Knowledge of the anatomy of the heart, the physiology of the heart, and the electrocardiography of the heart make a paramedic an
Emergency Cardiac Care Provider.
As primary point of care field paramedics, we have been taxed with the new challenge of diagnosing 12 lead cardiographs in the field and making a definitive diagnosis of acute myocardial infarction. This overwhelming task has been at sometimes difficult to manage and to complicate matters, we do not use this skill regularly enough to maintain the skills.
So, I hope the following information is helpful...
The 12 lead EKG measures electrical potential
Depolarization: The shift in electrolytes which reverses the charge
Repolarization: Return to the resting state
Three kinds of leads:
Einthoven's Triangle represents the leads that we all use with our monitors on a regular basis:
1) Standard Limb Leads
Leads I, II & III & AVR, AVL, and AVF.
2) Augmented Leads
3) Precordial Leads
The precordial views make up a cross section view of the heart in a transverse horizontal plane projecting a view across the AV Node.
Einthoven's Triangle and the four limb leads make up the "HEXAXIAL
VIEW!" This view is a vertical/frontal-posterior - ventral/dorsal plane making
a star with 6 points intersecting through the heart in a flat frontal plane
across the patients chest.
The PRECORDIAL views are used to make up the other six views of the heart for a total of twelve views.
So, adding this up: lead I, II and III, lead AVR, AVL, AVF, and the 6
precordial leads equals 12 leads... RIGHT?????
Correct, however, we only need 10 electrodes placed.
( 4 on the limbs and 6 on the chest )
The cardiac monitor uses the four Limb Leads to make up Lead I, II, III & AVR, AVL, AVF; six views...
12 lead Quick Triage
The following situation constitutes activation of the cardiac response team at the hospital by reporting the field diagnosis of AMI!
The following situation will result in the 12 Lead ECG being reported as "normal". No subsequent activation of the cardiac response team.
Other Signals to use as a diagnostic tool:
AMI diagnosis criteria: 1mm. or more of ST elevation in 2
or more contiguous leads.
Anterior wall requires 2mm. or more of ST elevation (V1-V4)
|INFARCT LOCATION:||ST ELEVATION FOUND IN:|
|Anterior - Septal||V1, V2, V3, and V4 -- 0.2mV or more in leads|
|Posterior||V1, and V2 -- 0.2mV or more in leads|
|Inferior||II, III, and aVF -- 0.1mV or more in 2 leads|
|High Lateral||I, and aVL -- 0.1mV or more in 2 leads|
|Low Lateral||V5, and V6 -- 0.1mV or more in 2 leads|
|Region of ST Elevation||Region of ST Depression|
|Anterior (leads V1-V4)||Inferior (true posterior)|
|Inferior (leads II, III, aVF)||Anterior (leads V1-V3 or lateral lead 1. aVL)|
|Lateral ( leads I, aVF, V5, V6)||Inferior ( leads II, III, aVF)|
|True Posterior||Anterior (leads V1-V3)|
If you can comprehend which way the current is expected to flow in
The HEXAXIAL VIEW and The PRECORDIAL VIEW of the heart, then you can diagnose
which area is effected if it is an abnormal flow...
See the information below.
|Iscehmia=Inverted T waves
|Injury=Elevated ST segment
Making the accurate Field Diagnosis:
Pathological Q waves:
If the Q wave ( the first downward "negative" deflected wave ) is more than 1/3 the size of the R wave ( the first upward deflected "positive" wave ) it is pathological and indicative of an A.M.I.
If no R wave is recorded, then the infarct is extremely acute. There is no electrical activity of the ventricle during polarization and contraction.
A LBBB may result from an acute myocardial infarction (AMI), but field paramedics cannot diagnose AMI in the presence of LBBB. The presence of LBBB negates meaning ful interpretation of other EKG criteria
A LBBB pattern prior to the onset of clinical findings of AMI with marked reduction in voltage of the QRS complex may offer clues to the diagnosis of an infarction.
LBBB obscures the pattern of AMI since the initial QRS vector is abnormally directed in a LBBB pattern. It will obscure the infarction vector and abnormal Q waves will not appear. The most diagnostic feature of AMI is the abnormal direction of the initial 0.04 sec of the QRS vector (ie; the abnormal Q wave).
The last 0.04 seconds of deflection on the QRS complex is used to determine
the direction of the block.
In V1 or MCL1, if the QRS duration is greater that 0.12 seconds (usually 0.14 - 0.20 seconds) and the last 0.04 second segment of the complex is pointing down (negative deflection), the block is LEFT.
If the last 0.04 seconds of the QRS complex is pointing up and is positively deflected, the block is RIGHT.
Certain easily identifiable ECG changes that are observed in the presence of cardiogenic chest pain, reveal some strong presumptive evidence toward the positive diagnosis of AMI. This pattern of changes is referred to as the "evolution of Myocardial Infarction."
It is often suggested that the first observable evolutionary change is the ischemia we associate with T-wave inversion or ST segment depression. Then, onto what is referred as the hyperacute phase. In the hyperacute phase of the MI, (usually the first few minutes) the T-wave may simply increase in height, and/or the ST segment becomes elevated. The finale phase is the acute phase. In the acute phase, (usually the first hour or more) the ST segment elevation is accompanied by the development of a pathological Q wave. This Q-wave confirms the diagnosis of MI.
This evolution is not precise, however. Often times the T-wave may invert in the presence of ST segment elevation during the end of the hyperacute phase. In any event... the most critical observation should be the recognition of ST elevation in 2 or contiguous leads. This is most important to paramedic in the pre-hospital phase because the development of the Q-wave may take hours and could easily be missed in the field.
Eventually, the ST segment will return to its baseline and the T-wave resumes its normal position, leaving only the Q-wave as evidence that an infarction has occurred. Recent research and studies have produced 95% accuracy in field diagnosis by paramedics. Perhaps some reasons would include other indications for ST changes. They would include simple angina, drug effects, and electrolyte imbalance.
Use Lead I, II, and aVF to diagnose Axis Deviation
Vectors and Axis
For pre-hospital purposes, the axis is either "normal" or "not normal."
As stated above. the electrical current should flow to the positive lead. If it does not flow in a positive direction, the heart is pointing toward the upper right or the left. So, if the QRS is negative in aVF, the heart is pointing more to the left than normal; hence, Left Axis Deviation. If the QRS is negative in Lead I, the heart is pointing more to the right than normal; hence, Right Axis Deviation.
This is very complicated and difficult to explain in this forum. If you need info on AXIS deviation or 12 lead diagnosis, please send E-Mail and information will be provided by E-Mail or conventional postage.
By, Mitch Mendler E.M.T. paramedic, San Diego Paramedic.
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