**2. Normal physiology**

Normal sinus rhythm is when the heart beats in an orderly predetermined sequence. The atria contract initially in response to the firing of an impulse by the Sino-Atrial (SA) node located at the junction of the superior vena cava and the right atrium. The SA node contains specialized tissue with 'pacemaker cells', which can initiate repetitive rhythmic action potentials. These potentials then travel via internodal atrial pathways to the AtrioVentricular (AV) node located at the right posterior portion of the interatrial septum. The AV node slows conduction into the bundle of HIS which then leads to its right and left branches. The left bundle branch further divides into anterior and posterior fascicles. The final pathway of conduction is the Purkinje system, which consists of a network of fibers that transmit the electrical impulse to the myocardium near the apex of the heart. (1)

The Electrocardiogram (ECG) is a reliable and practical way to document the underlying cardiac rhythm. It essentially consists of a recording obtained by 12 surface leads which trace the electrical activity of the heart from different directions. The 12 leads include 6 limb and 6 precordial leads. The limb leads include 3 bipolar leads (I, II, III) meaning they have 2 electrodes of opposite polarity. The other limb leads are aVR, aVL, aVF which are the unipolar leads meaning they have only one electrode connecting to a central terminal. The precordial leads are all unipolar and include V1-V6.

The limb leads are the frontal plane leads representing electrical current along the coronal plane of the heart, i.e. right/left and superior/inferior. The precordial leads represent the horizontal plane of the heart measuring transverse currents, i.e. right/left and anterior/posterior. Lead I traces currents from right shoulder to left shoulder, lead II from

Post Operative Arrhythmias 243

Step 4. Identify the P waves – upright in lead II and III and negative in aVR usually

The above steps should help one to identify the salient features of any rhythm and place it in one of the following mentioned categories. (Figure 1) We would like to point out that this scheme is only one of many and sometimes more than one arrhythmia can be present in a patient. This scheme also, at times over simplifies natural heart rhythms. For example, a heart rate of 40 beats per minute (bpm) during sleep or in an athletic patient can be normal, while patients with an abnormal conduction system can have supraventricular arrhythmias

identifies sinus rhythm, P waves are absent in atrial fibrillation, saw tooth

Step 1. Determine the ventricular rate – tachycardia is >100 / Bradycardia is < 60 Step 2. Measure the QRS complex – Narrow is < 0.12ms / Broad is > 0.12 ms Step 3. Determine the regularity of the QRS complex – Regular/irregular

appearance at an atrial rate of 300 bpm may indicate atrial flutter

 │¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯│ │¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯│ Regular Irregular Regular Irregular ↓ ↓ ↓ ↓ Sinus tach AF VT VF A Flutter\*\* MAT SVT \* AF \*

(AF – Atrial Fibrillation, AT – Atrial tachycardia, MAT – Multifocal atria tachycardia, SVT – Supra

ventricular tachycardia, VF – Ventricular Fibrillation, VT – Ventricular tachycardia)

The Brugada criteria can be used to identify any broad complex tachycardia (4)

Step 4. Is there any typical bundle branch morphology in leads V1 or V6?

\*\* Flutter can be irregular occasionally when the AV block is variable Fig. 1. Various cardiac rhythm disturbances noted in clinical practice

Step 1. Are there RS complexes in any of the chest leads? Step 2. Is the onset of the R wave to the nadir of S > 100 ms?

Step 5: Measure the PR interval – helps identify AV delay

with heart rates less than 100 bpm.

 ↓ ↓ Sinus Brady Heart blocks Sinus pause Atrial Fibrillation

 AVRT AVNRT AT

Step 3. Is there any AV dissociation?

\*With Aberrancy

Rate

 <60 │ >100 │¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯│ Bradycardia Tachycardia │ <0.12ms │ >0.12ms │¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ │ │¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯│ Regular Irregular Narrow Broad

right shoulder to left leg, and lead III from left arm to left leg. Lead aVF traces from central terminal, which corresponds to zero potential to the left leg, aVL from centre to left arm and aVR from centre to right arm. The precordial leads work in a similar fashion in that leads V1 – V6 trace their axis center out from right to left respectively, so that V1 represents right of the interventricular septum, V2 and V3 the interventricular septum (anterior wall), V4 the apex (anterolateral), V5 and V6 the lateral ventricular wall. Any current flowing towards the lead causes a positive deflection and current flowing away from the lead causes a negative deflection and vice versa. The strength of the deflection depends on the amount of potential recorded and is affected by cardiac and extracardiac structures. To understand the electrophysiological basis of the 12-lead tracings on an ECG is important, because it gives clues about the origin of an arrhythmia and sometimes guides their therapies.

The first deflection on an ECG is the P wave which represents atrial depolarization. In sinus rhythm without any discernable atrial pathology, P wave is an upright, smooth, rounded wave with relatively low voltage. The PR interval consists of the P wave and the normally isoelectric segment up to the initial deflection of the QRS complex. The PR interval represents the conduction through atria, AV node, bundle branches and Purkinje system. The QRS complex follows the PR segment. The initial negative deflection is the Q wave, a positive deflection which can occur either initially or after the Q is the R wave while any negative deflection which follows the R is the S wave. QRS complex represents intraventricular conduction and depolarization. The J point represents the junction between QRS and the ST segment. The ST segment corresponds with the end of ventricular depolarization and start of the ventricular repolarization. The T wave, which follows the ST segment, represents ventricular repolarization. As such the QT interval represents the complete ventricular depolarization and repolarization period. Occasionally a small hump-like U wave follows the T wave, and is felt to be due to repolarization of the purkinje system. (2,3)

The bedside monitors which are routinely used for continuous cardiac monitoring are typically wireless, i.e. telemetry systems. These can be either 5 lead wire or 3 lead wire systems. The 5 lead wire system allows for monitoring all of the limb leads or the precordial leads while the 3 lead wire system allows monitoring one lead at a time, usually lead II, because the P wave is best visible in this lead. Depending on the monitoring system available it is essential for health care providers to be able to recognize the cardiac rhythm changes based only on a few select leads seen on the monitor as there may be no time to record a 12 lead ECG.

### **3. Recognition of arrhythmias**

Postoperative arrhythmias though transient are usually sudden in onset. It is essential to recognize a rhythm disturbance and institute treatment as quickly as possible in most cases. A 12 lead ECG is recommended but may be impractical if the rhythm disturbance is an immediate threat to the patient's life. The wave forms visible on the telemonitor or a rhythm strip in one lead tracing may be the only available clue.

It is worthwhile to note some salient points early. Is the patient stable as assessed by the blood pressure, oxygen saturation or mental status? If deemed unstable then more aggressive steps are warranted.

A rapid and accurate interpretation of the ECG can be tricky and readers are advised to develop a personal strategy to identify any given cardiac tracing so that a quick diagnosis can be made. One approach is to identify and describe 5 basic features of the electrocardiogram (2):

right shoulder to left leg, and lead III from left arm to left leg. Lead aVF traces from central terminal, which corresponds to zero potential to the left leg, aVL from centre to left arm and aVR from centre to right arm. The precordial leads work in a similar fashion in that leads V1 – V6 trace their axis center out from right to left respectively, so that V1 represents right of the interventricular septum, V2 and V3 the interventricular septum (anterior wall), V4 the apex (anterolateral), V5 and V6 the lateral ventricular wall. Any current flowing towards the lead causes a positive deflection and current flowing away from the lead causes a negative deflection and vice versa. The strength of the deflection depends on the amount of potential recorded and is affected by cardiac and extracardiac structures. To understand the electrophysiological basis of the 12-lead tracings on an ECG is important, because it gives

The first deflection on an ECG is the P wave which represents atrial depolarization. In sinus rhythm without any discernable atrial pathology, P wave is an upright, smooth, rounded wave with relatively low voltage. The PR interval consists of the P wave and the normally isoelectric segment up to the initial deflection of the QRS complex. The PR interval represents the conduction through atria, AV node, bundle branches and Purkinje system. The QRS complex follows the PR segment. The initial negative deflection is the Q wave, a positive deflection which can occur either initially or after the Q is the R wave while any negative deflection which follows the R is the S wave. QRS complex represents intraventricular conduction and depolarization. The J point represents the junction between QRS and the ST segment. The ST segment corresponds with the end of ventricular depolarization and start of the ventricular repolarization. The T wave, which follows the ST segment, represents ventricular repolarization. As such the QT interval represents the complete ventricular depolarization and repolarization period. Occasionally a small hump-like U wave follows the

The bedside monitors which are routinely used for continuous cardiac monitoring are typically wireless, i.e. telemetry systems. These can be either 5 lead wire or 3 lead wire systems. The 5 lead wire system allows for monitoring all of the limb leads or the precordial leads while the 3 lead wire system allows monitoring one lead at a time, usually lead II, because the P wave is best visible in this lead. Depending on the monitoring system available it is essential for health care providers to be able to recognize the cardiac rhythm changes based only on a few select leads seen on the monitor as there may be no time to

Postoperative arrhythmias though transient are usually sudden in onset. It is essential to recognize a rhythm disturbance and institute treatment as quickly as possible in most cases. A 12 lead ECG is recommended but may be impractical if the rhythm disturbance is an immediate threat to the patient's life. The wave forms visible on the telemonitor or a rhythm

It is worthwhile to note some salient points early. Is the patient stable as assessed by the blood pressure, oxygen saturation or mental status? If deemed unstable then more

A rapid and accurate interpretation of the ECG can be tricky and readers are advised to develop a personal strategy to identify any given cardiac tracing so that a quick diagnosis can be made. One approach is to identify and describe 5 basic features of the

clues about the origin of an arrhythmia and sometimes guides their therapies.

T wave, and is felt to be due to repolarization of the purkinje system. (2,3)

record a 12 lead ECG.

**3. Recognition of arrhythmias** 

aggressive steps are warranted.

electrocardiogram (2):

strip in one lead tracing may be the only available clue.


The above steps should help one to identify the salient features of any rhythm and place it in one of the following mentioned categories. (Figure 1) We would like to point out that this scheme is only one of many and sometimes more than one arrhythmia can be present in a patient. This scheme also, at times over simplifies natural heart rhythms. For example, a heart rate of 40 beats per minute (bpm) during sleep or in an athletic patient can be normal, while patients with an abnormal conduction system can have supraventricular arrhythmias with heart rates less than 100 bpm.

(AF – Atrial Fibrillation, AT – Atrial tachycardia, MAT – Multifocal atria tachycardia, SVT – Supra ventricular tachycardia, VF – Ventricular Fibrillation, VT – Ventricular tachycardia)

\*With Aberrancy

\*\* Flutter can be irregular occasionally when the AV block is variable

Fig. 1. Various cardiac rhythm disturbances noted in clinical practice

The Brugada criteria can be used to identify any broad complex tachycardia (4)


Post Operative Arrhythmias 245

definition for postoperative AF includes the need of medical treatment or electrical cardioversion (14) or confirmation on a 12 lead ECG (13) altering the incidence rates further. The peak incidence of postoperative AF (POAF) has been consistently described on day 2 and 3 after surgery. (10) It usually is transient with 80% of the patients converting to sinus rhythm within 24 hours. The recurrence rate has been quoted to be as much as 50% but still only 10% of patients are still in AF at 6 weeks post operation. (13) Late Postoperative AF is less frequent but was seen in nearly 5% of the postoperative patients after discharge from the hospital. This was found when patients were being followed for cardiac rehabilitation and documented in the ISYDE and ICAROS registries in Italy. (15) Despite better coordinated postoperative care and advances in cardiothoracic surgical and anesthetic practices, the incidence of AF seems stable with no reduction over the last 2

AF is generally due to reentry of multiple wavelets circling the atria. It is likely that a preexisting substrate is needed to allow peri-operative triggers to initiate AF. It is thus a specific

Age is the most consistent risk factor seen in past studies. Advancing age increases the risk with each decade. The incidence of POAF is around 6% when less than 40y of age, 18% in less than 60y olds and increasing to as much as 50% in patients older than 80 years. (13,14,16) Other risk factors include male gender, history of prior AF, heart valve disease (especially if the mitral valve is affected), prior cardiac surgery, prior cardiac structural changes like increased left atrial size and left ventricular hypertrophy. Preexisting medical conditions like obesity, chronic lung disease, peripheral vascular disease, hypertension, prior stroke are associated with increased incidence of POAF. However certain other morbid factors like preexisting diabetes, chronic kidney disease, hyperlipidemia, smoking have not been shown to be individual risk factors for POAF in some studies. (17) Pericarditis which is usually a consequence of the cardiac surgery itself is mechanistically involved. Other unique factors such as preoperative use of Digoxin or Dopamine, raised Brain natriuretic peptide (BNP) and right-sided coronary artery disease have been associated as well. ECG features like increased P wave duration of more than 140 ms, which is suggestive of atrial conduction delay can increase the susceptibility to AF. (18) Withdrawal of preoperative Angiotensin converting enzyme inhibitor (ACE I) or Beta blocker therapy is also contributory if not

Certain operative features like aortic cross clamping, pulmonary venting, bicaval venous cannulation, increased length of cardiopulmonary bypass time and mitral valve surgery can increase the propensity for POAF. It has also been noted that at times cardioplegia via coronary sinus does not stun the atria completely and may be associated with occurrence of POAF(19) Direct cardiac injury due to operative techniques causing inflammation is

The postoperative period is a critical stage as the body is yet to recover from the operative stress completely. Many proarrhythmic features such as pericardial inflammation, acute blood pressure or volume changes, acute cardiac ischemia, electrolyte imbalances,

interaction of preexisting and perioperative risk factors which can lead to AF.

decades.

**Preexisting factors:** 

**Pathophysiology of Postoperative AF:** 

immediately initiated after the surgery. (13)

perceived as plausible cause as well.

**Intraoperative factors:** 

**Postoperative features:** 

If the answer to any of questions 1-3 is yes or to question 4 is no, the rhythm is probably ventricular tachycardia. There are several additional ways to distinguish between supra ventricular and ventricular arrhythmias, which exceed the objectives of this review.
