Detecting cryptogenic (or hidden) bleeding is crucial in many clinical settings. Traditional markers of bleeding like low blood pressure are late and sinister, as patients can compensate for hemorrhage by vasoconstricting their blood vessels. Although vasoconstriction helps maintain blood pressure, it does so at the expense of organ perfusion. Accordingly, non-invasive, early, and accurate methods for assessing stroke volume (SV) and fluid responsiveness (and increase in SV by 10% from a fluid bolus) are crucial for early intervention.
Among the non-invasive techniques to assess for SV changes, changes in corrected carotid flow time (ccFT) is a valuable non-invasive predictor of fluid responsiveness, showing promise in both elective surgical and critically ill patients. A recent meta-analysis suggests ccFT is highly accurate in predicting fluid responsiveness, however, methodological concerns about reference standards and analysis of the carotid beats themselves have been raised. To address this and improve the strength of evidence supporting ccFT in predicting fluid responsiveness, Kerrebijn et al. recently published “Correlation between changing carotid corrected flow time and ascending aortic Doppler flow velocity” in the British Journal of Anesthesia.

Study Design
Kerrebijn et al. studied fourteen healthy adults who underwent a rigorous lower body negative pressure protocol to simulate varying degrees of central hypovolemia and compared changes in ccFT with changes in ascending aortic velocity time integral on ultrasound cardiac output monitoring (USCOM). So what did they find?

Key Results
The authors demonstrated a strong linear correlation (Rrm = 0.85) between ccFT change and SV change. Specifically, a 10% change in SV correlates with a 6.5ms absolute and 2.1% relative ccFT difference. This high correlation suggests that changes in ccFT can be used to estimate changes in SV.

Key Take Homes
#1) ccFT changes can reliably predict changes in stroke volume.
#2) Analyzing many cardiac cycles (as opposed to 3 or 4) improves the correlation between ccFT and SV
#3) Carotid Doppler devices may have practical benefits over USCOM, particularly for surgical patients or trauma patients, as they do not require significant space on the patient’s chest.

Conclusions
Personalized medicine and tailored resuscitation is a goal worth striving for. Coupling evidence-based medicine with physiology informed practice has the potential to improve outcomes, but relies on accurate, non-invasive, and easy-to-use bedside tools. Wearable Carotid Doppler devices may serve as a valuable tool to provide personalized care.

Singla D, Gupta B, Varshney P, Mangla M, Walikar BN, Jamir T. Role of carotid corrected flow time and peak velocity variation in predicting fluid responsiveness: a systematic review and meta-analysis. Korean J Anesthesiol. 2023;76(3):183-193. doi:10.4097/kja.22385