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Reference invasive tests of microvascular injury in myocardial infarction
  1. Annette Marie Maznyczka1,2,
  2. Peter McCartney1,2,
  3. Colin Berry1,2
  1. 1 British Heart Foundation Glasgow Cardiovascular Research Centre, Institute of Cardiovascular and Medical Sciences, University of Glasgow, Glasgow, UK
  2. 2 West of Scotland Heart and Lung Centre, Golden Jubilee National Hospital, Glasgow, UK
  1. Correspondence to Professor Colin Berry, British Heart Foundation Glasgow Cardiovascular Research Centre, Institute of Cardiovascular and Medical Sciences, Glasgow, G12 8TA, UK; colin.berry{at}glasgow.ac.uk

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In patients with ST-segment elevation myocardial infarction (STEMI), primary percutaneous coronary intervention (PCI) successfully restores normal antegrade flow in the infarct-related artery in nearly 99% of patients. However, approximately half of all STEMI patients have failed microcirculatory reperfusion, as reflected by microvascular obstruction (MVO), and one-third have myocardial haemorrhage, reflecting severe, ‘downstream’, potentially irreversible, microvascular injury.1

MVO is the ‘Achilles Heel’ of primary PCI, yet clinicians are generally unaware of the occurrence of MVO and myocardial haemorrhage in their patients, unless cardiac magnetic resonance (CMR) is performed. However, CMR is not done routinely. Other established investigations for detecting failure of myocardial reperfusion, such as angiographic, or electrocardiographic parameters, lack sensitivity and reproducibility in clinical practice.

Immediate invasive measurement of microvascular resistance at the time of PCI has the potential to optimise the approach to therapeutic interventions by: (1) acutely identifying patients at high risk of MVO who are most likely to benefit from adjunct therapy, for example, with glycoprotein IIb/IIIa inhibitors, (2) targeting novel therapies in clinical trials to patients with evidence of microvascular dysfunction and (3) allowing immediate evaluation of the efficacy of reperfusion therapy. However, invasive tests of the efficacy of myocardial reperfusion in STEMI patients have been hampered by a number of methodological and technical shortcomings. The ideal acute invasive test of microvascular perfusion and dysfunction should be reliable and reproducible, operator independent and easy to perform with standard PCI equipment.

Currently, the index of microvascular resistance (IMR) has the most extensive evidence base to support its use as a reference test of culprit artery microvascular function in patients with acute STEMI.IMR is a thermodilution-derived index, measured using a guide wire that combines a pressure and temperature sensor. Specifically, IMR is defined as distal coronary pressure multiplied by the mean transit time of a 3 mL bolus of saline at room …

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