Relationship between myocardial oxygenation and blood pressure: Experimental validation using oxygenation-sensitive cardiovascular magnetic resonance

Abstract

BackgroundThe relationship between mean arterial pressure (MAP) and coronary blood flow is well described. There is autoregulation within a MAP range of 60 to 140 mmHg providing near constant coronary blood flow. Outside these limits flow becomes pressure-dependent. So far, response of myocardial oxygenation to changes in pressure and flow has been more difficult to assess. While established techniques mostly require invasive approaches, Oxygenation-Sensitive (OS) Cardiovascular Magnetic Resonance (CMR) is a technique that can non-invasively assess changes in myocardial tissue oxygenation. The purpose of this study was to follow myocardial oxygenation over a wide range of blood pressure variation within and outside known coronary autoregulatory limits using OS-CMR, and to relate these data to coronary hemodynamics.MethodsTen anaesthetized swine (German Large White) underwent left-sided thoracotomy and attachment of a perivascular flow probe to the proximal left anterior descending (LAD) coronary artery for continuous measurement of blood flow (QLAD). Thereafter, animals were transferred into a 3T MRI scanner. Mean arterial pressure (MAP) was varied in 10–15 mmHg steps by administering alpha1-receptor agents phenylephrine or urapidil. For each MAP level, OS-CMR images as well as arterial and coronary sinus blood gas samples were obtained simultaneously during brief periods of apnea. Relative changes (Δ) of coronary sinus oxygen saturation (ScsO2), oxygen delivery (DO2) and demand (MVO2), extraction ratio (O2ER) and excess (Ω) from respective reference levels at a MAP of 70 mmHg were determined and were compared to %change in OS-signal intensity (OS-SI) in simultaneously acquired OS-CMR images.ResultsQLAD response indicated autoregulation between MAP levels of 52 mmHg (lower limit) and127 mmHg (upper limit). OS-CMR revealed a global myocardial oxygenation deficit occurring below the lower autoregulation limit, with the nadir of OS-SI at -9.0%. With MAP values surpassing 70 mmHg, relative OS-SI increased to a maximum of +10.6%. Consistent with this, ΔScsO2, ΔDO2, ΔMVO2, ΔO2ER and ΔΩ responses indicated increasing mismatch of oxygenation balance outside the autoregulated zone. Changes in global OS-CMR were significantly correlated with all of these parameters (p≤0.02) except with ΔMVO2.ConclusionOS-CMR offers a novel and non-invasive route to evaluate the effects of blood pressure variations, as well as of cardiovascular drugs and interventions, on global and regional myocardial oxygenation, as demonstrated in a porcine model. OS-CMR identified mismatch of O2 supply and demand below the lower limit of coronary autoregulation. Vasopressor induced acute hypertension did not compromise myocardial oxygenation in healthy hearts despite increased cardiac workload and O2 demand. The clinical usefulness of OS-CMR remains to be established.

The blood oxygen level dependent (BOLD) effect of in-vitro myoglobin and hemoglobin

“…coronary artery disease, heart failure, hypertrophic cardiomyopathies, it can be expected that this effect may in fact occur at much lower heart rates, when patients feel exertion. Guensch et al showed in a swine model that BOLD-SI only drops when oxygen demand outweighs the supply 36 . In patients with cardiovascular disease, this balance may be much more delicate and shifted, with fewer reserves on the supply side.…”

Assessment of Myocardial Function During Blood Pressure Manipulations Using Feature Tracking Cardiovascular Magnetic Resonance

“…This study was approved by the Veterinary Services at the Department of Agriculture and Nature of the Canton Bern, Switzerland [#BE 103/14] ( 7 ). The study was carried out in accordance with national and local animal care regulations and adheres to the ARRIVE guidelines.…”

“…Inside of a clinical 3T magnetic resonance imaging scanner (MAGNETOM TRIO, Siemens Healthineers, Erlangen, Germany), MAP was adjusted in steps of 10–15 mmHg from a baseline level of 70 mmHg using urapidil and phenylephrine ( Figure 1B ), as published by our group previously ( 7 ). At each MAP-level, blood gas samples from the coronary sinus and the femoral artery were obtained, along with OS-CMR images.…”

“…Multiple studies investigating myocardial oxygenation responses in healthy animal models as well as in various disease models (7,14,15,(17)(18)(19)(20)(21) have emerged over the last decade. As we use a cine variant, it measures a cardiac phase every 40 ms, thus strain can be analyzed from the same set of images.…”

“…Techniques such as oxygenation-sensitive cardiovascular magnetic resonance (OS-CMR) can non-invasively interrogate changes in myocardial oxygenation on a tissue level with high spatial resolution based on local deoxyhemoglobin fractions ( 5 , 6 ). In 2019, we reported that left ventricular myocardial oxygenation measured by OS-CMR had a curvi-linear association with mean arterial pressure ( 7 ). However, the impact of blood pressure regulation and these oxygenation changes on biventricular myocardial function defined by strain have not been systematically examined so far.…”