acute myocardial infarction (AMI/STEMI/NSTEMI)

see also:

• cardiology
• Mx of chest pain in ED
• thrombolytics
• Myocardial Infarction With Nonobstructive Coronary Arteries (MINOCA)
• atherosclerosis and primary prevention
• web resources:
  • AMI probability calculator (Goldman)
  • Probability of bleeding from thrombolysis Rx in AMI - scoring tool
  • EMedicine
  • sandnsurf's blog Mx of STEMI - with references
  • Aust Presc 2007 - Antiplatelet therapy after coronary occlusion
  • Aust. Heart Foundation 2011 treatment algorithm chart for ACS and STEMI (pdf)

• following data from a 2025 US study of 4,116 myocardial injury events in 2,780 patients which excluded periprocedural MI¹⁾
• atherothrombosis
  • acute obstructive blood clot in a coronary artery associated with atherosclerosis and acute plaque rupture
  • the cause in 75% of men under age 65yrs but only accounts for ~ half of women under age 65yrs
  • 2025: Viridans Streptococcal Biofilm Evades Immune Detection and Contributes to Inflammation and Rupture of Atherosclerotic Plaques
    • see also the oral microbiome
• supply/demand mismatch secondary myocardial infarctions (SSDMs)
  • imbalance of oxygen supply and demand caused by other stressors on the body, such as PE, anaemia or an infection
  • the most common cause in women aged under 45yrs and accounts for a third of all cases in women under 65yrs
  • accounts for ~19% of cases in men
  • is associated with higher 5yr mortality (33% compared to 8% for atherothrombotic cause) - usually due to underlying non-cardiac disease
• spontaneous coronary artery dissection (SCAD)
  • accounts for 11% of cases in women aged under 65yrs and 0.7% of cases in men aged under 65yrs
  • often don't have risk factors for heart disease, such as high blood pressure, high cholesterol or diabetes
  • risk factors include:
    • intense physical exercise and severe emotional distress
    • pregnancy or puerperium
    • fibromuscular dysplasia (FMD)
    • Ehlers-Danlos and Marfan syndromes
    • severe high blood pressure (eg. using cocaine or other sympathomimetics)
• embolism
  • accounts for 2% of cases in both men and women aged under 65yrs
• vasospasm
  • accounts for 3% of cases in women aged under 65yrs and 1% of cases in men aged under 65yrs
• myocardial infarction with non-obstructed coronary arteries not meeting another category (MINOCA-U)
  • accounts for 3% of cases in women aged under 65yrs and 2% of cases in men aged under 65yrs
  • see Myocardial Infarction With Nonobstructive Coronary Arteries (MINOCA)

• 1 in 300 patients with acute ischaemic chest pain transported to hospital via private car will have a cardiac arrest en route
  • thus the community needs to be aware of the need to call emergency ambulance services
• if ambulances have access to 12 lead ECG's then direct communication with an angiography lab potentially bypassing ED may be the most expeditious Mx once AMI has been diagnosed on the ECG.
• if 12 lead ECG is not available, then ambulance protocols usually consist of:
  • oxygen if hypoxic or in shock
  • cardiac monitor, aspirin 300mg o, GTN s/lingual and immediate transfer to an ED, preferably at a hospital with interventional angiography services.

• see Mx of chest pain in the ED
• oxygen if SaO2 < 93% or if in shock (oxygen is no longer advised to be given routinely!)
• cardiac monitoring & Rx any cardiac arrhythmias as appropriate
• iv access, bloods sent for FBE, U&E, glucose, CK, troponin +/- clotting
  • treat significant electrolyte abnormalities, especially hypokalaemia & hyperkalemia
  • treat severe anaemia with blood transfusion
  • early stabilisation of blood glucose
• avoid excessive venipunctures which may cause problems after thrombolysis - take blood at time of cannulation
• with-hold all caffeine-containing foods (may worsen ischaemia via tachycardia)
• 12 lead ECG:
  • in particular, look for:
    • LBBB
    • ECG Diagnosis of AMI - particularly RV infarct which may C/I GTN Rx and may require iv fluid Rx to maintain BP.
  • if anterior MI:
    • higher risk of cardiogenic shock, LVF, cardiac thrombus
  • if inferior MI:
    • be prepared for temporary AV block:
      • with-hold verapamil, diltiazem, mibeframil, beta blockers, digoxin
      • may need atropine, isoprenaline or temporary pacemaker
      • if complete heart block with idioventricular escape rhythm:
        • DO NOT give lignocaine as may cause asystole!
        • if rate < 40 or compromised then try atropine 0.6-1.2mg
        • if rate still < 40 try isoprenaline boluses +/- infusion (avoid causing tachycardia if possible)
        • if no success then:
          • pt unconscious → external pacemaker
          • otherwise call cardiologist for temporary transvenous pacemaker
    • look for RV involvement (30-50% of inf. AMIs; do right side ECG leads, JVP often high):
      • if RV AMI then:
        • avoid nitrate Rx as may cause profound hypotension
        • may need fluid Rx to maintain BP
    • look for posterior involvement:
      • check for R wave in V1
• rapid history and examination to help exclude confounding diagnoses or complications of AMI:
  • aortic dissection
  • subarachnoid haemorrhage (SAH)
  • stroke (CVA)
  • pulmonary embolism (PE)
  • pericarditis
  • myocarditis - especially in adolescents and young adults, eg. viral, mRNA vaccines
  • LVF or APO
  • cardiogenic shock
  • cardiac tamponade
• aspirin (acetylsalicylic acid) 300mg o if not already given aspirin that day and no allergy
• aim to maintain systolic BP > 90mmHg
• GTN - sublingual +/- patch +/- infusion if ongoing pain
  • C/I to GTN includes:
    • RV infarct
    • hypotension
    • recent Viagra use
• iv morphine titrated in 2.5mg doses for pain relief
• CXR - to further exclude chest pathologies such as dissection, pneumonia, LVF and as a baseline
  • postpone CXR if patient clearly needs PTCA and a CXR will prolong the door-to-balloon time

• if < 12hrs since onset of maximal pain, consider either:
  • Percutaneous transluminal coronary angioplasty (PTCA or primary PCI) (balloon angioplasty)
    • door to balloon time should be < 90min
    • primary PCI reduces serious events including death from 12-14% in those given thrombolysis, to 7-9% in those Rx with PCI within 60min
    • primary PCI within 120min had 30 day mortality 3.8% vs intrahospital thrombolysis 30 day mortality of 8.6% ²⁾
    • hence primary PCI is preferable to thrombolysis if available within 120min.
    • ensure patient is given:
      • aspirin (acetylsalicylic acid) 300mg if not had aspirin that day
      • clopidogrel 600mg stat dose orally or NGT if intubated
      • iv heparin
      • GTN infusion if ongoing pain
      • inotropes if cardiogenic shock
      • elective intubate if transfer to cath lab and:
        • patient has acute pulmonary oedema (APO) (as patient will not be able to lie flat for PCI)
        • BUT note that intubation is NOT indicated just for cardiogenic shock due to STEMI as risk of rapid sequence induction (RSI) for emergency intubation is too high and will only delay PCI
    • Western Health STEMI angioplasty/thrombolysis process
    • if there is delay in obtaining primary PCI, then facilitated PCI may be better than thrombolysis and rescue PCI:
      • half-dose Reteplase + Abcx prior to transfer for PCI
  • thrombolysis if timely PCI not available within 2 hours or is contraindicated, and there is no C/I to thrombolysis
    • door to needle time should be < 30min
    • failed thrombolysis patients may need early rescue angioplasty:
      • development of cardiogenic shock or heart failure
      • persistent ischaemia after 90min post lysis
      • failure of the lead with greatest ST elevation to resolve to 50% of max. height or less by 90min post lysis
• if > 12hrs, or C/I to the above, then Mx as for non-ST elevation myocardial infarct (NSTEMI)

• early discussion with cardiology to decide on medical Mx or early angiography
• if ongoing pain consider IV GTN (and heparin as below) and if not settling may be an indication for emergent angiography
• anti-thrombotic agents as per cardiology, eg:
  • clopidogrel or perhaps iv GPIIb/IIIa receptor antagonists such as ticagrelor (Brilinta) if NSTEMI going for PCI
  • if ongoing pain and chance of early angiography then heparin, otherwise use enoxaparin - may need initial iv dose then sc - reduce dose in elderly and those with impaired creatinine clearance.
• commence a statin if not already on one, eg. atorvastatin 80mg daily
• consider oral beta adrenergic blockers (NOT iv unless severe HT or arrhythmia!!) in the 1st 24 hours (no hurry) unless C/I
  • (level I evidence, grade A recommendation)
  • contra-indications to beta blockers in acute coronary syndromes:
    • risk of cardiogenic shock or heart block:
      • age > 70, systolic BP < 120, HR < 60, HR > 110 (as evidence of decreased cardiac reserve), 1st degree heart block with PR > 240msec or 2nd/3rd degree HB.
    • acute asthma or reactive airways disease
• if angiogram shows normal vessels, consider:
  • pulmonary embolism (PE)
  • substance-abuse related “NSTEMI” such as cocaine
  • Takotsubo cardiomyopathy (TCM) / stress cardiomyopathy / "broken heart"
    • stress-induced “AMI”, 90% are in post-menopausal women and features often last 4-8wks and may recur
  • Myocardial Infarction With Nonobstructive Coronary Arteries (MINOCA)

• see thrombolytics

• heparin or enoxaparin may be considered to reduce risk of complete occlusion of coronary vessel

• heparin is usually required as adjunctive Rx for t-PA thrombolysis and post-angioplasty to prevent re-occlusion

• see Mx of lower limb DVT

• A large proportion of pts with transmural anterior infarcts develop mural thrombi which are the cause of 90% of cerebral emboli in patients post-AMI (overall incidence post-AMI is 3-4%).
• Most cerebral emboli post-AMI occur in the 1st 3months.
• Other risk factors for cerebral emboli post-AMI are:
  • large size of infarct, heart failure, ventricular aneurysm, & AF
• Thus, pts with transmural anterior infarcts are often anticoagulated with full doses of heparin, followed by Rx with warfarin for 3mths.

• see AF for more details