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 (β = −0.216, p = 0.036) in OS-CMR and both systolic and diastolic dysfunction are linked to reduced coronary blood flow (peak strain: β = −0.028, p = 0.047, early diastolic strain rate: β = 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.

Resolution of severe secondary mitral valve regurgitation following aortic valve replacement in infective endocarditis

We present the case of a patient with infective endocarditis anesthetized for replacement of severely regurgitant aortic valve. Intraoperative transesophageal echocardiography revealed a new diagnosis of severe secondary mitral regurgitation. After aortic valve replacement and tricuspid valve repair, severe mitral regurgitation resolved rapidly without any intervention. In multivalvular disease, instant spontaneous resolution of secondary mitral regurgitation is possible after surgical correction of an aortic regurgitation causing left ventricular volume overload.

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

Background

The 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.

Methods

Ten 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 (Q_LAD). 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.

Results

Q_LAD 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.

Conclusion

OS-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.

Transvalvular pressure gradients for different methods of mitral valve repair: only neochordoplasty achieves native valve gradients

OBJECTIVES

Many surgical and interventional methods are available to restore patency for patients with degenerative severe mitral valve regurgitation. Leaflet resection and neochordoplasty, which both include ring annuloplasty, are the most frequently performed techniques for the repair of posterior mitral leaflet flail. It is unclear which technique results in the best haemodynamics. In this study, we investigated the effect of different mitral valve reconstruction techniques on mitral valve haemodynamics and diastolic transvalvular pressure gradient in an ex vivo porcine model.

METHODS

Eight porcine mitral valves were tested under pulsatile flow conditions in an in vitro pulsatile flow loop for haemodynamic quantification. Severe acute posterior mitral leaflet flail was created by resecting the posterior marginal chorda. The acute mitral valve regurgitation was corrected using 4 different repair techniques, in each valve, in a strictly successive order: (i) neochordoplasty with polytetrafluoroethylene sutures alone and (ii) with ring annuloplasty, (iii) edge-to-edge repair and (iv) triangular leaflet resection, both with ring annuloplasty. Valve haemodynamics were measured and quantified for all valve configurations (native, rupture and each surgical reconstruction). The results were analysed using a validated statistical linear mixed model, and the P-values were calculated using a 2-sided Wald test.

RESULTS

All surgical reconstruction techniques were able to sufficiently correct the acute mitral valve regurgitation. Neochordoplasty without ring annuloplasty was the only reconstruction technique that resulted in haemodynamic properties similar to the native mitral valve (P-values from 0.071 to 0.901). The diastolic transvalvular gradient remained within the physiological range for all reconstructions but was significantly higher than in the native valve for neochordoplasty with ring annuloplasty (P < 0.000), edge-to-edge repair (P < 0.000) and leaflet resection (P < 0.000). Neochordoplasty without ring annuloplasty resulted in a significantly better pressure gradient than neochordoplasty with a ring annuloplasty (P < 0.000). Additionally, neochordoplasty with a ring annuloplasty resulted in significantly lower transvalvular pressure gradients than edge-to-edge repair (P < 0.000) and leaflet resection (P < 0.000).

CONCLUSIONS

Neochordoplasty with or without ring annuloplasty was the reconstruction technique that almost achieved native physiological haemodynamics after repair of posterior mitral leaflet flail after acute isolated chordal rupture in our ex vivo porcine model.

Right atrial pressure and venous return during cardiopulmonary bypass

The relevance of right atrial pressure (RAP) as the backpressure for venous return (QVR) and mean systemic filling pressure as upstream pressure is controversial during dynamic changes of circulation. To examine the immediate response of QVR (sum of caval vein flows) to changes in RAP and pump function, we used a closed-chest, central cannulation, heart bypass porcine preparation ( n = 10) with venoarterial extracorporeal membrane oxygenation. Mean systemic filling pressure was determined by clamping extracorporeal membrane oxygenation tubing with open or closed arteriovenous shunt at euvolemia, volume expansion (9.75 ml/kg hydroxyethyl starch), and hypovolemia (bleeding 19.5 ml/kg after volume expansion). The responses of RAP and QVR were studied using variable pump speed at constant airway pressure (PAW) and constant pump speed at variable PAW. Within each volume state, the immediate changes in QVR and RAP could be described with a single linear regression, regardless of whether RAP was altered by pump speed or PAW ( r2 = 0.586–0.984). RAP was inversely proportional to pump speed from zero to maximum flow ( r2 = 0.859–0.999). Changing PAW caused immediate, transient, directionally opposite changes in RAP and QVR (RAP: P ≤ 0.002 and QVR: P ≤ 0.001), where the initial response was proportional to the change in QVR driving pressure. Changes in PAW generated volume shifts into and out of the right atrium, but their effect on upstream pressure was negligible. Our findings support the concept that RAP acts as backpressure to QVR and that Guyton’s model of circulatory equilibrium qualitatively predicts the dynamic response from changing RAP.

NEW & NOTEWORTHY Venous return responds immediately to changes in right atrial pressure. Concomitant volume shifts within the systemic circulation due to an imbalance between cardiac output and venous return have negligible effects on mean systemic filling pressure. Guyton’s model of circulatory equilibrium can qualitatively predict the resulting changes in dynamic conditions with right atrial pressure as backpressure to venous return.

Simplified Approach for Repair of Early Pseudoaneurysm of the Left Coronary Button Following Composite Graft Due to Acute Type A Aortic Dissection

We present a simplified surgical technique that was performed on a 37-year-old man who presented with a pseudoaneurysm of the left coronary ostium two months after repair of acute Type A aortic dissection with a composite graft. Intraoperatively, the surgical sites showed extreme adhesions. The leakage at the level of the coronary suture line was exposed from inside the aortic graft. Repair was performed using 7.0 polypropylene sutures, and the postoperative course was uneventful. The patient was discharged on postoperative day six without further complications.

Ideal site for ventricular anchoring of artificial chordae in mitral regurgitation

OBJECTIVE

Surgical treatment of mitral leaflet prolapse using artificial neochordae shows excellent outcomes. Upcoming devices attempt the same treatment in a minimally invasive way but target the left ventricular apex as an anchoring point, rather than the tip of the corresponding papillary muscle. In this study, cine cardiac magnetic resonance imaging was used to compare these 2 different anchoring positions and their dynamic relationship with the mitral leaflets.

METHODS

Eleven healthy volunteers (mean age, 31 years; 6 female; mean ejection fraction, 62%) were examined by cardiac magnetic resonance imaging (3 Tesla, cine steady free precession technique with retrospective gating), whereby dedicated software enabled assessment of the physiologic distances among 3 anchoring sites (anterior papillary muscle, posterior papillary muscle, and apex) and the plane of the mitral annulus at the level of leaflet coaptation. These distances were measured in systole and diastole, and the performance of virtual neochordae was analyzed for the 3 potential anchoring sites.

RESULTS

Length difference between systole and diastole for the 3 measured distances were 0.19 ± 0.11 cm (5.9% ± 3.4%) for the anterior papillary muscle, 0.19 ± 0.09 cm (6.7% ± 3.6%) for the posterior papillary muscle, and 1.52 ± 0.18 cm (17.8% ± 2.8%) for the left ventricular apex (P = .001). Virtual neochordae between the leaflet and the left ventricular apex were first adjusted in systole to achieve leaflet coaptation. Leaflet tear in diastole can only be avoided if the width of the attached leaflet is larger than the systole-diastole length difference. On the other hand, if virtual neochordae are adjusted in diastole to avoid leaflet tear, residual leaflet prolapse during systole can result. Because the systole-diastole length difference for papillary muscle anchored chordae is smaller than for apical chordae by a factor 10, there is a strongly reduced risk of prolapse or tearing and the leaflet width is unimportant. Furthermore, if the neochordae attached to the anterior mitral leaflet uses the apex as a distal anchoring site, the angle α between the aortic valve plane and this mitral leaflet is significantly reduced in diastole and therefore increases the risk of systolic anterior motion.

CONCLUSIONS

Anchoring of neochordae at the papillary muscles, thereby mimicking the real anatomy, should be preferred over the left ventricular apex. Further analysis of dilated hearts and papillary muscle displacement is necessary to include the whole spectrum of pathologies.

Aortic arch morphometry in living humans

Anatomical features of the aortic arch such as its steepness, the take-off angles and the distances between its supra-aortic branches can influence the feasibility and difficulty of interventional and/or surgical maneuvers. These anatomical characteristics were assessed by means of 3D multiplanar reconstruction of thoracic angio-computed tomography scans of 92 living patients (79 males, 13 females, mean age 69.4 ± 9.9 years) carried out for various indications (gross pathology of the thoracic aorta excluded). There was a significant variation of all measured parameters between the subjects - a standard aortic arch (i.e. with all measured parameters within 2 SD) does not seem to exist. There were no significant differences between genders but some of the parameters correlated significantly to age.