The baseline (resting) electrocardiogram gives many details about the myocardium.  Interpretation requires that five features of the ECG are inspected in this sequence: Rate, Rhythm, Axis, Hypertrophy, Infarction

RATE   The heart rate is expressed in beats (cycles) per minute. If the SA node (pacemaker) fails to function normally, ectopic pacemakers take over the pacesetting activity. Many idionodal pacemakers are scattered within all parts of the heart including the atria, ventricles, and AV node and are normally electrically quiet. **

RHYTHM  In a normal cardiac rhythm, there is a constant distance between similar waves. Ectopic foci occasionally emit an electrical impulse in non-emergency situations, especially in heart disease. Arrhythmias may be broken down into large general categories: varying rhythms, extra beats and skips, rapid rhythm, and heart blocks.

Arrhythmia denotes abnormal rhythm, or breaks in the regularity of a normal rhythm. Impulses which cause atrial contraction originate in the Sino Atrial node... hence the name "Sinus" Rhythm.

Varying Rhythms have a normal sequence (P-QRS-T), but the rhythm changes continuously. They include: Sinus Arrhythmia, Wandering Pacemaker, Atrial Fibrillation

Extra beats are waves appearing earlier than expected, caused by a premature firing of various ectopic foci. These include Premature Atrial and Ventricular Contractions. Skips refer to blank areas of baseline.

Premature Atrial Contraction (PAC): premature stimulation from an atrial ectopic focus produces an abnormal P wave earlier than expected which does not appear like the other P waves in the same lead.

Premature Ventricular Contractions (PVC): originate from an ectopic focus in a ventricle occurring very early in the cycle which does not follow the usual Bundle Branch conduction system. Conduction of ventricular impulses through the myocardium is slow, showing a very wide QRS with tall and deep deflections. PVCs often indicate that the heart's coronary blood supply is inadequate or poorly oxygenated. Poor oxygenation and high CO2 make ventricular ectopic foci discharge frequently. Premature discharges may couple with normal beats in patterns (bigeminy and trigeminy). When a run of more than four PVCs occurs in rapid succession, it is called Ventricular Tachycardia.

The appearance of numerous multifocal PVCs is dangerous and requires rapid treatment. If a PVC falls on a T wave, it occurs during a vulnerable period and dangerous arrhythmias may result.

Rapid Rhythm- Paroxysmal Tachycardia, Atrial Flutter, Ventricular Flutter, Atrial Fibrillation, Ventricular Fibrillation

Ventricular Flutter- a ventricular focus discharging electrical stimuli at a rate of 200-300 per minute and which into deadly arrhythmias. The ventricles do not have time to fill with blood resulting in no effective cardiac output, and the heart itself has no blood supply.

Atrial Fibrillation- many ectopic atrial foci fire at different rates, causing a chaotic, irregular atrial rhythm, with no one discharge carried far. Atrial fibrillation may cause such small shallow spikes that it appears as an irregular baseline without visible P waves.

Ventricular Fibrillation, a type of cardiac arrest, is created by stimuli from many ventricular ectopic foci causing a chaotic twitching of the ventricles. There is no effective cardiac pumping. The other type of cardiac arrest is standstill (or "asystole"), occurring when there is no cardiac activity (flat baseline on EKG).

Heart Blocks (SA Block, AV Block, Bundle Branch Block)

Bundle Branch Block is caused by a block of the impulse of the right or left Bundle Branch. The Right Bundle Branch quickly transmits the stimulus of depolarization to the right ventricle. The Left Bundle Branch does the same to the left ventricle. This stimulus is transmitted to both ventricles at the same time, depolarizing them simultaneously. In Bundle Branch Block one ventricle fires slightly later than the other, causing two "joined QRS's". In Bundle Branch Block the QRS is 3 small squares wide (.12 sec) or greater and two R waves (R and R') are seen.

In RBBB the right ventricle fires late; in LBBB the left ventricle fires late.

In RBBB the left ventricle fires first, so the R' represents delayed activity from the right ventricle. In the LBBB the ventricular impulse is delayed, so the right ventricle depolarizes first and is followed by the depolarization of the left ventricle.

In some individuals a Bundle Branch Block will not become evident until a certain rapid rate ("critical rate") has been reached. With Left BBB, infarct cannot be accurately diagnosed on EKG. Q waves originating from the left ventricle which signify infarction cannot be identified. The sudden appearance of an AV Block or Bundle Branch Block often indicates impending myocardial infarction.

AXIS  gives us clues about the position of the heart in the chest, the presence and location of scarring, and about thickness of the ventricular walls. Axis refers to the direction of depolarization spreading throughout the heart to stimulate the muscle fibers to contract. Depolarization of the ventricles essentially proceeds from the endocardium to the outside surface through the thickness of the ventricular wall in all areas at once. The "Mean QRS Vector" demonstrates that depolarization runs from the AV node and points downward and to the patient's left side.

HYPERTROPHY  in the heart refers to an increase in thickness of the wall. Most of the information concerning hypertrophy of the heart chambers is gained from lead V1.

To examine the atria for signs of hypertrophy, study the P wave in V1 (which lies directly over the atria). With atrial hypertrophy, the P wave is diphasic (both positive and negative), having deflections both above and below the baseline.

To evaluate for Right Ventricular Hypertrophy, study lead V1. Normally, in the QRS complex, the S wave is larger than the R wave. In RVH, there is a large R wave in V1 with a smaller S wave. The large R wave of V1 gets progressively smaller in V2, V3, V4, etc., progressing toward the left chest leads.

In Left Ventricular Hypertrophy, the LV wall is very thick causing great QRS deflections. With LVH there is a large S in V1 and a large R in V2. If depth (in mm) of S in V1 plus the height of R in V5, is greater than 35 mm, this is diagnostic for LVH.

INFARCTION   Coronary occlusion causes myocardial infarction. Only the thick left ventricle suffers myocardial infarction. There is no electrical activity in an area of infarct. The classic triad of an acute myocardial infarction in ISCHEMIA, INJURY and INFARCTION, but any of these three may occur alone.

Ischemia (decreased blood supply) is characterized by symmetrical inverted (upside-down) T waves, which may vary from a slightly flat or depressed wave to deep inversion. Check V1 through V6 for T wave inversion. In the left chest leads (V5 or V6) the T wave often shows wave inversion and asymmetry.

Injury indicates the acuteness of an infarct: ST segment elevation indicates that the infarct is acute (fresh). The ST segment may be elevated as much as ten or more millimeters above the baseline.

NOTE: Pericarditis may elevate the ST segment, however, the T wave is usually elevated off the baseline also. An aneurysm may also cause ST elevation, but it does not return to the baseline with time.

The ST segment may be depressed in certain conditions: digitalis, subendocardial infarction (an infarction that does not involve the full thickness of the left ventricle), or during an exercise stress test in a patient suspected of having coronary ischemia.

The Q wave makes the diagnosis of infarction. Q waves are tiny or absent in most of the leads in the tracing of a normal person. The Q wave is the FIRST DOWNWARD part of the QRS complex and is never preceded by anything in the complex. A significant Q wave is one small square wide (0.04 sec) or one-third the size of the QRS complex.

ANTERIOR WALL INFARCTION: Q waves present in V1, V2, V3, V4. Elevated ST segment indicates acute infarction.
LATERAL WALL INFARCTION: Q waves in leads I and aVL.
INFERIOR INFARCTION: significant Q waves in leads I, III, and aVF.
POSTERIOR INFARCTION: a large R wave in V1, V2, and or V3. ST depression indicates acute infarction.

The diagnosis of infarction by EKG is generally not valid in the presence of Left Bundle Branch Block. Hemiblocks are commonly associated with infarction and a resultant diminished blood supply to the Bundle Branch conduction system. The RCA usually renders blood to the AV node, Bundle of His, and a variable twig to the posterior division of the Left Bundle Branch. The LCA also sends a variable twig of blood supply to the posterior division of the Left Bundle Branch.

MISCELLANEOUS EFFECTS: Pulmonary, Electrolytes, Patterns, and Drugs can all produce changes in electrocardiograms.

**RAPID RATE DETERMINATION: In emergency situations, it is crucial to find the rate rapidly. Find an R-wave which peaks on a heavy line. Next, count off "300, 150, 100" for every heavy line that follows in succession. Count off the next three lines after "300, 150, 100" as "75, 60, 50". The logic behind these unusual rate denominations for the heavy lines follows. The distance between the heavy lines represents 1/300 min. So two 1/300 min. units = 2/300 min. = 1/150 min. (or 150/min. rate) and three 1/300 units = 3/300= 1/100 min. (or 100/min. rate). The number of time units between five consecutive heavy lines is 4. So this represents 4/300 minute or a rate of 75 per minute.

BRADYCARDIA : For rates of less than 60 per minute, there is another fast method of calculation. Since each page of our tracings represents a "6 second strip", simply count the number of complete cycles (R wave to R wave is one cycle) in this strip. Obtain the rate by multiplying the number of cycles in the six-second strip by 10.