering has recently been introduced 1] and shown to offer a tremendous reduction in radiation dose 2-8], which makes its widespread clinical use feasible. Scanning in CTCA with prospective ECG triggering is exclusively performed during a short phase in diastole, called “diastasis”. The new technique appears to be more prone to artefacts caused by coronary motion. This is especially true in higher heart rates because diastasis disproportionally shortens with increasing heart rates 9] and the length of diastasis becomes shorter than the time required for image acquisition when the heart rate exceeds a certain threshold. Furthermore, when low-dose CTCA with prospective ECG triggering is performed with the smallest possible acquisition window to achieve the lowest possible radiation dose, it does not permit image reconstruction in other phases of the cardiac cycle to compensate for a possible reduction in image quality. An aggressive reduction in heart rate with β-blockers below a target heart rate of 63 bpm appears to be a prerequisite for low-dose CTCA 2,10].CTCA is generally performed during the patient''s breath-hold to avoid motion artefacts caused by movement of the thorax during image acquisition. However, the breath-hold and, particularly, the level (or depth) of breath-hold can affect heart rate 11]. Generally, heart rate is lowest at full vital capacity (deep inspiration) and highest at residual volume (normal inspiration) 11]. However, other factors, such as intrapleural and intrathoracic pressure and neural reflexes also determine the heart rate during breath-hold 11]. The lowest heart rates are not always reached at deep inspiration level during breath-hold. Furthermore, during CTCA, very deep inspiration for breath-hold may lead to an increased intrathoracic pressure (Valsalva effect), which may itself impair contrast material flow from the arm veins to the coronary arteries, and thus cause reduced coronary artery attenuation and decreased image quality.Accordingly, the purpose of this study was to describe and characterise the effect of breath-hold on heart rate and on coronary artery attenuation in CTCA with prospective ECG triggering.