cardiac rhythm and anaesthesia: basics of ecg abnormal rhythms relevant to anaesthetic practice...

Cardiac Rhythm and Anaesthesia: Basics of ECGAbnormal rhythms relevant to anaesthetic practiceAntidysrhythmic agentsManagement of perioperative arrhythmiasAbnormal rhythms relevant to anaesthetic practiceManagement of perioperative arrhythmiasAntiarrhythmic agents

Dr. B. Uma

University College of Medical Science & GTB Hospital, Delhi

Basics of ECG

Objectives

1. What is an ECG

2. ECG indications

3. Review of the conduction system

4. ECG recording

ECG paper

ECG leads

5. ECG interpretation

What is an ECG?

The electrocardiogram (ECG) is a graphic recording of the electrical potentials generated by the heart. The signals are detected by means of metal electrodes attached to the extremities and chest wall and are then amplified and recorded by the electrocardiograph

Introduced by William Einthoven in 1901

ECG Indications

Monitoring

Diagnosis

ECG Indications…

Preoperative period

Risk Assessment• Assess the baseline cardiac status• Information regarding MI, conduction or rhythm

abnormalities• Reveals findings related to life threatening

metabolic disturbances or susceptibility to sudden cardiac death

ECG Indications…

Intraoperative period

Monitoring to detect changes in rate and rhythm or myocardial ischaemia

Monitoring of proper function of pacemakers or implantable cardiac defibrillators in patients undergoing surgery with these devices in place

ECG Indications…

Postoperative period

Monitoring

• Important in high risk patients when new ischemic or rhythm changes are suspected

ECG Paper

The Normal Conduction System

P wave caused by atrial

depolarization

Right atrial activation reflected by

ascending limb of P wave

Duration=0.02-0.04s

Left atrial activation reflected by

descending limb of P wave

Duration=0.05-0.06s

Hence, total duration=0.08- 0.10s

(maximum duration=0.11s)

Maximum normal amplitude=2.5mm

QRS complex is caused by the ventricular depolarization

QRS duration 0.05 - 0.11 sec

QRS amplitude

S amplitude in V1 + R amplitude in V6 < 3.5 millivolt (mV)

R in V5 or V6 < 2 mV

The ST segment and T wave reflect ventricular repolarization

ST segment duration=0.08-0.12s

T wave duration=0.16 s

T wave should be at least ⅛th but less than ⅔rd of the amplitude of the corresponding R wave

ECG Leads

Leads are electrodes which measure the difference in electrical potential between either:

1. Two different points on the body (bipolar leads)

2. One point on the body and a virtual reference point with zero electrical potential, located in the center of the heart (unipolar leads)

ECG Leads

The standard ECG has 12 leads: 6 Frontal plane leads

6 Horizontal plane leads

Frontal plane leads - oriented in frontal or coronal plane of the body and consist of standard leads I, II, III and augmented limb leads AVR, AVL, AVF.

Horizontal plane leads - oriented in transverse or horizontal plane of the body and are formed by precordial leads V1-V6.

Standard Limb Leads

Standard Limb Leads

Augmented Limb Leads

All Limb Leads

Precordial leads

Summary of Leads

Limb Leads Precordial Leads

Bipolar I, II, III(standard limb leads)

-

Unipolar aVR, aVL, aVF (augmented limb leads)

V1-V6

Anatomic Groups(Summary)

3 Electrode ECG monitoring

Electrodes placed on the torso to reduce artifacts from limb movement

RA/LA electrodes placed in right and left infraclavicular fossae

LL leg electrode below the left rib cage

Most common mode in OR and ICU

Good enough to detect HR and VF

Inadequate for detecting complex arrhythmias and ST segment monitoring

5- Electrode ECG monitoring

Modified Chest leadsModified chest leads (MCL) are useful in detecting bundle branch blocks and premature beats.

Lead MCL1 simulates chest lead V1 and views the ventricular septum.

Lead MCL6 simulates chest lead V6 and views the lateral wall of the left ventricle

CS 5 (RA electrode placed under the right clavicle and LA electrode placed in the V5 position) for detection of anterior myocardial wall ischaemia

CB5 (RA electrode over the center of the right scapula and LA electrode in the V5 position) for detection of ischaemia and supraventricular arrhythmias

Ten-Electrode, Twelve-Lead ECG Monitoring

The Right-Sided 12-Lead ECG

The limb leads are placed as usual but the chest leads are a mirror image of the standard 12-lead chest placement

Clinical significance: Patients with an acute inferior MI should have right-sided ECGs to assess for possible right ventricular infarction

Invasive Electrocardiographic Monitoring

Esophageal electrocardiogram: Much closer to atria. Hence better option when p waves recording is uncertain

Detection of posterior wall ischaemia

Esophageal electrodes incorporated into esophageal stethoscopes and welded to conventional electrocardiographic wires

:Intracardiac electrocardiogram:Multipurpose pulmonary artery catheter with 3atrial and 2ventricular electrodes for intracavitary ECG

Relatively insensitive to electrocautery

Endotracheal electrocardiogram:Endotracheal tube with 2 electrodes embedded

Diagnosis of atrial arrhythmias in pediatrics

Intracoronary electrocardiogram:Coronary guide wire during angioplasty is used

Greater detection of acute ischaemia

ECG Interpretation

Rate

Rhythm

QRS axis

P Wave

PR Interval

QRS Complex

QT Interval

ST Segment

Documentation: name of the patient and the date and time it was recorded.

Calibration signal: The amplifier gain is normally adjusted so that a 1 millivolt signal through the ECG amplifier results in a vertical deflection of 10 mm (two large ECG squares). All voltage measurements on the ECG depend entirely on the accuracy of this calibration signal.

Determining the Heart RateRule of 1500

Heart rate=1500/no. of small boxes between adjacent RR intervals

Rule of 300Heart rate=300/no. of big boxes between adjacent QRS complexes

6/3 second ruleNo. of RR intervals in 3sec multiplied by 20

or

No. of RR intervals in 6sec multiplied by 10

What is the heart rate?

1500/30 = 50 bpm

www.uptodate.com

What is the heart rate?

Count number of large boxes between first and second R waves=7.5. 300/7.5 large boxes = rate 40

What is the heart rate?

Count 30 large boxes, starting from the first R wave. There are 8 R-R intervals within 30 boxes. Multiply 8 x 10 = Rate 80

Rhythm

Normal: Each QRS preceded by a P wave with a regular PR and RR interval and a rate between 60 and 100 bpm

Irregular– Regularly Irregular – Irregularly Irregular

The QRS Axis

The QRS axis represents the net overall direction of the heart’s electrical activity

Direction of the axis determined on the basis of the hexaxial reference system

The QRS Axis

By near-consensus, the normal QRS axis is defined as ranging from -30° to +90°.

-30° to -90° is referred to as a left axis deviation (LAD)

+90° to +180° is referred to as a right axis deviation(RAD)

Movement of the electrical impulse towards the positive electrode will result in a positive deflection on the ECG.

Movement of the electrical impulse towards the negative electrode will result in a negative deflection on the ECG.

Movement of an electrical impulse perpendicular to a line between the positive and negative electrodes results in a biphasic deflection on the ECG.

Determining the Axis

Predominantly Positive

Predominantly Negative

Equiphasic

The Equiphasic Approach

1. Determine which limb lead contains the most equiphasic QRS complex. The fact that the QRS complex in this lead is equally positive and negative indicates that the net electrical vector (i.e. overall QRS axis) is perpendicular to the axis of this particular lead.

2. Examine the QRS complex in whichever lead lies 90° away from the lead identified in step 1. If the QRS complex in this second lead is predominantly positive, than the axis of this lead is approximately the same as the net QRS axis. If the QRS complex is predominantly negative, than the net QRS axis lies 180° from the axis of this lead.

Equiphasic Approach: Example 1

Equiphasic in aVF Predominantly positive in I QRS axis ≈ 0°

The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/

Equiphasic Approach: Example 2

Equiphasic in II Predominantly negative in aVL QRS axis ≈ +150°

The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/

Two lead approach

Look for net QRS deflection in leads I and aVF

If both +ve – Normal axis

If I +ve & aVF predominantly –ve – Left axis deviation

If I –ve & aVF +ve – Right axis deviation

Calculation of Axis – Third Approach

Calculate the net QRS deflection in lead I and aVF

E.g. in lead I, Q wave measures 3 small squares & R wave measures 6 small squares - net deflection is +3

Similarly, net deflection in aVF is -5

Cardiac vector is thus sum of individual vectors from leads I and aVF

P Wave

Best evaluated in standard lead II and lead V1

In standard lead II P wave is pyramidal with a rounded apex

In lead V1 biphasic P wave with an initial positive and a terminal negative deflection

Axis within 40°to 60°

P Wave – Clinical Significance

P Pulmonale

-Tall peaked P wave

-Amplitude in lead II >2.5 mm

-Duration WNL

-Expression of right atrial enlargement

P Mitrale

-Double peaked, notched or

camel humped P wave

-Negative deflection in V1> 1mm

-Duration of notch > 0.04s

-Expression of left atrial enlargement

PR Interval

PR interval - between beginning of P wave and beginning of QRS complex

Duration = 0.12 to 0.20 sec

Reflects time taken for conduction of impulse from SA node to the ventricles through the AV node

Long PR interval: first degree heart block, hyperkalemia

Short PR interval: WPW syndrome

QRS Complex - Interpretation

QRS interval

QRS voltages

Precordial R-wave progression

Abnormal Q waves

The QRS Complex

QRS Complex – Clinical Significance

LVH

RVH

Ventricular ectopics

RBBB

LBBB

Hemiblocks

RVH

LVH

Q waves

Pathologic Q waves are a sign of previous myocardial infarction.

The precise criteria for pathologic Q waves:• Q-wave 0.04 s in duration • Deeper than one fourth of the

following R wave in voltage• Q wave is deep usually greater

than 4mm in depth

ST Segment

Represents greater part of ventricular repolarization

Merges smoothly and imperceptibly with the proximal limb of T wave

Leaves the baseline immediately after its origin; hence not isoelectric

Flat, downsloping, depressed: coronary ischaemia, hypokalemia, digitalis toxicity

Elevation: MI

T Wave

Represents the repolarization of the ventricles

Last half represents relative refractory period

Positive deflection in most leads

Negative deflection in AVR

Biphasic in V1

T wave inversion: coronary ischaemia

Tall tented T wave: hyperkalemia

Flat T wave: hypokalemia

Heavily notched: pericarditis

ST-T segment: Significance

ST-T segment is the component of ECG most sensitive to acute myocardial ischaemia

Transmural ischaemia→ ST elevation accompanied by tall positive (hyperacute) T waves

Subendocardial ischaemia→ ST depression

Myocardial infarction→ pathologic Q waves

QT Interval

Interval from the beginning of the QRS complex to the end of T wave

Represents the total duration of ventricular electrical activity

Corrected QT interval (Q-Tc)= QT/√R-R

Normal values= 0.39±0.04 s.

QT Interval –Clinical Significance

Prolonged QTc interval:Acute myocarditis

Acute MI

Torsades de pointes

Romano Ward syndrome

Shortened QTc interval:Digitalis effect

U Wave

Small rounded deflection (< 1 mm) that occurs immediately after T wave and has same polarity as T wave

Best seen in leads V2-V4

?repolarization of papillary muscles or purkinje fibres

Prominent U wave: hypokalemia, drugs e.g. dofetilide, amiodarone, sotalol, quinidine, procainamide, disopyramide

Inverted U wave: Sign of ischaemia

J Point

The J point in the ECG is the point where the QRS complex joins the ST segment.

It represents the approximate end of depolarization and the beginning of repolarization as determined by the surface ECG.

There is an overlap of 10 milliseconds.

Some Practical Points

Skin prepared by-• Hair should be shaved• Skin cleaned with spirit • Abrade skin lightly

Muscle tremor presents as artifact. Electrode should be placed over bony prominence.

Electrodes – pregelled, disposable

Broken electrodes to be discarded

Artifacts

Equipments interfering with ECG:• 60 hz power lines• Electrocautery (mc)• Cardiopulmonary equipment• Defibrillators• Transplanted heart(pseudoartifact)

What is the axis?

Outline

Factors causing perioperative arrhythmias

Mechanisms of arrhythmogenesis

Sinus Rhythms

Atrial Rhythm disturbances

Junctional Rhythms

Ventricular Rhythm disturbances

Conduction blocks

Arrhythmias

Abnormality of cardiac rate, rhythm or conduction

Most frequent perioperative cardiac abnormality

Both during cardiac and non cardiac surgeries

With general and regional anaesthesia

Factors contributing to perioperative arrhythmias

Patient related

Anaesthesia related

Surgery related

Patient related factors

Pre existing cardiac disease – higher incidence, more fatal

CNS stimulation – SAH → ST-T changes

Old age – Post-op AF frequent due to degenerative changes in atrial anatomy

Anaesthesia related factors

Endotracheal intubation

General anaesthetics Halothane (reentry, sensitization to catecholamines), enflurane,

sevoflurane (severe bradycardia), desflurane (QTc prolongation) Drugs blocking reuptake of catecholamines- ketamine

Local anaesthetics Inadvertent intravenous injection of large dose - cardiac arrest High thoracic levels of regional anaesthesia- bradyarrhythmias

Abnormal arterial blood gas and electrolyte levels

Hypercarbia, hypoxia, hypocarbia a/w hypokalemia

Surgery related factors

Autonomic reflexes Peritoneal traction, pressure on vagus - sinus bradycardia, AV

block, asystole Oculocardiac reflex- bradycardia, asystole IJV cannulation

Cardiac surgery Release of aortic cross clamp Placement of venous cannulae Retraction of the heart Suturing over the atrium