By Mark Favot, MD, FACEP and Scott Sparks, MD, FACEP
Scenario: 58 year-old female arrives to you with post arrest, but with a questionable cause of whether it was after seizure activity, drug induced, or actual cardiac induced arrest. She is tachycardic, hypotensive, and unresponsive. She is a former smoker, obese and with poor cardiac windows during her initial EFAST and TTE. How much fluid to give? How best to find the answer? A classic question, but what is the right answer?
As resuscitationists, we constantly struggle to balance sepsis guidelines, CMS quality measures (ugh), and best practices with each sick patient. This particular patient made cardiac output (CO) by TTE difficult due to her obesity and barrel chest. Not wanting to increase mortality by over resuscitating her using only blood pressure and heart rate as a guide, Ma and colleagues have given us more evidence for a method for measuring CO with carotid blood flow (CBF) and carotid flow times (CFT) that is far less challenging from an image acquisition standpoint. We now have a more robust study comparing it to an invasive gold standard (Pulmonary Artery Catheter).
They performed an observational study of 51 spontaneously breathing, euvolemic patients undergoing elective right heart catheterizations who could tolerate a passive leg raise (PLR). Repeat carotid ultrasound measurements were taken before and 1 minute after PLR. The common carotid artery was scanned in transverse and longitudinal planes within 2 to 3 cm proximal to the carotid bulb in longitudinal plane with a 0.5mm sample gate through the center of the vessel to create spectral Doppler tracings. Angles of insonation greater than 60 degrees were excluded due to underestimation of flow velocity. CFT was calculated as systole time/square root of the cycle time with systole time measured from the start of the systolic upstroke of the dicrotic notch, while cycle time was the duration of 1 cycle (Figure 1a). CBF was calculated as π x (carotid diameter)2/(4 x VTI x heart rate) with VTI being the Velocity Time Integral of a single cycle. Intimal to intimal carotid diameter measured at the level of the sample gate (Figure 1b).
a. Carotid systole time, as measured from the start of the systolic upstroke to the start of the dicrotic notch (left). Carotid cycle time, as measured from start of one systolic upstroke to the next (right).
b. Velocity time integral tracing of the spectral Doppler signal (left). Carotid diameter (right)
CFT single and three wave form measurements did not correlate with CO measurements. CBF single and three wave form measurements did moderately correlate with CO. A subgroup analysis of patients in atrial fibrillation was performed and CFT and CBF single and three wave form measurements did not correlate with CO. There were no differences in CO, corrected CFT or CBF following PLR maneuvers in this study.
Having the ability to reliably estimate stroke volume & CO can be important to determine fluid responsiveness. The extensive training required, the angle dependent nature of doppler and patient body habitus can limit accurate TTE estimates of CO. The primary benefit of the method studied by Ma et al is that the ease of carotid ultrasound would help the resuscitationist determine fluid responsiveness by providing a simpler image acquisition than TTE. This study was limited by the euvolemic status of the patients and the lack of significant change with PLR, which limits the conclusions that can be drawn from this paper related to fluid responsiveness using auto-transfusion. Instead, this may require further study in a hypovolemic patient population, or better yet, a mixed volume status patient population in the early phases of resuscitation (i.e. a population more similar to what is seen in an ED).
In the meantime, the patient above had a limited, sub optimal TTE, as well as a carotid US performed with serial TTE and CBF CO measurements for fluid responsiveness. These seemed to match on two attempts, so further resuscitation was guided by CBF. Her subsequent cardiac catheterization showed significant LAD stenosis. After PCI and stent placement, she had early recovery of function. This is only anecdotal evidence for population based studies being applied to an individual patient, but, pending further study of CFT & CBF, gives hope to those ultrasound-guided resuscitationists out there searching for a less technically challenging answer than TTE determined CO.