Necrotic Q-wave

ECG1A: Sinus rhythm, normal PR interval; deep Q-wave and poor R wave progression in the lower territory (leads II, III and aVF) and in the anterior territory (from V1 to V6);

 Comments: This patient presents an electrocardiogram characteristic of severe ischemic cardiomyopathy with prior large infarction (inferior and anterior territories) and highly altered left ventricular ejection fraction (< 25%).

A Q-wave corresponds to the first negative deflection of the QRS-complex not preceded by a positive wave. It is deemed a Q-wave when it is wide and/or deep, a q-wave when it is narrow and shallow, a qR complex when the Q-wave is less tall than the R wave and a Qr complex in the opposite case. In an acute coronary syndrome setting, the Q-wave reflects the presence of necrosis resulting from myocardial anoxia sufficiently prolonged to have abolished all electrical activity in a large muscle sector. During a coronary occlusion, the onset delay of a Q-wave varies according to the patient (from 6 to 9 hours on average but can occur earlier after 2 hours and even after a few minutes) depending on the presence or absence of a collateral network. The findings on the electrocardiogram of very early Q-waves during an episode of acute coronary syndrome should not be considered as a marker indicating that it is too late to propose revascularization, the Q-waves being able to regress if reperfusion is proposed in a timely manner. The presence of a Q-wave is, however, a factor of poor prognosis since it reflects a significantly large infarction with increased intra-hospital mortality.

Various explanations have been proposed to explain the presence of Q-waves following myocardial infarction. The necrosis Q-wave is the result of the depolarization of the healthy myocardium and the electrical silence of the necrotic zone. The necrotic wall behaves like an open window, an electric hole, which allows recording the normal activation vector of the opposite wall with a directed depolarization of the endocardium toward the epicardium which moves away from the exploratory electrode. The initial negative deflection (QR pattern) or exclusive deflection (QS pattern) therefore corresponds to the positive depolarization wave that appears on the opposite myocardial region. Conversely, an abnormal R wave can be observed when the electrode is located opposite of the infarcted area.

We can thus find

1. a so-called central image, with a QS pattern at the level of the electrodes facing the necrotic wall,
2. a so-called marginal image, with a QR pattern, at the edge of the infarction, the amplitude of the Q-wave decreasing as the distance from the necrotized area decreases
3. an indirect or reciprocal image recorded by the electrodes located opposite to the infarcted wall with recording of an R wave of abnormal amplitude and width in the lead concerned. The loss of the vectors of the infarcted zone alters the equilibrium of the electrical forces between the various walls and results in an increase in the amplitude of the initial vectors of the opposite wall (increase in the R wave amplitude). This type of pattern is found in the setting of a posterior infarction with amplitude of the R waves increased in V1-V2.

The highlighting of a Q-wave can be 1) physiological: Q-waves that are narrow (< 30 ms) and shallow (< 1/4 of the ensuing R wave) in the lateral leads corresponding to the normal first septal depolarization from left to right; 2) positional: an isolated Q-wave in lead III and disappearing at deep inspiration can be observed in the case of a horizontalization of the heart in obese patients, in sitting position or in pregnant women; 3) pathological: a Q-wave can correspond to a sequela but can also be observed in various diseases.

A wide and deep Q-wave in at least two contiguous leads should evoke a developing infarction or remnant of necrosis, especially since it is associated with a ST-segment deviation or ischemic inverted T-wave. The necrosis Q-wave is therefore differentiated from a physiological Q-wave by: a) a longer duration (> 40 ms); b) a depth which is a function of the depth extension of the infarction with a Q-wave greater than 5 mm (if a QR pattern) or a Q-wave exceeding 25% of the R wave amplitude (if a qR pattern); at maximum, the R wave disappears (QS pattern); c) a slurred or notched morphology in particular on its initial descending branch; d) the pathological value of a Q-wave is reinforced by the anatomical correspondence of a coronary territory and its presence on several contiguous leads (leads I, aVL, V5, V6; leads II, III, aVF; V4 to V6); e) the persistence of a repolarization disorder (lesion current and/or negative T-waves) increases the probability that the Q-wave corresponds to a sequela of necrosis.

Non-physiological Q-waves can be observed in other diseases:

1. in hypertrophic cardiomyopathy, it is common to observe narrow Q-waves, often deep, associated with an increase in R wave voltage (positive Sokolow) and to T-waves that may be positive or negative and peaked (apical hypertrophic cardiomyopathy);
2. in the presence of a pre-excitation (Wolff-Parkinson-White syndrome), the delta wave can, according to its orientation, simulate necrosis Q-waves;
3. in a left bundle branch block, a QS pattern is common in V1-V2;
4. in a left hemibloc, a Q-wave can be observed in leads I, aVL (anterior fascicular block) or in lead III (posterior fascicular block);
5. a narrow Q-wave preceding a large R wave in V1, V2 is indicative of a right ventricular hypertrophy;
6. in the acute phase of pulmonary embolism, a fleeting S1Q3 (Q-wave in lead III) pattern can be observed;
7. in a left pneumothorax, the heart can be pushed to the right with absence of anterior R wave progression simulating a lateral infarction;
8. in certain infiltrative diseases such as amyloidosis, the presence of Q-waves with poor R wave progression reflects the presence of advanced myocardial disease.

In a normal electrocardiogram, we find an rS aspect in V1 with progressive growth of the R wave from V1 to V6 and a decrease in the amplitude of the S wave. There is no precise definition of the « poor progression » of the R waves although the absence of progression of the amplitude of the R wave in the precordium (small-sized r in V4) can evoke a previous sequela of necrosis. The poor R wave progression is weakly specific and can be observed if the electrodes are poorly positioned in the presence of dextrocardia, right bundle branch block, emphysema, dilated or infiltrative cardiomyopathy and certain accessory pathways.

Take-home message: A wide and deep Q-wave in at least two contiguous leads should evoke an infarction in the process of development or a sequela of necrosis especially since it is associated with a ST-segment deviation or an ischemic inverted T-wave.