The automated working station Medscanner BIORS and the hardware-software complex Bioscanner Wellness make it possible to carry out the ECG contour analysis. This module is intended for finding diagnostically significant points on ECG graph as well as calculation of the cardiogram parameters. The obtained data help to detect different heart diseases.
ECG graph appearance of the normal person depends on their body type, level of training and other factors. However, normal sequence and position of certain waves and segments are always similar. To evaluate ECG it is necessary to compare the values of wave height, duration and deviation of segments to normal values.
To work successfully with the module of contour analysis you should understand the basic principles of the cardiosignal structure. Standard ECG graph includes a lot of repetitive, similar segments, called cardiointervals. Each cardiointerval in turn consists of set of peaks and lows (waves) specific to the different time intervals of cardiac activity.
There are segments, waves and intervals on the ECG graph. A section of a straight line (isoline) between two neighboring waves is called an ECG segment. Cardiogram waves are indicated by Latin letters P, Q, R, S, T, according to the order in which they appear from left to right. There are negative waves (Q or S waves; besides, T and P waves can be negative as well), i. e. below the isoline, or positive waves (T, P, R,), i. e. above the isoline. An interval consists of a wave (wave complex) and a segment. Thus, an interval = a wave + a segment. The most diagnostically significant are the P-Q and S-T segments, the most important intervals are P-Q and Q-T.
The P wave represents an atrium activation. The P wave is registered first. It is a small, spherical, not steep deflection, preceding the serrated QRS complex. It is better to see the atrium state in V1 and V2 leads, because chest leads are located closer to these cardiac compartments than standard leads. The beginning of the P wave corresponds to activation of the right atrium, the middle part represents the end of this process and beginning of the activation of the left atrium. The end of the P wave is generated by the left atrium.
Normal P wave is always positive in I, II, aVF, V2–V6 leads. In III, aVL, V1 leads the P wave can be positive or diphasic (one wave part is positive and another part is negative). In aVR lead the P wave is always negative.
Normal P wave duration does not exceed 0.1 sec, normal amplitude (height) is between 1.5 and 2.5 mm or no more than 0.25 mV (in standard calibration 1 mV = 10 mm). Usually the deviation of these parameters from normal values points to the atrial hypertrophy.
The top of the P wave can be split, while the distance between waves must not exceed 0.02 sec. Time of activation of the right atrium is measured from the beginning of P wave to its first top (no more than 0.04 sec). Time of activation of the left atrium is measured from the beginning of P wave to its second top or to the highest point (no more than 0.06 sec).
In significant damage of the atrial myocardium this wave is usually shortened, elongated and split. In so-called atrial fibrillation, when atria contract very quickly and irregularly, there are irregular isoline waves instead of P wave.
The P-Q interval is a distance (time interval) between the beginning of the P wave and the beginning of the Q wave (or the R wave if the Q wave is absent; in that case it is the P-R interval). It corresponds to conduction time through atria and atrioventricular node to ventricle myocardium. The P-Q interval (P-R) depends on age, body weight, heart rate. It is elongated in AV block and shortened in WPW syndrome.
The normal P-Q interval is between 0.12 and 0.18 sec (up to 0.2 sec), it is 6–9 squares. P-Q interval is elongated with age.
Ratio of wave P duration to segment P-Q duration is called Macruz Index. Normal value of Macruz Index is 1.1-1.6. This index is used in diagnosis of atrial hypertrophy.
QRS complex is a ventricular complex, that is registered during activation of the ventricles. It is the largest complex on the ECG. The complex consists of several spiky waves: positive (above the baseline) and negative (below the baseline).
N point is a transition point from the isoline to the Q wave (the beginning of the QRS complex). J point is a transition point from the S wave to the S-T segment (the end of the QRS complex).
The width of the QRS complex indicates duration of intraventricular activation. Its normal value is 0.06–0.08 sec (up to 0.1 sec). The width of QRS complex decreases slightly with the heart rate increase and vice versa. The shape of the complex can change in ventricular premature beats (ventricular extrasystoles) and other conduction disorders. QRS complex is enlarged in His bundle branch blocks.
The Q wave (initial wave of the QRS complex) is registered during activation of the left side of the ventricular septum. It must be present in chest leads V4, V5, V6. The Q wave is not registered (otherwise it is considered as abnormality) in chest leads V1, V2, V3. Normal width of the Q wave must not exceed 0.03 sec. The Q wave amplitude in each lead must be less than 1/4 of value of amplitude of following R wave in the same lead. The amplitude must not exceed 0.2 mV (with the exception of the standard lead III). Normal Q wave shall not be split.
The R wave (the main wave on the ECG) represents a ventricular activation. The R wave amplitude in standard and unipolar limb leads depends on the position of the electric axis of the heart.
This wave, the same as Q wave, can be registered in all standard and unipolar limb leads. Amplitude increases in leads from V1 to V4: RV4>RV3>RV2>RV1 (while RV1 wave can be absent) and then it decreases in V5 and V6 leads. R wave amplitude in each standard and unipolar limb lead (in adults) must not exceed 2 mV. In I lead its value must not exceed 1.5 mV. In any of chest leads R wave amplitude must not exceed 2.5 mV.
The S wave (inconstant wave) represents a final activation of a base of the left ventricle. It is the deepest negative wave on ECG. It gradually decreases from V1 to V6. It is normal if this wave is absent in V5 and V6 leads. S wave amplitude may vary, but in I, II, aVF leads must not exceed 0.5 mV.
The S-T segment is very important for diagnostics of cardiac disorders. It is necessary to pay special attention to the S-T segment in IHD (ischemic heart disease), because it reflects the oxygen deficit (ischemia) of myocardium. The S-T segment is measured from the J point to the beginning of the T wave. The J point (from a word junction) can be determined on the cardiogram by change in the slope of the vertical curve of the end of QRS complex and its transition to the horizontal part, i. e. the beginning part of the ST segment.
Normal elevation (a deviation above the isoline) of the segment is allowed to be no more than:
0.1 mV in limb leads, 0.3 mV in V1 and V2 leads, 0.2 mV in V5 and V6 leads.
Depression (a deviation below isoline) of S-T segment is allowed to be no more than:
0.05 mV in limb leads.
Deviation of the S-T segment is measured according to the formula J+60 or 80 ms (it depends on pulse rate). Deviation of the S-T segment is considered diagnostically significant if its duration from the J point is between 0.06 and 0.08 sec.
T wave represents a repolarization process (the recovery of resting potential or the resting phase) of the ventricles. It normally begins on isoline after the S-T segment. Normal T wave is not split and its first half has a more gradual slope than the second half. Normal T wave is always positive in leads I, II, aVF, V2-V6, while TI > TIII and TV6 > TV1. T wave is always negative in aVR lead. T wave amplitude (standards have not developed) in standard and unipolar limb leads is usually between 0.3 and 0.6 mV (up to 0.8 mV). T wave duration shall not be less than 1/8 or more than 2/3 of amplitude of the preceding R wave. T wave duration varies from 0.16 to 0.24 sec and has no great diagnostic value.
The Q-T interval is called an electrical systole of the ventricles, because during this time period all ventricular compartments are activated. It is a time interval from the beginning of the QRS complex to the end of the T wave. The Q-T interval duration depends on gender, age, heart rate. Normal QT interval shall not be more than 50% of the preceding R-R interval. It is possible to determine upon the Bazett’s formula if the Q-T interval is normal or abnormal on the ECG of the patient. The Q-T interval is considered abnormal if its value exceeds 0.42:
QTb = QT (measured on ECG) / √(R-R) (an interval between two neighboring R waves, measured on ECG)
The possible cause of the QT interval elongation is hypokalemia (hypocalcaemia), the cause of the QT interval shortening is hyperkalemia (hypercalcaemia).
It is an interval between the end of the T wave and the beginning of the P wave. It represents a cardiac relaxation period (a flat line on the ECG).
Electrical axis of the heart
The standard leads of cardiac impulses from the bode surface register the potential difference between two limbs. The first standard lead is a potential difference between electrodes on the right and left arms. The second standard lead is a potential difference between a left leg and a right arm. The third standard lead is a potential difference between a left leg and a left arm (the negative electrode). These three leads form an equilateral triangle (it is called the Einthoven Triangle) with corners on the limbs on which electrodes are placed. In the middle of the triangle the electrical centre of the heart is located, that is equidistant from every lead.
The electrical axis of the heart is the projection of the resulting vector of ventricular excitation in the frontal plane. Direction of the electrical axis of the heart represents the total value of bioelectrical changes that occur in myocardium with each contraction. The position of the heart axis is only an additional parameter in the diagnosis of diseases.
The deviation of the electrical axis of the heart is determined from the angle alpha and measured in degrees. The angle alpha is formed by the electrical axis of the heart and the horizontal line that is drawn through the conditional electrical center of the heart, i. e. lead I axis, shifted towards the centre of the Einthoven Triangle.
The angle alpha in normal individuals may vary from 0° to +90°, depending on the body type. There are three types of position of the electrical axis of the heart depending on the physique of the person:
- normal position: alpha angle is between +30°and +70°;
- horizontal position: alpha angle is between 0°and +30° (can point to hypersthenic body type);
- vertical position: alpha angle is between +70°and +90° (can point to asthenic body type);
For people older than 40 years, the normal angle of the heart axis is between –30 and +90°, for people younger 40 years it is between 0 and +105°. Condition itself, in which the electrical axis of the heart is deviated, is not a diagnosis. However, such changes on the electrocardiogram can indicate different cardiac disorders. Most often the deviation of the electrical axis of the heart is related to ventricular hypertrophy, but to specify the nature of a heart condition it is necessary to analyse other parameters. For example, the left axis deviation can point to left ventricular hypertrophy or overload. The right axis deviation can point to right ventricular hypertrophy or overload. This condition is a sign of age-old chronic disorder, so usually the emergency medical assistance of cardiologist is not required. However, the deviation of the electrical axis of the heart related to His bundle branch block poses a danger. This situation calls for emergency medical assistance of cardiologist and treatment in a specialized hospital.
Cardiac rhythm analysis
Rhythm regularity is evaluated by R-R intervals. If the waves are situated at equal distances from each other, rhythm is called regular. The duration range of different R-R intervals is allowed no more than ± 10% of their average duration. If rhythm is sinus, it is usually a regular beat rate.
It is a normal rhythm, whereas other are abnormal (i. e. they indicate cardiac disorders). The pacemaker is situated in the sinoatrial node. ECG signs:
- In II standard lead P waves are always positive and situated before each QRS complex,
- P waves in the same lead have a constant same shape.
P wave in sinus rhythm.
If a pacemaker is situated in the lower atrial regions, the cardiac impulse spreads to atria from bottom to top (retrogradely), so:
- P waves in II and III leads are negative,
- there is a P wave before each QRS complex.
Atrial rate is an abnormal heart rhythm. Characteristics of an atrial rate: a pacemaker is situated in the lower atrial regions, the cardiac impulse spreads to atria from bottom to top (retrogradely), so P waves in II and III leads are negative.
P wave in atrial rate.
Rhythm from AV node
If a pacemaker is situated in theatrioventricular node, ventricles activate as usual (from top to bottom), but atria activate retrogradely (i. e. from bottom to top). In that case on the ECG:
- P waves may be absent because they can be buried in normal QRS complexes.
- P waves can be negative and located after each QRS complex.
The rhythm from AV node, P wave is buried in QRS complex.
The rhythm from AV node, P wave is located after QRS complex.
The rhythm from AV node is associated with HR which is slower than sinus rhythm. It is about 40 to 60 beats per minute.
Ventricular or idioventricular rhythm
In that case the pacemaker is a ventricular conduction system. The impulse spreads through the ventricles by abnormal pathways and because of that it spreads slower.
Characteristics of idioventricular rhythm:
- QRS complexes are wide and deformed. Normal QRS complex duration is 0.06–0.10 sec. This rhythm is associated with duration of QRS complex more than 0.12 sec.
- There is no fixed ratio between QRS complexes and P waves, because AV node does not release impulses from ventricles, but atria can be activated from the sinus node, as in normal conditions.
- HR is less than 40 beats per minute.
Idioventricular rhythm. There is no connection between P wave and QRS complex.
The examination procedure
To facilitate analysis it is advisable to enable Flattening and Filter 50 Hz in Settings. It is sufficient to record ECG in three leads.
The accuracy of diagnosis depends heavily on the selected part of the ECG, so you should select the cardiointerval without interference and artifacts.
The program detects control points automatically, but selection by the program can be incorrect due to the complexity of the calculations and unclear shape of the ECG graph.
Because of that the operator should correct the marker position (position of waves) of the specific diagnostic points on the curve by dragging them with the mouse and moving towards the wanted direction.
The accuracy of diagnosis depends heavily on how correct the markers are set. Besides, the contour analysis takes into account the base ECG characteristics only and cannot be considered as a ground for the establishment of clinical diagnosis. In case you suspect a disorder, a cardiologist must read the ECG.