Coronary Blood Flow Velocity Responses to Oxygen and Carbon Dioxide in Humans
Bibliographic record
Abstract
In humans, coronary blood flow (CBF) is tightly regulated by microvessels within the myocardium. While there is clear evidence that CBF rises to match myocardial workload, there is conflicting evidence regarding the inherent sensitivity of the coronary vasculature to changes in the partial pressure of arterial carbon dioxide (PaCO 2 ) and oxygen (PaO 2 ) within the physiological range. The purpose of this study was to investigate the changes in CBF associated with isooxic hypo‐ and hyper‐capnia and isocapnic hypoxia. We hypothesized that a modest systemic increase in end‐tidal partial pressure of CO 2 (PetCO 2 ) or decrease in end‐tidal partial pressure of O 2 (PetO 2 ) would result in a significant increase in coronary blood flow velocity (CBF V ) due to downstream vessel dilatation. Likewise, we hypothesized that hypocapnia would cause a reduction in CBF V . Finally, we hypothesized that normalizing for cardiac workload (W C ) or mechanical energy (E M ) would diminish the observed reactivity. Young men free of cardiovascular disease (age = 23.4 ± 1.3 years, n=14) were exposed to two ventilatory challenges: (1) an isooxic CO 2 test that consisted of six, 8‐min steps of PetCO 2 (−8, −4, 0, +4, and +8 mmHg from baseline values) and (2) an isocapnic hypoxic test that involved three, 8‐min steps of hypoxia (PetO 2 = 64, 52 and 45 mmHg). CBF V was measured using Doppler echocardiography in the mid segment of the left anterior descending coronary artery (LAD). W C and E M were estimated using the rate pressure product (RPP; systolic blood pressure × heart rate) and the area of a derived pressure‐volume loop (PV A ) respectively. The PV A was calculated using volumes derived from echocardiographic measurements, a validated estimation of left ventricular end‐systolic elastance and beat‐by‐beat blood pressure from photoplethsmography. A PetCO 2 of +8 mmHg evoked a 34.6 ± 8.5% increase in mean peak diastolic CBF V (P<0.01). CBF V increased by 38.3 ± 12.6% and 51.4 ± 8.8% in response to a PetO 2 of 52 and 45 mmHg, respectively (P<0.05). RPP and PV A were significantly elevated during hypercapnia (+4, +8 mmHg PetCO 2 ; P<0.05), hypoxia (52 and 45 mmHg PetO 2 ; P<0.01) and were not different during hypocapnia (P>0.05). When CBF V was indexed against either measure of RPP and PV A , the CBF V responses to hypercapnia and hypoxia were abolished. Estimated coronary vascular resistance (CBF V /mean arterial pressure) did not change significantly across any protocols (P>0.05). With respect to absolute changes in CBF V , hypercapnia and hypoxia elicit increases in peak diastolic velocity; though when corrected for changes in RPP and PV A associated with each condition the changes in CBF V were absent (P>0.05). In summary, the lack of sensitivity of the coronary vasculature supplied by the LAD is underscored by the lack of both a CBF V and cardiac workload response in hypocapnia. It can be surmised that the regulation of coronary vessels is mediated primarily through metabolic changes within the heart rather than an inherent sensitivity to changes in arterial blood gases within a physiological range. Support or Funding Information Funding: NSERC, CFI
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".