MicroEKG Manual

MicroEKG Details

Diagnosis of Q waves

As you read the ECG, you must decide which Q waves indicate pathology, and which Qs are normal. For example in lead I, a Q less than 1/4 of the R height, and less than one box wide, is considered normal.

“Normal” Qs indicate activation of the intraventricular septum. They will therefore appear in the leads that “look left” — the rightward electrical activity across the septum will cause a negative deflection in these leads.

“Septal Qs” are normal in I, F, V5 and V6 (left or lateral leads). Small Qs are also generally innocent in lead III and lead V1 if no other abnormality is seen.

mi_norm.gif (5546 bytes)
EKG showing normal Qs in I, L, V5 and V6

Q waves are “significant” if they are greater than 1 box in width (longer than 0.04 msec) OR are larger than 1/4 of the R wave. Significant Q waves indicate either myocardial infarction or obstructive septal hypertrophy (IHSS).

A Q wave in lead III alone is not diagnostic of infarction, even if it is otherwise “significant” in size and width. Qs in III are ignored unless other abnormalities are seen.

In transmural myocardial infarction, significant Q waves (1 box wide or 1/4 the R) appear in the leads “looking at” the area of infarction: II, III, and F for inferior infarction; I, L, and V5-V6 for lateral; and V2-V4 for anterior.

mi_inf_o.gif (5380 bytes)
Q waves of old infarction in II, III, and F

Q waves that occur in the setting of LBBB or LVH are less reliable for diagnosis of myocardial infarction.

The presence of ST or T wave abnormality in the same lead(s) as “borderline” Qs makes these Qs suspicious for infarction.
In idiopathic hypertrophic subaortic stenosis (IHSS) the Q waves tend to appear in the same leads in which normal “septal” Qs are seen — because the pathology is thickening of the septum.

These “significant” Qs of IHSS are almost always accompanied by evidence of marked left ventricular hypertrophy.

Q Waves
Causes: Septal, Infarction, IHSS
Septal: I, L, V5-V6, occasionally inferior leads
Significant: Q > 1/4 R, or
                 Q > 1 box wide
                 and NOT in lead III
IHSS: increased “septal” Qs, evidence of LVH

Go to Chapter 7, ST and T wave abnormalities

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All material referenced through this menu is excerpted from copyrighted works by Bruce Argyle, MD. You are welcome to use selected portions, as long as appropriate credit is given. The credit for the text referenced through this menu is:

Argyle, B., MicroEKG Computer Program Manual.
Mad Scientist Software, Alpine, Utah

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