(1) Background: The inspiratory collapse of the inferior vena cava (IVC), a non-invasive surrogate for right atrial pressure, is often used to predict whether a patient will augment stroke volume (SV) in response to a preload challenge. There is a correlation between changing stroke volume (SV_∆) and corrected flow time of the common carotid artery (ccFT_∆). (2) Objective: We studied the relationship between IVC collapsibility and ccFT_∆ in healthy volunteers during preload challenges. (3) Methods: A prospective, observational, pilot study in euvolemic, healthy volunteers with no cardiovascular history was undertaken in a local physiology lab. Using a tilt-table, we studied two degrees of preload augmentation from (a) supine to 30-degrees head-down and (b) fully-upright to 30-degrees head down. In the supine position, % of IVC collapse with respiration, sphericity index and portal vein pulsatility was calculated. The common carotid artery Doppler pulse was continuously captured using a wireless, wearable ultrasound system. (4) Results: Fourteen subjects were included. IVC % collapse with respiration ranged between 10% and 84% across all subjects. Preload responsiveness was defined as an increase in ccFT_∆ of at least 7 milliseconds. A total of 79% (supine baseline) and 100% (head-up baseline) of subjects were preload-responsive. No supine venous measures (including IVC % collapse) were significantly related to ccFT_∆. (5) Conclusions: From head-up baseline, 100% of healthy subjects were ‘preload-responsive’ as per the ccFT_∆. Based on the 42% and 25% IVC collapse thresholds in the supine position, only 50% and 71% would have been labeled ‘preload-responsive’.

Background: Change in the corrected flow time of the common carotid artery (ccFT _Δ ) during a preload challenge (PC) is a surrogate for stroke volume change (SV _Δ ) to infer preload responsiveness (P _R ). Methods: This is a retrospective cohort study comprising patients with suspected sepsis in a large, publicly funded, academic emergency department; no patient health information is linked to the Doppler assessments. Carotid Doppler spectrograms from a wireless, wearable critical care Doppler ultrasound system are analyzed for ccFT _Δ during a PC (either infusion of 300 mL of crystalloid over 3 min or a Trendelenburg maneuver). The primary objectives of this study are to describe the proportion of P _R patients by previously described ccFT _Δ thresholds (i.e., both 4 and 7 ms ccFT _Δ ), describe time-to-peak (TTP) ccFT _Δ , and evaluate the ability of baseline ccFT to detect ccFT _Δ during PC, using both the equations of Wodey and Bazett. Comparisons of related outcomes were adjusted for multiple comparisons using the Holm procedure, baseline ccFT discrimination was quantified using area under the receiver operating characteristic curve, and TTP was estimated using patient-level bootstrap resampling. Threshold-agnostic ccFT _Δ trajectories were additionally explored using unsupervised k-means clustering of patient-level temporal responses for both correction equations. Results: A PC was attempted in 4367 patients and an adequate ccFT _Δ was collected from 4,128 patients (95%). In total, 982,937 Doppler cardiac cycles are included. The median ccFT _Δ was 7.3 milliseconds (ms). 51.9% of patients had a ccFT _Δ ≥ 7 ms, while 68.6% had a ccFT _Δ ≥ 4 ms by the equation of Wodey. For Bazett, these fractions were 61.2% and 73.5%, respectively. For patients identified as P _R based upon either a 4 ms or 7 ms ccFT _Δ threshold, the time-to-peak ccFT _Δ was 27.5 s for both equations. Baseline absolute ccFT did not discriminate P _R from P _UR patients under both correction formulae (AUROC 0.56–0.57). Conclusions: To our knowledge this is the largest reported dataset of ED patients with suspected sepsis receiving P _R assessments by Doppler ultrasound, comprising nearly 1 million Doppler cardiac cycles obtained from over 4100 ED patients. Both 7 ms and 4 ms ccFT _Δ thresholds described clinically-accepted rates of P _R . The TTP was roughly 30 s into PC and baseline ccFT did not accurately detect ccFT _Δ during PC. Finally, a hypothesis-generating analysis identified four distinct ccFT _Δ trajectories during PC which may represent unique preload response patterns during P _R testing.