Pulsed-wave Doppler for ultrasound-based tip location using bubble test: A pilot study

MicroBubbles Time test (MBT), consisting in the rapid infusion of saline with addition of air microbubbles, visualized by B-mode echocardiography, represents a potential alternative to Intracavitary ECG (IC-ECG) and chest X-ray for central venous catheters (CVCs) tip location. Even if promising, this technique lacks of standardization: a clear time cut-off between bubble infusion and their detection in heart's right chambers hasn't been yet established. At these regard, microbubbles could be also detected as microembolic signals (MES) with an alternative ultrasound technique: the pulse wave Doppler (PW).

The first aim of this pilot study is to establish agreement of MBT with PW test (MBT-PW) compared with reference standard IC-ECG and normal MBT for tip location on CVCs. Corrected tip's position was established through reference standard IC-ECG, afterward MBT-PW was performed, with the sample volume placed at tricuspid valve to detect MES simultaneously with micro-bubbles injection in CVCs. The second aim was to evaluate inter-observer variability for MES detection and grading.

Eight patients were enrolled; we obtained three records for each patients (24 with MBT and 24 with MBT-PW, the two techniques were acquired simultaneously). Inter-methods agreement through reference standard IC-ECG versus MBT-PW and MBT versus MBT-PW methodic was satisfying (Cohen's kappa value = 1). MBT-PW and MBT signals were recorded within the first heart beat after microbubble infusion in all patients. Mean time delay thorough MBT-PW and MBT was 0.76 ±0.07 and 0.78 ± 0.07 s respectively; Intraclass correlation coefficient was 0.992 (95% CI: 0.981-0.996) suggesting excellent correlation. Inter-observer variability for positive MBT-PW evaluation was optimal (Cohen's kappa value was 1), while indicated substantial agreement for MES grade evaluation (Fleiss' Kappa value was 0.704; 95% CI: 0.328-1.000).

Our study supports agreement between MBT-PW and reference standard IC-ECG for tip location. Satisfactory agreement was observed also for MBT-PW and MBT.