CARDIAC MONITORING

1. CARDIAC MONITORING

2. Electrocardiography (EKG) and phonocardiography are the two most important techniques for observing the condition of the heart and its associated arteries. • Since the electrical and the acoustic signals are generated by the same events, it makes sense to discuss both of them at the same time.

3. Plots of the action of the heart, the associated heart sounds, and the accompanying ECG signals.

4. The P wave is associated with atrial contraction. • The QRS complex signals the atrial repolarization and ventricular contraction sequence. • The T wave is generated by the repolarization of the ventricles. • The U wave is not completely understood, but it may be associated with some after-potential phenomenon.

5. The R-wave frequency is normally taken as a measure of the heart rate. • The P-wave frequency should equal that of the R waves, and the P-to-R interval, which is normally about 0.12 to 0.22 second should not vary from beat to beat.

6. The appearance of ectopic R waves or missing R waves is usually taken as a serious sign.

7. The measurement and recording of acoustic signals associated with the action of the heart is known as phonocardiography. • Listening to the sounds of various phenomena inside the body is one of the oldest medical arts. • Today, the hearing of the nurse or physician may be aided by various electrical and electronic devices, and a graphic record can be obtained of the sounds heard on monitoring the heart.

8. PHONOCARDIOGRAPHIC TECHNIQUES • The acoustic signals that accompany the action of the heart can be detected with either a stethoscope or a microphone. • The use of a microphone provides some significant advantages in that the signals can be

9. An electrocardiogram is a recording of the rhythmic electrical activity of the heart. • The abbreviation for electrocardiogram (EKG) is derived historically from the German spelling "electrokardiogram".

10. The electrical activity of the heart is based on the ability of excitable tissue, such as heart muscle (myocardium), to change its membrane permeability to sodium (Na+) and potassium ions (K+). • When these ions move across the cell membrane, a changing electric field (dipole) results which is recorded as electrical activity of the heart.

11. Metal electrodes in contact with the skin surface are used to pick up weak EKG signals that are amplified and displayed by an oscilloscope and/or Strip Chart Recorder. • Cardiac muscle goes from relaxed (diastole) to the contractile (systole) state after the onset of "electrical depolarization". • The return from contraction to relaxation occurs after "electrical repolarization".

12. The normal heart rhythm is established by a specialized bundle of cells called the "pacemaker" or the sinoatrial (SA) node of the heart. • Electrical impulses are generated spontaneously by the pacemaker, initiating the heart cycle.

13. At the onset of the heart cycle, impulses from the SA node induce the right atrium to depolarize. • This depolarization spreads across the atrial muscle causing atrial contraction, increasing atrial pressure and forcing blood into the ventricles.

14. The ventricular contraction phase of the heart cycle is brought about by depolarization of the ventricles via the atrioventricular (AV) node, Bundle of His and the Purkinje fibers. • The AV node provides a delay time allowing the atria to pump blood into the ventricles before ventricular contraction.

15. This Bundle of His and Purkinje fibers permit the ventricles to be depolarized in a relatively short time. • If the heart relied on impulse conduction through ventricular musculature, total contraction would not occur in a short interval due to long conduction delay of muscle. • Impulse velocity through the conduction system is much faster than through the cardiac muscle itself.

16. The heart goes through a periodic sequence of electrical depolarizations and repolarizations that initiate the mechanical events of pumping blood. • Mechanically the heart cycle can be divided into two phases, diastole and systole. • During diastole the atria contract, emptying blood into the ventricles. • During systole the ventricles contract.

17. THE STANDARD ELECTROCARDIOGRAPHIC LEADS

18. A complete EKG consists of 12 tracings, • The standard three leads (I, II, and III), • The precordial or chest lead (V1 - V6) and • The augmented unipolar limb leads (aVR, aVL and aVF).

19. To record the electrical activity of the heart, the use of three standard leads has long been routine. • Electrodes are placed on the right arm, left arm and left leg, and voltage differences constitute the three standard leads as shown.

20. Einthoven Triangle • In summary, the three voltages I, II, and III are measured on the body as: • I—The voltage drop from left arm to right arm. • Il—The voltage drop from left leg to right arm. • Ill—The drop from left leg to left arm. • Anatomically, these points form a triangle on the body, known as the Einthoven triangle.

21. The voltages may be represented by vectors, having the tail on the negative pole of the potential and the arrowhead on the positive pole of the potential.

22. Thus, in clinical practice, the three voltages are represented as vectors, which are called frontal-plane vectors and illustrated.

23. The algebraic relationship between these voltages comes from circuit theory applied to the human thorax. • It is the fundamental law of voltage drops that the voltage drop between two points is the same regardless of the path traveled between those two points. • This is known as Kirchhoff’s law of voltages.

24. LEAD I • In recording limb lead I, • The negative (-) terminal of the electrocardiograph is connected to the right arm (RA), • The positive terminal (+) to the left arm (LA), and • The ground (G) to the left leg (LL).

25. Therefore, when the point on the chest where the right arm connects to the chest is electronegative with respect to the point where the left arm connects, the electrocardiograph records positively (i.e. above the zero voltage line in the electrocardiogram). • When the opposite is true, the electrocardiograph records below the line.

26. LEAD II • In recording limb lead II, • The negative terminal of the electro-cardiograph is connected to the right arm (RA), • The positive terminal to the left leg (LL), and • Ground to the right leg (RL). • Thus, when the right arm is negative with respect to the left leg, the electrocardiograph records positively.

27. LEAD III • In recording limb lead III, • The negative terminal of the electro-cardiograph is connected to the left arm • The positive terminal to the left leg and • Ground to the right leg. • This means that the electrocardiograph records positively when the left arm is negative with respect to the left leg.

28. These three leads give electro-cardiograms that show the same component waves, but the amplitude (height) and direction of the waves are different

29. Kirchhoff’s law applied to the figure implies that the voltage drop as one travels from the left arm to the right arm equals the drops measured as one travels from the left arm to the left leg and then to the right arm.

30. In equation form, this implies that • The minus sign appears because III is a negative drop, in accordance with the polarities assigned in the figure. • The three voltages as arranged on the figure have traditionally been called Einthoven’s tnangle, in honor of Willem Einthoven, the physiologist and inventor, who studied ECG voltages in 1903.

31. The equation means that only two leads are needed to gather all of the information available to the three leads. • This follows from the fact that, the voltage on any one of the leads can be calculated from the other two. • In other words, one of the leads is redundant.

32. This is not wasteful, though, because if one of the leads is poorly connected, the information will still be available for diagnosis. • This is especially important in ECG units that do diagnosis automatically.

33. CHEST LEADS (PRECORDIAL LEADS)

34. Often electrocardiograms are recorded with one electrode placed on the anterior aspect (front) of the chest over the heart.

35. This electrode (exploring) is connected to the positive terminal and the negative electrode (i.e. indifferent electrode) is normally connected simultaneously through electrical resistances to the right arm, left arm, and left leg.

36. Usually six different standard chest leads are recorded from the anterior chest wall, the chest electrode being placed respectively at the six points.

37. The sites of the six possible precordial leads, are as follows: • V1—Fourth intercostal space, on the right sternal margin. • V2—Fourth intercostal space, on the left sternal margin. • V3—Midway between V2 and V4. • V4—Fifth intercostal space on the midclavicular line (MCL). • V5—Fifth intercostal space on the anterior axillary line. • V6—Fifth intercostal space on the midaxillary line.

38. The different leads recorded by the method are known as leads V1, V2, V3, V4, V5, and V6. • Because the heart surfaces are close to the chest wall, each chest lead records mainly the electrical potential of the cardiac musculature immediately beneath the electrode. • Therefore, relatively minute abnormalities in the ventricles, particularly in the anterior ventricular wall, frequently cause marked changes in the electrocardiograms recorded from chest leads.

39. In leads V1 and V2, the QRS recordings of the normal heart are mainly negative because the chest electrode in these leads is nearer the base of the heart than the apex, which is the direction of electronegativity during most of the ventricular depolarization process. • On the other hand, the QRS complexes in leads V4, V5, and V6 are mainly positive because the chest electrode in these leads is near the apex, which is the direction of electropositivity during depolarization.

40. AUGMENTED UNIPOLAR LIMB LEADS

41. Another system of leads in wide use is the "augmented unipolar limb lead". • In this type of recording, two of the limbs are connected through electrical resistance to the negative terminal of the electrocardiograph while the third limb is connected to the positive terminal.

42. When the positive terminal is on the right arm, • The lead is known as the aVR lead;

43. When on the left arm, as the aVL lead;

44. When on the left leg, as the aVF lead.

45. Normal recordings of the augmented unipolar limb leads are similar to the standard limb lead recordings except that the aVR lead is inverted. • The reason for this inversion is that the polarity of the electrocardiograph in this instance is connected backward to the major direction of current flow in the heart during the cardiac cycle.

46. Each augmented unipolar limb lead records the voltage of the heart on the side nearest to the respective limb. • Thus, when the recording in the aVR lead is negative, the side of the heart nearest to the right arm is negative in relation to the remainder of the heart.

47. Generally speaking, the P wave results from electrical currents generated as the atria depolarize prior to contraction, and the QRS complex is caused by currents generated when the ventricles depolarize prior to contraction.

48. Therefore, both P wave and the components of the QRS complex are DEPOLARIZATION WAVES.

49. The T wave is caused by currents generated as the ventricles recover from the state of depolarization. • This process occurs in the ventricular muscle about 0.25 sec after depolarization, and this wave is known as a REPOLARIZATION WAVE. • Thus, the electrocardiogram consists of depolarization and repolarization waves.

50. The P Wave. --The P wave represents the spread of excitation and contraction of atrial tissue of both atria. • It is normally upright, of amplitude about 0.2 mV (between 0.1 - 0.3 mV) and lasts about 0.1 sec.