Introduction
The diagnosis and treatment of acute myocardial infarction (AMI) in the operational
setting can be both difficult and challenging. The underlying principle in the General
Medical Officers (GMO) treatment of AMI should be the rapid stabilization and
earliest possible medical evacuation of the patient to a medical facility with the
appropriate Intensive Care Unit/Cardiac Care Unit (ICU/CCU) and support capabilities. The
maximum effort should be made to ensure, whenever possible, that the patient with AMI is
electrically and hemodynamically stable and pain free before transfer. Treatment of
cardiac arrest or hemodynamically significant arrhythmias should follow the advanced
cardiac life support (ACLS) guidelines.
History
The diagnosis of AMI is primarily based on an appropriate clinical history. The
clinician must maintain a high index of suspicion as patient complaints of chest
discomfort may vary considerably. Usually lasting for greater than 30 minutes, the
discomfort generally is described as a severe retrosternal squeezing, choking, or heavy
pressure sensation. It may radiate to the shoulders, down the ulnar aspect of the left
arm, into the neck or jaw. The patient will often report associated diaphoresis or
shortness of breath. A history of antecedent exertional chest discomfort can often be
elicited.
Physical exam
During the physical examination particular attention should be paid to the pulse,
blood pressure, and respiratory rate, signs of heart failure to include elevated jugular
venous pulse, rales on chest auscultation, the presence of an S3, or the development of a
new heart murmur on cardiac examination.
Laboratory
Serial 12-lead electrocardiogram (ECG) monitoring is helpful in establishing the
diagnosis of AMI, but the presence of an initially normal ECG does not exclude the
diagnosis. Serial creatinine kinase (CK) determinations should be obtained every 6 to 8
hours. If laboratory facilities are not available, serum should be collected, spun down,
if possible, and kept on ice until sent with the patient at the time of medical
evacuation. Spectralä cards for qualitative measurement of Myoglobin, CKMB and Troponin may be available on some platforms. A right-sided V4r lead
ECG recording is essential in all patients with suspected AMl as it will allow diagnosis
of RV infarction. In the precepts of a right ventricular (RV) infarct, hypotension may be
a problem when nitroglycerin is administered, and may require 35 liters of fluid in the
first 24 hours.
Initial treatment
All patients with diagnosed or suspected AMI should be placed on a continuous ECG
monitor (one lead is sufficient) with a large bore intravenous line established. All
patients should be placed on supplemental oxygen. Patients should chew at least 160 mg of
aspirin and then be treated with 325 mg of
aspirin orally each day. Blood pressure should
be monitored every 5 minutes until stable and then hourly. The patient should be placed on
complete bed rest. In the absence of contraindications (table 1), patients should receive B-blockers. Metoprolol can be given by intravenous push in 3 separate 5-mg doses. Then
give 50mg PO every 6 hours or a dose of 1 to 3 mg of IV propranolol followed by 60 to 80
mg PO TID. If an IV preparation of atenolol is not available, begin the oral (PO) dosing.
Atenolol 50 mg BID PO can alternatively be used.
A careful assessment of the patients condition should be made between each dose
of beta-blocker and the dose held if hypotension (systolic pressure <90mmHg), severe
bradycardia (HR < 50), or signs of congestive heart failure develop.
Pain relief
In the absence of severe hypotension (systolic BP < 90 mmHg) and in the presence
of on-going chest pain, the patient should receive 0.4 mg of sublingual nitroglycerin
every 5 minutes until chest pain is relieved or hypotension ensues, usually no more than 3
doses are needed. Additional therapy may be instituted with intravenous nitroglycerin
at a
dose of 10 mcg/min if a controlled infusion pump is available. The dosage of IV nitroglycerin may be increased 10 mcg/min every 5 minutes until clinical symptoms decrease
or significant hypotension develops. If an infusion pump is not available 1/2 to 2 inches
of nitroglycerin paste may be used. For pain refractory to nitroglycerin and beta-blocker,
intravenous morphine sulfate in doses of 1-2 mg every 5-10 minutes may be used.
Dysrythmia
Experts no longer recommend lidocaine prophylaxis for suppression of premature
ventricular contractions as its use is associated with an increase in asystolic deaths. In
the presence of ventricular fibrillation or tachycardia, intravenous lidocaine
is
indicated with a bolus and infusion pump dosing per the ACLS protocol. If an infusion pump
is not available lidocaine
should not be given by continuous infusion unless the drip is
monitored continuously. The Israeli's demonstrated that 100 mg of lidocaine
IM before
transport decreased VT in the ambulance. This may be particularly relevant to patients
transported in helicopters. IM injections had been discouraged in the past due to the rise
in CK, however the rise in CK is all MM and does not interfere with diagnosis by the
isoenzymes. Symptomatic bradycardia can be treated with atropine 0.6 mg IV. The dose may
be repeated if needed, every 5 minutes until a total dose of 2 mg is reached.
Anticoagulation
The following considerations should be evaluated before anticoagulation
is initiated.
- Does the patient have any contraindications to systemic anticoagulation?
- Recent severe head injury.
- Coagulopathy.
- Bleeding ulcer or other inaccessible bleeding site.
- Uncontrollable hypertension.
- Severe hepatic or renal disease.
- Is an infusion pump is available?
- Is the laboratory capable of supporting the analysis of complete blood
counts (CBC) and (partial
thromboplastin time) PTT?
If these considerations are met, then the patient should receive an
initial bolus of 5,000 units of heparin IV followed by a continuous infusion of 1,000
units/hour. The PTT should be checked 6 hours after beginning the infusion, and the drip
rate adjusted to maintain a PTT at 1.5 to 2.0 times the control value. An excellent
alternative is the use of Low Molecular Weight heparin, which does not require dose
titration by PTT monitoring. Enoxaprin 1 mg/kg subcutaneous every 12 hours is equally
effective to adjusted dose heparin in unstable angina and non-Q wave infarction (ESSENCE
trial).
Thrombolytic Therapy
Multiple clinical trials have shown that thrombolytic therapy reduces mortality
from myocardial infarction. The three currently available thrombolytic agents,
Streptokinase (SK), rt-PA, and APSAC have been shown to be equally effective in reducing
mortality. SK is the most economical drug to use but given APSAC's ease of administration
(30 units IV push over 5 minutes), this drug may have a role in the operational setting.
Reteplase is new to the market and preliminary data shows it to be equally effective as
rt-PA in achieving an open artery. It is given as a double bolus making it more convenient
for use at sea. Depending on the operational setting and the ability to medically evacuate
(MEDEVAC) the patient to an appropriate facility within 6 hours, the use of thrombolytic
therapy in a patient presenting with an AMI should be strongly considered.
Only give thrombolytics after assessing the indications for thrombolytic therapy (table
2). Ensure no contraindications to therapy exist (table 3). Recent evidence shows that
thrombolytic therapy can be given safely in a secondary treatment facility (such as a
large deck) without the need for emergent, or even subsequent transport to a tertiary care
facility.
Transportation
When transporting a patient, the GMO must ensure that the appropriate staff and
equipment accompany the patient to provide advanced cardiac life support (ACLS) in route.
Contraindications to Beta blocker
administration
Table 1
CONTRAINDICATIONS TO BETA
BLOCKADE |
Heart rate < 50 |
Signs of peripheral
hypoperfusion |
Systolic Blood Pressure <
100mmHg |
AV conduction abnormalities: PR
> 0.20 |
Moderate to severe left
ventricular dysfunction |
Severe COPD or history of
asthma |
Indications for Thrombolytic Therapy
Table 2
Patient age alone is not
a contraindication. New evidence shows that those over 76 have a greater benefit, despite
a greater risk for stroke (CVA).
Patients who present with pain characteristic for myocardial infarction of
greater than 30 minutes duration but less than 12 hours duration and at least 0.1 mV of ST
segment elevation in at least two contiguous ECG leads. If there is ongoing pain and ST
elevation, thrombolytic therapy may provide a benefit out to 24 hours. |
Absolute
Contraindications to Thrombolytic Therapy
Table 3
ABSOLUTE
CONTRAINDICATIONS TO THROMBOLYTIC THERAPY |
Active internal bleeding within
10 days |
Recent head trauma or history
of any cerebrovascular event |
Suspected aortic dissection |
Pregnancy |
Traumatic CPR lasting greater
than 10 minutes |
Severe uncontrolled
hypertension (systolic BP > 200 mmHg or diastolic > 120 mmHg) |
Recent severe trauma or surgery |
Active hemorrhagic diabetic
retinopathy |
References
ACC/AHA Guidelines for the Management of Patients with Acute
Myocardial infarction., JACC, November 1, 1996 Vol. 28, No. 5: pp 1328-1428.
The U.S. Department of Health and Human Services- Agency for Health Care Policy and
Research Clinical Number 10; Unstable Angina: Diagnosis and Management, March 1994.
Reviewed by CAPT K. F. Strosahl, MC, USN, Cardiology/Computer
Assisted Program of Cardiology Specialty Leader, Cardiovascular Disease
Division, Portsmouth Naval Hospital, Portsmouth, VA (1999).