Coronary Artery Anatomy as related to EKG Interpretation

Patients undergoing Percutaneous Transluminal Coronary Angioplasty (PTCA) are at an increased risk of coronary artery occlusion related to clot formation (thrombosis) or plaque rupture, especially in the acute period after procedure. It is imperative that individuals providing care for this patient population are knowledgeable in reading EKG’s and are able to correlate EKG changes with acute coronary artery occlusions and / or spasms.

When a patient presents with chest pain, pressure, tightness, shortness of breath or other related symptoms status post PTCA, a STAT EKG should be ordered. When caring for a patient post PTCA, is important to know which coronary artery was ballooned and/or stented and be able to identify the EKG leads which reflect those coronary arteries.

The best way to learn how the EKG reflects the coronary anatomy is to take a systematic approach.  Think about where the EKG leads are placed on the patient’s chest and visualize the heart within the chest.  This article assumes that the reader already knows how to read an EKG strip and understands what the P, Q, R, S and T represent on the EKG.

The limb leads are I, II and III. Lead I is between the right arm and left arm; Lead II is between the right arm and left leg; Lead III is between the left arm and left leg.

            The precordial leads are V1- V6. V1 is the fourth intercostal space to the right of the septum; V2 is the fourth intercostal space to the left of the sternum; V3 is directly between leads V2 and V4; V4 is the fifth intercostal space at midclavicular line; V5 is at the same level as V4 but at the left anterior axillary line; V6 is level with V5 but is at the midaxillary line.

The right coronary artery (RCA) provides oxygen supply to the inferior portion of the heart. Therefore, it is leads II, III, and aVf that reflect this artery.

            Next let’s look at the anterior and septal leads. These are V1 , V2, V3 and V4. These leads lay directly over the anterior and anterioseptal area of the heart.  The left anterior descending artery (LAD) lies down the front of the heart, providing the septal and anterior walls with oxygen. Leads V1 and V2 reflect the LAD and septum. The LAD and diagonal branches are reflected in the EKG leads V3 and V4.

            The lateral wall is exactly that. The part of the heart that faces the lateral part of the chest wall. This area is reflected in the leads V5 and V6, I and aVL. These leads reflect the circumflex artery.

            Myocardial ischemia and injury are reversible (2). This is due to an increase in demand, or a decrease in supply of oxygen (2).  An acute MI will have the following progression: Tall T waves in the first few minutes; ST elevation or T wave inversion in the first hour(s); possibly pathologic Q waves in the hours after onset; Q waves remain and ST and T waves return to normal in the days following acute myocardial infarction (AMI) (2).

            Acute myocardial infarction (AMI) must be 1 mm in the inferior leads and 2 mm in the anterior leads (2).  There MUST always be a presence of ST elevation in 2 or more contiguous leads (2).

By definition, a myocardial injury has ST segment elevation of at least one mm above the baseline (1). Other signs may be: straightening of the ST segment that slopes up to the peak of the T wave without any time spent at baseline; tall, peaked T waves; or symmetric T wave inversion (1).

            Myocardial ischemia has a classic pattern of T-wave inversion (1). The ST segment may be depressed 0.5 mm or more below baseline, or the ST segment may remain at baseline longer than 0.12 seconds, or from a sharp angle with the upright T wave (1). The EKG may also present tall, wide-based T waves and inverted U waves.

            A myocardial infarction (MI) may present with the development of Q waves on the EKG in the leads reflecting the affected coronary artery (1). Decreased R-wave amplitude, ST segment depression and T-wave inversion are also sometimes noted on the EKG (1).

References

1        Microsoft Power Point – The NEW 12-Lead ECG. Retrieved from www.wccems.com/Files/12_Lead_ECGs.pdf on June 15, 2010.

2        www.ambulancetechnicianstudy.co.uk/ecgbasics.html Ambulance Technician Study : ECG Basics. Retrieved June 15, 2010.