MYOCARDIAL INFARCTION | THROMBOLYSIS

MYOCARDIAL INFARCTION

·      Clinical features

Acute myocardial infarction is a clinical diagnosis.  Features include typical crushing, pressing or constricting chest pain or tightness, although many patients with established AMI will have atypical pain.  ECG is usually abnormal with localised ST elevation is 30%.  Q waves indicate full thickness infarction.  Patients with normal ECGs who do have AMI have a much lower complication rate. 

One-off cardiac enzymes are not helpful in excluding AMI. Serial enzymes in the ED over 6 hours will exclude over 95% of AMIs.

Complications include arrhythmias, failure and cardiogenic shock.

·        Management

Treatment of the patient with suspected ischaemic chest pain needs to encompass some or all of the following areas:

·      Supportive treatment - including 02, position, reassurance

·      Treat life threatening problems - basically arrhythmias

·      Monitoring - ECG, NIBP and Sa02

·      IV access - if thrombolysis is planned then two lines are required of at least 18G so that one can be used to give drugs and the other to take blood samples.  In the Emergency Department K+ is the most important test.  Group and hold and cardiac enzymes may be indicated. 

·      History is only brief and should focus on the questions of “Is this ischaemic chest pain?” and “Is there a need for or contraindications to thrombolysis?”

·      The most valuable investigation is the ECG.  Compare to old ones if available.  The ECG should be repeated at 30 minute intervals if the history suggests cardiac ischaemia.

·      CXR is not a priority and really only indicated in the Emergency Department if treatable LVF is suspected.

·      The criteria for thrombolysis are constantly evolving and depend on a host of factors.  The contraindications are relative rather than absolute and apart from the patients pre morbid health depend on the site and age of the infarct.

      The important criteria are:

·        ischaemic chest pain < 12 hours duration

·      ST elevation in 2 contiguous leads (or new onset BBB)

·      no unacceptable contraindications

·        Streptokinase is still generally the thrombolytic of choice however t-PA is becoming increasingly used especially for patients under 75 years of age with anterior infarcts of less than four hours duration, in the repeat use situation, and in patients with coronary artery bypass grafting.

·      You may need to use any or all of the following:

· heparin (if TPA is to be used)

                                            · nitrates (s/ , spray, topical, IV)

· morphine IV

· beta-blockers

· diuretics, inotropes, ACE inhibitors, Ca channel blockers

·        Aspirin 300 mg is routinely given to all patients with myocardial ischaemia.

·      Although not readily available in most centres at present there is clear evidence that acute angioplasty is an excellent therapy for reperfusion in acute AMI. Its role will increase with availability. It is the Rx of choice in AMI with cardiogenic shock, and should be considered where there are contraindications to thrombolysis.

 THROMBOLYSIS PROTOCOL FOR A.M.I.

  

INDICATION:

• Acute myocardial infarction < 12 hours of pain

• ECG changes - either new onset BBB, or

                           - ST ­ in 2 anatomically contiguous leads (>1mm in limb leads, >2mm in V leads)

• benefits of treatment thought to outweigh risks

 

MONITORING

• Continuous ECG

• Heart rate & blood pressure every five minutes during infusion

 

AT HIGH RISK OF COMPLICATIONS - NEED TO BALANCE BENEFITS AGAINST POTENTIAL RISKS

ALLERGY (if using streptokinase):

         • SK in the past

         • documented strep. throat in the last 1/12

         • known allergy to streptokinase

 

BLEEDING:

         • congenital bleeding disorder (eg haemophilia)

         • acquired bleeding disorder (eg liver disease)

         • recent major trauma (eg subdural)

         • recent surgery (eg cholecystectomy)

         • medical conditions that may be complicated by bleeding

           (eg.haemorrhagic CVA, peptic ulcer)

 

NOTE: prolonged CPR and age are not themselves contraindications

 ADMINISTRATION:

STREPTOKINASE

RECONSTITUTE:    1,500,000 unit vial with 5 ml NaCI 0.9%

 

INFUSION:                Add to 95ml NaCI 0.9%

                                  þ 15,000 units per ml

 

TIME:  Infuse over 60 minutes

tPA

100mg (2 vials) in 100ml N Saline

15mg IV bolus over 2 minutes (then)

0.75mg 1kg over 30 minutes (max 50mg) (then)

0.5mg 1kg over 60 minutes (max 35mg)

SIDE EFFECTS (most are more common with streptokinase):

• Hypotension (treated by head tilt down, fluid bolus, stopping then slowly restarting if necessary)

• Haemorrhage

• Arrhythmias

• Anaphylaxis

• Headache, nausea

• Fever, chills, rashes

 SK vs tPA:

·        Use SK only in >75 year olds

tPA may be preferred in young patients with anterior AMI < 4 hrs old.