Repeatability of End‐Tidal Carbon Dioxide and Internal Carotid Artery Blood Flow Responses During Steady‐State Hypercapnia

Abstract

Cerebrovascular reactivity to hypercapnia is commonly utilized to assess cerebral vascular function. Systemic hypercapnia can be achieved through various methods, including steady-state (SS) carbon dioxide (CO2) delivery. Additionally, in recent years, investigators have begun to more frequently utilize duplex ultrasonography of extracranial vessels (e.g., internal carotid artery) during cerebrovascular reactivity testing due to its ability to simultaneously measure blood velocity and vessel diameter for the calculation of blood flow and thus provide a more complete picture of cerebral vascular regulation. However, to our knowledge, the repeatability of the partial pressure of end-tidal CO2 (PETCO2) and internal carotid artery (ICA) blood flow response to SS hypercapnia has not been evaluated. Therefore, we tested the between-visit and within-day repeatability of PETCO2 and ICA blood flow responses to a SS hypercapnia stimulus. Methods Six young, healthy males participated in the study (age: 21.3 ± 0.3 years; BMI: 24.1 ± 0.6 kg/m2). Three SS hypercapnia tests were performed over the course of two days (one trial on one day and two trials, separated by ~15 min, on another day). PETCO2 (capnography) and right ICA blood velocity and vessel diameter (duplex ultrasonography) were measured at baseline (3 min) and during the administration of 6% CO2 (3 min). Beat-to-beat blood pressure, respiration, and ECG data were also collected throughout the protocol. Results For the between-visit comparison, baseline PETCO2 was similar for Visit 1 and Visit 2 (47.3 ± 0.6 mmHg vs. 47.0 ± 0.6 mmHg; p = 0.67), and the increase in PETCO2 during the hypercapnic stimulus exhibited excellent repeatability (Visit 1: Δ9.5 ± 0.5 mmHg vs. Visit 2: Δ9.0 ± 0.5 mmHg; ICC: 0.94, p < 0.01). ICA blood flow was similar at baseline for Visit 1 and Visit 2 (502 ± 56 ml/min vs. 426 ± 32 ml/min; p = 0.11) and exhibited excellent between-visit repeatability during hypercapnia (Visit 1: 697 ± 75 ml/min vs. Visit 2: 610 ± 73 ml/min; ICC: 0.91, p < 0.01). For the within-day comparison, baseline PETCO2 was similar for Trial 1 and Trial 2 (47.0 ± 0.6 mmHg vs. 46.3 ± 0.7 mmHg; p = 0.14), and the increase in PETCO2 during the hypercapnic stimulus exhibited moderate repeatability (Trial 1: Δ9.0 ± 0.5 mmHg vs. Trial 2: Δ10.0 ± 0.5 mmHg; ICC: 0.64, p = 0.06). ICA blood flow was similar at baseline for Trial 1 and Trial 2 (426 ± 32 ml/min vs. 475 ± 45 ml/min; p = 0.20) and exhibited good within-day repeatability during hypercapnia (Trial 1: 610 ± 73 ml/min vs. Trial 2: 618 ± 47 ml/min; ICC: 0.74, p = 0.03). Conclusion These preliminary data suggest that 1) systemic hypercapnia elicited by a fixed concentration of inspired CO2 exhibits better between-visit than within-day repeatability and 2) between-visit and within-day repeatability of internal carotid artery blood flow during a SS hypercapnia stimulus of 6% inspired CO2 is good to excellent.