Introducing a free-breathing MRI method to assess peri-operative myocardial oxygenation and function: A volunteer cohort study.

BACKGROUND: Induction of general anaesthesia has many potential triggers for peri-operative myocardial ischaemia including the acute disturbance of blood gases that frequently follows alterations in breathing and ventilation patterns. Free-breathing oxygenation-sensitive cardiovascular magnetic resonance (OS-CMR) imaging may provide the opportunity to continuously quantify the impact of such triggers on myocardial oxygenation. OBJECTIVE: To investigate the impact of breathing patterns that simulate induction of general anaesthesia on myocardial oxygenation in awake healthy adults using continuous OS-CMR imaging. DESIGN: Prospective observational study. SETTING: Single-centre university hospital. Recruitment from August 2020 to January 2022. PARTICIPANTS: Thirty-two healthy volunteers younger than 45 years old were recruited. Data were analysed from n  = 29 (69% male individuals). INTERVENTION: Participants performed a simulated induction breathing manoeuvre consisting of 2.5 min paced breathing with a respiration rate of 14 breaths per minute, followed by 5 deep breaths, then apnoea for up to 60s inside a magnetic resonance imaging scanner (MRI). Cardiac images were acquired with the traditional OS-CMR sequence (OS bh-cine ), which requires apnoea for acquisition and with two free-breathing OS-CMR sequences: a high-resolution single-shot sequence (OS fb-ss ) and a real-time cine sequence (OS fb-rtcine ). MAIN OUTCOME MEASURES: Myocardial oxygenation response at the end of the paced breathing period and at the 30 s timepoint during the subsequent apnoea, reflecting the time of successful intubation in a clinical setting. RESULTS: The paced breathing followed by five deep breaths significantly reduced myocardial oxygenation, which was observed with all three techniques (OS bh-cine -6.0 ±â€Š2.6%, OS fb-ss -12.0 ±â€Š5.9%, OS fb-rtcine -5.4 ±â€Š7.0%, all P  < 0.05). The subsequent vasodilating stimulus of apnoea then significantly increased myocardial oxygenation (OS bh-cine 6.8 ±â€Š3.1%, OS fb-ss 8.4 ±â€Š5.6%, OS fb-rtcine 15.7 ±â€Š10.0%, all P  < 0.01). The free-breathing sequences were reproducible and were not inferior to the original sequence for any stage. CONCLUSION: Breathing manoeuvres simulating induction of general anaesthesia cause dynamic alterations of myocardial oxygenation in young volunteers, which can be quantified continuously with free-breathing OS-CMR. Introducing these new imaging techniques into peri-operative studies may throw new light into the mechanisms of peri-operative perturbations of myocardial tissue oxygenation and ischaemia. VISUAL ABSTRACT: http://links.lww.com/EJA/A922.

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

Background: Coronary autoregulation is a feedback system, which maintains near-constant myocardial blood flow over a range of mean arterial pressure (MAP). Yet in emergency or peri-operative situations, hypotensive or hypertensive episodes may quickly arise. It is not yet established how rapid blood pressure changes outside of the autoregulation zone (ARZ) impact left (LV) and right ventricular (RV) function. Using cardiovascular magnetic resonance (CMR) imaging, measurements of myocardial tissue oxygenation and ventricular systolic and diastolic function can comprehensively assess the heart throughout a range of changing blood pressures. Design and methods: In 10 anesthetized swine, MAP was varied in steps of 10-15 mmHg from 29 to 196 mmHg using phenylephrine and urapidil inside a 3-Tesla MRI scanner. At each MAP level, oxygenation-sensitive (OS) cine images along with arterial and coronary sinus blood gas samples were obtained and blood flow was measured from a surgically implanted flow probe on the left anterior descending coronary artery. Using CMR feature tracking-software, LV and RV circumferential systolic and diastolic strain parameters were measured from the myocardial oxygenation cines. Results: LV and RV peak strain are compromised both below the lower limit (LV: Δ1.2 ± 0.4%, RV: Δ4.4 ± 1.2%, p < 0.001) and above the upper limit (LV: Δ2.1 ± 0.4, RV: Δ5.4 ± 1.4, p < 0.001) of the ARZ in comparison to a baseline of 70 mmHg. LV strain demonstrates a non-linear relationship with invasive and non-invasive measures of oxygenation. Specifically for the LV at hypotensive levels below the ARZ, systolic dysfunction is related to myocardial deoxygenation (ß = -0.216, p = 0.036) in OS-CMR and both systolic and diastolic dysfunction are linked to reduced coronary blood flow (peak strain: ß = -0.028, p = 0.047, early diastolic strain rate: ß = 0.026, p = 0.002). These relationships were not observed at hypertensive levels. Conclusion: In an animal model, biventricular function is compromised outside the coronary autoregulatory zone. Dysfunction at pressures below the lower limit is likely caused by insufficient blood flow and tissue deoxygenation. Conversely, hypertension-induced systolic and diastolic dysfunction points to high afterload as a cause. These findings from an experimental model are translatable to the clinical peri-operative environment in which myocardial deformation may have the potential to guide blood pressure management, in particular at varying individual autoregulation thresholds.

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

The presence of deoxygenated hemoglobin (Hb) results in a drop in T2 and T2* in magnetic resonance imaging (MRI), known as the blood oxygenation level-dependent (BOLD-)effect. The purpose of this study was to investigate if deoxygenated myoglobin (Mb) exerts a BOLD-like effect. Equine Met-Mb powder was dissolved and converted to oxygenated Mb. T1, T2, T2*-maps and BOLD-bSSFP images at 3Tesla were used to scan 22 Mb samples and 12 Hb samples at room air, deoxygenation, reoxygenation and after chemical reduction. In Mb, T2 and T2* mapping showed a significant decrease after deoxygenation (- 25% and - 12%, p < 0.01), increase after subsequent reoxygenation (+ 17% and 0% vs. room air, p < 0.01), and finally a decrease in T2 after chemical reduction (- 28%, p < 0.01). An opposite trend was observed with T1 for each stage, while chemical reduction reduced BOLD-bSSFP signal (- 3%, p < 0.01). Similar deflections were seen at oxygenation changes in Hb. The T1 changes suggests that the oxygen content has been changed in the specimen. The shortening of transverse relaxation times in T2 and T2*-mapping after deoxygenation in Mb specimens are highly indicative of a BOLD-like effect.