Carotid Doppler
One investigator (SD'A) with previous formal training in critical care ultrasound insonated the common carotid artery in a longitudinal view approximately 1–2 cm before the bifurcation using a 5–10-MHz linear array transducer (Vivid™ iq, General Electric Healthcare, Chicago, Illinois, USA) and performed a pulse-wave Doppler analysis. The sample volume was in the vessel’s middle, with the caliper positioned parallel to flow, angled at no more than 60°. We averaged five consecutive measurements to limit the effects of respiratory-induced changes in CDPV and cFT.The ΔCDPV was expressed as a percentage and was calculated with the following formula:The FT was measured from the beginning of the flow tracing’s upstroke to the nadir of its dicrotic notch on a pulse waveform analysis. We used the Wodey’s formula: cFT = FT + 1.29*(HR-60) to correct measurements for the heart rate [