Ebook Clinical examinations in cardiology: Part 2

(BQ) Part 2 book Clinical examinations in cardiology presents the following contents: Basic investigations - Clinical electrocardiogram, basic investigations - Radiology of the heart and great vessels.

BASIC INVESTIGATIONS

6A CLINICAL ELECTROCARDIOGRAPHY

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■ ■ ■ C HAPTER 22

Table 22.1 History of development of ECG

1 Animal electricity Bancroft (1769), Walsh (1773)

and Galvani (1780)

2 Electric current accompanies each Matteucci (1842), Koelliker and

3 Capillary electrometer Gabriel Lippmann (1872)

5 Attempt to record ECG in humans Alexander Muirhead (1869)

6 Recorded first human ECG Augustus D Waller (1887)

7 Naming of deflections in ECG as “PQRST” Willem Einthoven (1895)

a) Electrophysiology of the Heart 461

b) Electrocardiographic Instrument,

Recording Electrodes and Lead 466

c) Precautions to be Taken for

● The existence of electricity in the animal (“animal electricity”) was first suggested byEdward Bancroft (1769, in torpedo fish)1and substantiated by John Walsh (1773 ineel)2and Luigi Galvani (1780 in dissected frog) (see Table 22.1)

● However; it was in 1842, when Carlo Matteucci, Professor of Physics at the University

of Pisa first showed that an electric current accompanies each heart beat in a dissectedfrog,3 which was later confirmed by Rudolph von Koelliker and Heinrich Muller

in 1856.4

● First successful attempt to record an ECG in humans was made by Alexander Muirhead,

a Telegraph engineer in 1869 at St Bartholomew’s hospital, London, using ThomsonSiphon recorder

● French Physicist Gabriel Lippmann invented (1872) capillary electrometer, a glass tubewith a column of mercury beneath the sulphuric acid, for which he was awarded

a Nobel Prize in 1908

● British Physiologists John Burden Sanderson and Frederick Page in 1878 first recordedthe heart’s electric current with a capillary electrometer which consisted of two phasesfrom the ventricle of a frog.5

● But it was in 1887 when Augutus D Waller, British Physiologist, at St Mary’s Medicalschool, London, recorded the first human ECG with a Lippmann’s capillary elec-trometer and labeled the deflections as ‘V1’, ‘V2’ and the third wave which was laterdiscovered as ‘A’.6

● After witnessing the Waller’s demonstration in 1889, the Dutch PhysiologistWillem Einthoven recorded ECG with an improved Lippmann’s electrometer

in 1895 and named the deflections as “ABCD” and with a correction formula

as “PQRST”7 as per the mathematical convention derived from the FrenchPhilosopher Descartes points on the curves (1662).8 His invention of string gal-vanometer later in 1901 provided a reliable and direct method for recording ECG,and by 1910 the string galvanometer emerged from the research laboratory in theclinic.9

● Subsequent improvement of the instrument and better understanding of the ECGresulted in the wide use of ECG and has become an invaluable clinical tool for thedetection and diagnosis of a broad range of cardiac conditions

● Though at present not a sine qua non for the diagnosis of the heart diseases, it tinues even 100 yrs after its inception to be the most commonly used cardiologictest

con-– For the diagnosis of the cause of chest pain– As a reliable tool for the diagnosis of acute myocardial infarction and dictates thetimely administration of life saving thrombolytic therapy

– For the diagnosis and the management of cardiac arrhythmias– Can help with the diagnosis of the cause of breathlessness– For the diagnosis of pericarditis and

– For assessing the electrolyte disorders, drug effects and toxicity (see Table 22.2)

A patient with an organic heart disorder may have a normal ECG and a perfectly normal individual may show non-specific ECG abnormalities Hence, a patientshould not be given an unwarranted assurance of the absence of heart disease solely onthe basis of a normal ECG

Table 22.2 Utility of ECG in the current era

1 Chest pain 1 Acute myocardial infarction 1 Electrolyte disorders

2 Acute myocardial infarction 2 Cardiac arrhythmias 2 Drug effects and toxicity

3 Pericarditis

4 Cardiac arrhythmias

BASIC CONCEPTS

The basic concepts are described as follows:

● Electrophysiology of the heart

● Electrocardiographic instruments, recording electrodes and leads

● Precautions to be taken for recording an ECG

a) Electrophysiology of the Heart

1) The Conduction System of the Heart

See Part-1: Basic anatomy and physiology: chapter 6

2) The Contractile or Working Myocardial Cell

See Part-1: Basic anatomy and physiology, chapter 7:

a) Sarcolemma—see p57 b) Intercalated discs—see p58 c) Sarcotubular system—see p59 d) Diadic cleft—see p60

e) Contractile proteins—see p61

3) Electrical Activity of the Heart

a) Properties of the transmembrane potentials: see Part-1: Basic anatomy and

physi-ology, chapter 8

b) Recording of the electrical potentials (electrogram) produced by the normal cardiac cell:

(i) Resting cell: In a resting cardiac muscle cell, molecules dissociate into positively

charged ions on the outer surface and negatively charged ions on the inner surface ofthe cell membrane, and the cell is in an electrically balanced or polarized resting state

If an electrode is placed on the surface of the resting cell, no deflection is recorded bythe galvanometer as entire cell surface has zero potential due to high impedance of thecell membrane (see Fig 22.1)

(ii) Depolarization: When the cell is stimulated (S) by an excitatory electrical wave,

the negative ions migrate to the outer surface of the cell and positively charged ionspass into the cell, this reversal of polarity is called depolarization

● If an electrode is placed so that the depolarization wave flows toward the electrode,

a galvanometer will record a positive or an upward deflection

● When a depolarization current is directed away from an electrode, a negative ordownward deflection is recorded

● If an electrode (E) overlies the mid portion of the cell (muscle strip), the deflectionwill be diphasic The initial deflection is upward due to an advancing positive charge,while the second deflection is downward due to the effect of passing negative charge(see Fig 22.2)

● If two cells (muscle strips) of approximately equal size are stimulated at a central point,

a positive of equal magnitude is recorded at either end (see Fig 22.3)

● If two cells (muscle strips) of different sizes (e.g RV and LV) are stimulated at a centralpoint, a large positive deflection is recorded over the large cell (muscle mass) and a smallpositive deflection followed by a deep negative deflection or entirely negative deflection

is recorded over the smaller cell surface (muscle mass) (see Fig 22.4)

Current flow towards the electrode Current flow away from the electrode

Electrode overlying the mid portion of a cell

Fig 22.2 | Depolarization wave in a single cell E: electrode, S: stimulation.

E E

S





Two muscle strips of equal size

Fig 22.3 | Depolarization wave in two cells of equal size E: electrode, S: stimulation.





Fig 22.1 | Electrical activity in resting cell and effect of depolarization.

(iii) Repolarization: During recovery period, positively charged ions return to the

outer surface and negatively charged ions move into the cell The electrical balance of thecell is restored; this process of return of the stimulated cell to the resting state is known

Depolarization towards the electrode Repolarization in the opposite direction

Fig 22.5 | Repolarization in the opposite direction of depolarization E: electrode, S: stimulation.

Depolarization towards the electrode Repolarization in the same direction

Fig 22.6 | Repolarization in the same direction of depolarization E: electrode, S: stimulation.

● The transfer of the Naand K ions across the cell membrane plays an importantrole in generating cardiac electrical activity.

● Intracellular concentration of K is 30 times greater than extracellular K Naconcentration is 30 times less inside the cell as compared to outside

● Because of this ionic composition, membrane of the resting cardiac fiber is in anelectrically balanced or polarized state

d) Origin and sequence of cardiac activation: see Part-1: Basic anatomy and physiology, chapter 8.

e) Phases of cardiac action potential: see Part-1: Basic anatomy and physiology, chapter 8.

f) The modifying transmission factors: These factors affect transmission of electrical activity of the heart throughout the body and are broadly grouped into four categories: (i) Cellular factors determine the intensity of the current flow They include:

● Intracellular and extracellular resistance

● Intracellular and extracellular ions: Lower ion concentrations reduce the intensity

of the current flow by reducing the movement of the ions and by lowering the cellular potentials

extra-(ii) Cardiac factors affect the transmission of current from one cardiac cell to another.

These include:

● Anisotropy: It is the property of the cardiac tissue to propagate more rapidly along the

length of the fiber than transversely Hence, the recording electrodes oriented alongthe long axis of a cardiac fiber register larger potentials than the electrodes orientedperpendicular to the long axis

● Connective tissue between the cardiac fibers: It disrupts the effective electrical

cou-pling between adjacent fibers The waveforms recorded from fibers with little or nointervening connective tissue are narrow and smooth in contour, whereas those re-corded from the fibers with abundant connective tissue (fibrosis) are prolonged andfractionated.10

(iii) Extracardiac factors include all tissues and structures that lie between the region

of cardiac electrical activity and the body surface These tissues alter the electrical ity due to differences in electrical resistances of the adjacent tissues i.e electrical inho-mogeneities within the torso e.g intracardiac blood has lower resistance of 162-cmthan the lungs (2150-cm)

(iv) Physical factors which affect the electrical activity are:

● The distance between the heart and recording electrode is governed by ‘inverse square

law’ i.e amplitude of the electrical potential decreases in proportion to the square of

the distance All electrodes placed at a distance 15 cm from the heart may be sidered to be equidistant from the heart in electrical sense as the amplitude of theelectrical potentials recorded will be the same in all electrodes

con-● Eccentric location of the heart i.e the heart is located eccentrically more anteriorly

so that the RV and anteroseptal portion of the LV are located closer to the anterior

of the chest than other parts of the LV and atria Hence, the ECG potentials andwave forms generated by the anterior regions of the heart are higher and greater thanthose generated by the posterior ventricular regions and atria

As a result of all these factors, body surface potentials have amplitude of only 1% of theamplitude of transmembrane potentials (see Table 22.3 and Fig 22.7)

Eccentric location

of the heart

Connective tissue between heart and recording electrode

Cardiac action potential Cellular factors

1 Lungs

2 Skeletal muscles

Fig 22.7 | Factors modifying cardiac action potential.

Table 22.3 Factors modifying transmission of action potential Modifying factors

1 Intracellular and extracellular resistance

2 Intracellular and extracellular ions

3 Anisotrophy

4 Connective tissue between the cardiac fibers

5 Electrical in-homogeneities within the torso

6 Distance between the heart and recording electrode

7 Eccentric location of the heart

b) Electrocardiographic Instrument, Recording Electrodes and Lead

1) Electrocardiographic Instrument

The two main types of apparatus used are:

● String galvanometer

● Radio amplifier

String galvanometer records on the photographic paper which has to be developed.

It requires experience to operate so as not to damage the valuable string

Radio amplifier is compact, light, easy to operate and has a direct writer Many

mod-ern machines record multiple leads simultaneously Other methods utilized clinicallyare (see Table 22.4):

● Oscilloscopic viewing of the ECG i.e ECG monitoring in coronary and intensive careunits This produces a constant ECG on a fluorescent screen with a facility to obtainthe ECG tracing

● Ambulatory ECG recording: A small ECG tape recorder is attached to the patientfor continuous recordings for 24 hrs while the patient is ambulatory which can bereviewed later by the attending physician for arrhythmias or myocardial ischemia

● Telemetry ECG: ECGs can be transmitted via telephone lines, for constant or porary monitoring and interpretation by a physician many miles away from the patient

tem-● Computer facilities are available not only for ECG interpretation but also for therecognition and quantitation of arrhythmias

2) Recording Electrodes and Leads

These are as follows:

● Bipolar standard or limb leads

● Bipolar chest leads

● Unipolar augmented limb leads

● Unipolar precordial or chest leads

● Monitor leads

● Unipolar esophageal leads

● The Mason-Liker modified standard leads

● Unipolar intracardiac leads

The standard clinical ECG consists of 12 leads: 3 bipolar limb leads (I, II, III), 3 ted limb leads (aVR, aVL, aVF) and 6 unipolar precordial leads (V1–V6) The bipolar

augmen-Table 22.4 Methods of ECG recording Methods

1 Standard method

2 ECG monitoring

3 Ambulatory ECG

4 Telemetry

II I

II



III



Fig 22.8 | Bipolar standard leads (I, II, III).

and augmented limb leads are oriented in the frontal or coronal plane of the body,while the precordial leads are oriented in horizontal or transverse plane of the body

(1) Bipolar standard or limb leads: These were introduced by Einthoven.11Theseleads record the potential difference between the two limbs and consists of three leads:

I, II and III with four electrodes: LA (left arm electrode), RA (right arm electrode), LL(left leg electrode), and RL (right leg electrode) which serves as a ground connection.The electrodes are usually placed just above the wrists and ankles or to the stump in theamputated limb

● Lead I represents the potential difference between LA (positive electrode) and RA

(negative electrode) (LA-RA) This lead with aVL is oriented to the left lateral wall

● Lead II represents the potential difference between LL (positive electrode) and RA

(negative electrode) (LL-RA)

● Lead III represents the potential difference between LL (positive electrode) and LA

(negative electrode) (LL-LA) Leads II and III with aVF are oriented to the inferiorsurface of the heart (see Fig 22.8)

The relation between these three leads is expressed algebraically by Einthoven’s equation

or equilateral traiangle: 11

Lead II  Lead I  Lead III i.e electrical potential recorded in Lead II equals the

sum of electrical potentials recorded in Leads I and III

This equation is based on Kirchhoff ’s Law i.e the algebraic sum of all potential

differences in a closed circuit equals zero Hence I II  III 0 or II I  III

(2) Bipolar chest leads: Presently, a special bipolar chest lead (Lewis lead) is times used to amplify the atrial activity and thereby to clarify the mechanism of anatrial arrhythmia

some-The RA electrode is placed in the 2ndintercostal (IC) space to right of the sternum,the LA electrode is placed in the 4thIC space to right of the sternum, and tracing isrecorded on lead I

(3) Unipolar augmented limb leads: The unipolar leads (limb leads: VR, VL, VF;multiple chest leads: V and esophageal leads E) were introduced by Wilson.12

● The unipolar leads represent the potentials in a given lead and not the differences inpotentials as in bipolar leads

● The unipolar lead system consists of a Wilson’s central terminal or indifferent lead and

an exploring lead The central terminal is formed by joining electrodes (RA, LA, andLL) together to 5000 resistor which is attached to negative terminal of the machine

● In unipolar limb leads, the central terminal is connected to the RA electrode of themachine (which acts as negative terminal) and exploring lead (positive terminal) isconnected to the LA electrode of the machine and the tracing is recorded on lead I.Although technically this system has two electrodes i.e bipolar leads, it represents aunipolar lead since one of the potentials is zero (central terminal has zero potential)

● At present, only augmented limb leads (aVR, aVL and aVF, introduced by EmanuelGoldberger in 1942) are in vogue as the amplitude of the deflections is 50% morethan the non-augmented leads (VR, VL and VF)

● To record aVR (a augmented, VR vector of right arm), the LA electrode (exploringlead) of the machine is placed on the right arm, while RA electrode of the machine(indifferent lead/central terminal through 5000 resistor) is placed on the left armand left leg This lead is oriented to the cavity of the heart and hence all deflections(P, QRS, and T) are normally negative

● To record aVL (a augmented, VL vector of left arm), the LA electrode of themachine is placed on the left arm, while RA electrode of the machine (indifferentlead through 5000 resistor) is placed on the right arm and left leg This lead is ori-ented to the anterolateral or superior surface of LV

● To record aVF (a augmented, VF vector for left leg), the LA electrode of themachine is placed on the left leg, while the RA electrode (indifferent lead through

5000 resistor) of the machine is placed on the right arm and left arms This lead

is oriented to inferior surface of the heart (see Fig 22.9)

The unipolar limb leads bear a definite mathematical relationship to the standard bipolarleads This relationship is derived from Einthoven’s formula: VR VL  VF 0

I 2/3 (aVL aVR) aVR I  II/2

II 2/3 (aVF aVR) aVL I III/2III 2/3 (aVF aVL) aVF II  III/2Bipolar and unipolar leads are not of equal lead strength An augmented lead is 87%

of the lead strength of a bipolar lead Therefore, the above equations must be corrected.When the strength (voltage) of an augmented unipolar lead is determined from thebipolar lead values, it is corrected by multiplying by 0.87, and when the strength (volt-age) of a bipolar lead is determined from an augmented lead values, it is corrected bymultiplying by 1.15 (100/87) (see Fig 22.10)

Hence, I 2/3 (aVL ... bymultiplying by 1.15 (100/87) (see Fig 22 .10)

Hence, I 2/ 3 (aVLaVR) (1.15) aVR III /2 (0.87)

II 2/ 3 (aVFaVR) (1.15) aVL IIII /2 (0.87)III 2/ 3 (aVFaVL) (1.15) aVF IIIII /2 (0.87)... 90.

● Inverted in a VR and frequently in V1and sometimes in V2< /small>

● Upright, diphasic or inverted in leads III and aVL

– P wave is upright in. .. class="text_page_counter">Trang 23

1 RR interval is the distance between the two consecutive R waves

● In regular sinus rhythm, the RR interval in