Equipped with a carefully formed clinical impression of the patient, a systematic approach to 12 lead ECGElectrocardiogram; also called an EKG; a representation of electrical voltage measured across the chest over a period of time. 1. Six Second ECG Guidebook (2012), T Barill, p. 196 interpretation makes the 12 lead ECG a reliable assessment tool. Mindful of false positives and false negatives, the patient’s old 12 lead ECG serves as an invaluable reference that greatly increases the likelihood of a correct interpretation.
The systematic ECG interpretation of a 12 lead ECG takes a bit more time than the six seconds necessary to correctly name most single lead rhythmOften refers to a discernible pattern in time or distance between QRS complexes and/or P waves. strips. While many excellent approaches to 12 lead interpretation exist, all methods include certain key points that are incorporated into a four-step system for 12 lead ECG analysis.
1. Begin with the four-step method of ECG interpretation to identify the rhythm.
Using lead II, is the rhythm too fast or too slow? If so, check the patient for poor cardiac outputWhat is it? Why is it Vital? The amount of blood pumped out of the ventricle in a minute (most often refers to the blood pumped by the left ventricle) What is cardiac output? Simply, cardiac output is the amount.... Are the QRSThe electrical representation of ventricular depolarization; the atrial repolarization is also a part of the QRS. ECG interpretation relies heavily on the QRS complex. The QRS complex represents the depolarization of the ventricles. The repolarization of the atria is also... complexes wide or narrow? Check the P wavesA wave generated from the depolarization of the atria; the P wave is upright when originating from the SA node. The P wave represents the depolarization of the right and left atria. The P wave begins with the first deviation.... If the ECG rhythm is irregular, identify the causes of the irregular patternPattern refers to the repeating components of an ECG rhythm in a predicative order. An ECG rhythm that is initiated by one of several intrinsic pacemaker sites of a heart will often generate a regular, even rhythm pattern. This can....
2. Hunt for indicators of cardiac ischemiaThe ability to identify cardiac ischemia, injury and infarction is vital in the management of the majority of cardiac emergencies. Most sudden cardiac deaths are associated with an ischemic episode. Patient deaths due to an acute myocardial infarction (MI) typically... and infarctionThe necrosis of tissue; acute myocardial infarction involves the acute death of myocardial cells. 1. Six Second ECG Guidebook (2012), T Barill, p. 198.
Q – prominent Q waves (25% the height of the R waveFollowing the depolarization of the interventricular septum, ventricular depolarization then progresses from the endocardium through to the epicardium across both ventricles producing an R wave and an S wave. An R wave is the first positive deflection of the QRS... and/or 1 mm in width)
ST segmentA line between waves. Segments measure time between waves (waves are not included in a segment). While waves are fairly self-explanatory, intervals measure time from the start of one wave to the start of another wave (an interval includes at... deviation (of 1 mm or more in two leads with similar lead viewsLocating the positive electrode is crucial to determining which area of the heart is viewed electrically. Metaphorically, the positive electrode serves as a mini-video camera aimed at the heart in the direction of the negative electrode. These leads and their...)
T wave changes such as T wave inversionAn inverted T wave can point to cardiac ischemia among other causes. Ischemia to the epicardium prolongs ventricular repolarization to this area. This extended repolarization of the epicardium removes the delay between the repolarization of the endocardium and the repolarization..., peaked T waves and biphasic T waves
Once you find an ischemic indicator in a single lead view, immediately complete the picture by proceeding to other leads that share the same lead view. For example, if lead I shows ST elevationThe upward placement of the ST segment greater than 1 mm from the isoelectric line; suggestive of cardiac infarction or ischemia. Between the QRS complex and the T wave, lies the ST segment. The ST segment usually follows the isoelectric..., proceed visually to leads aVL, V5 and V6 – all lateral leads.
Distinguish between signs of infarction and reciprocal ECG changes.
3. Determine if a right or left bundle branch blockA bundle branch block must satisfy two criteria - a wide QRS complex and a notch in the QRS complex. To distinguish between a RBBB and a LBBB, first make certain that the rhythm is indeed supraventricular (P waves before... is present.
The presence of a new left bundle branch blockSince the bundle branches are insulated – they are encapsulated with a fibrous sheath – an obstacle to conduction in any bundle (i.e. ischemia or infarct) results in the impulse not carried through to the ventricle; as a result, the... accompanied by symptoms of an acute myocardial infarctionThe necrosis or death of myocardial tissue due to insufficient supply of oxygen to the infarcted region. The ability to identify cardiac ischemia, injury and infarction is vital in the management of the majority of cardiac emergencies. Most sudden cardiac... is diagnostic. Both left and right bundle branch blocks also change the morphology (shape) of the ECG to resemble ischemic changes with and without the presence of cardiac ischemiaInsufficient supply of oxygen to meet the oxygen demands of tissue. Anaerobic metabolism becomes increasingly important during periods of ischemia. Ischemia results from an inadequate blood flow that fails to meet the oxygen demands (energy demands) of tissues. If tissues.... Again, an old ECG is quite useful here.
For example, the symptoms typical of cardiac ischemia are also typical of a long list of medical conditions such as cholecystitis and gastroesophageal reflux disease. Do not be fooled into moving down the cardiac ischemia road to the complete exclusion of other possibilities. Do not allow a chronic left or right bundle branch blockA bundle branch block must satisfy two criteria - a wide QRS complex and a notch in the QRS complex. To distinguish between a RBBB and a LBBB, first make certain that the rhythm is indeed supraventricular (P waves before... together with their accompanying ST elevation or depression move you into a one-dimensional treatment plan. Rule out the presence of bundle branch blocks.
For most clinicians, a generally reliable interpretation can be provided after only
these first three steps.
4. Revisit the waveforms, mapping normal and abnormal findings to regions of the heart in a systematic fashion, all the while evaluating:
P waves, looking for large and notched morphologies (i.e. atrial hypertrophy)
Q waves – normal or abnormal
R and S waves for amplitudeThe height or depth of waves and complexes of an ECG in millimetres; represents millivolts where 10 mm is 1 millivolt with a properly calibrated monitor. 1. Six Second ECG Guidebook (2012), T Barill, p. 190 (also to determine R wave progressionThe phenomenon of R wave progression utilizes the following six leads: V1, V2, V3, V4, V5, V6. The chest leads provide information on the sagittal plane from the anterior surface across to the lateral surface of the heart. Notice in... and electrical axisThe direction of an average (mean) vector is called the electrical axis. The overall direction of ventricular depolarization is called the QRS axis. Many novice practitioners of 12 lead ECG interpretation find identifying the QRS axis challenging and somewhat confusing....)
ST segment deviation (possible ventricular strainWith severe ventricular hypertrophy, the myocardium can thicken to such a degree that the blood supply to the subendocardium (inner lining of the heart just inside the endocardium) can diminish. As a result, the endocardium is particularly susceptible to hypoxia.... or digoxin dipST segment deviation (possible ventricular strain or digoxin dip) Between the QRS complex and the T wave, lies the ST segment. The ST segment usually follows the isoelectric line. The ST segment represents early repolarization of the ventricles. Early repolarization...)
T wave changes for signs of electrolyte imbalance
U wave may suggest electrolyte imbalance
Note the easy to remember PQRSTU progression to complete your descriptive analysis. By completing your 12 lead ECG systematically, more subtle findings are rarely missed.
Only for those with a keen interest in cardiology and a generous volume of 12-lead ECGs, the fourth step can alert you to (but not confirm) potential cardiac disease. This last step, while not diagnostic, can yield valuable clues when combined with a well established history. Further tests must be ordered to confirm these concerns.
The successful application of the first three steps is a solid foundation for the identification of cardiac ischemia and infarction – the most common use of a 12 lead ECG. While being redundant, it again is worth mention: for many, if not most practitioners, the fourth step is unnecessary. Begin to utilize step four only after the first three steps become a well established skill set.
The availability of previous ECGs, a thorough clinical assessment, and an established patient history are all necessary for a definitive 12 lead ECG interpretation. The moment is ideal to put the first three steps of 12 lead ECG interpretation into practice.
1. Six Second ECG GuidebookA Practice Guide to Basic and 12 Lead ECG Interpretation, written by Tracy Barill, 2012 Introduction The ability to correctly interpret an electrocardiogram (ECG), be it a simple six second strip or a 12 lead ECG, is a vital skill... (2012), T Barill, p. 153