Disturbances of Conduction

The wave of cardiac excitation which spreads from the sinoatrial node to the ventricles via the conduction pathways may be delayed or blocked at any point.

First Degree Block

There is a delay in the conduction from the sinoatrial node to the ventricles, and this appears as a prolongation of the PR interval ie greater than 0.2 seconds. It is normally benign but may progress to second degree block - usually of the Mobitz type I. First degree heart block is not usually a problem during anaesthesia.

Second Degree Block - Mobitz Type I (Wenkebach)

There is progressive lengthening of the PR interval and then failure of conduction of an atrial beat. This is followed by a conducted beat with a short PR interval and then the cycle repeats itself. This occurs commonly after an inferior myocardial infarction, and tends to be self limiting. It does not normally require treatment although a 2:1 type block may develop with haemodynamic instability.

Second Degree Block - Mobitz Type II

If excitation intermittently fails to pass through the AV node or the bundle of HIS, this is the Mobitz type II phenomenon. Most beats are conducted normally but occasionally there is an atrial contraction without a subsequent ventricular contraction. This often progresses to complete heart block and if recognised preoperatively will need expert assessment.

Second Degree Block - 2:1 Type

There may be alternate conducted and non-conducted beats, resulting in 2 P waves for every QRS complex - this is 2:1 block. A 3:1 block may also occur, with one conducted beat and two non-conducted beats. This may also herald complete heart block, and in some situations the placing of a temporary transvenous pacing wire pre-operatively would be recommended.

Complete Heart Block

There is complete failure of conduction between the atria and the ventricles. The ventricles are therefore excited by a slow escape mechanism from a focus within the ventricles. There is no relationship between the P waves and the QRS complexes, and the QRS complexes are abnormally shaped. This may occur occasionally as a transient phenomenon in theatre as a result of vagal stimulation, in which case it often responds to stopping surgery and intravenous atropine. When it occurs in association with acute inferior myocardial infarction, it is due to AV nodal ischaemia and is often transient. Very rarely it may be congenital! However if it occurs with anterior myocardial infarction it indicates more extensive damage including to the HIS - Purkinje system. It may also occur as a chronic state usually due to fibrosis around the bundle of HIS.

Management

• Isoprenaline given by intravenous infusion can be used to increase the ventricular rate
• In the acute situation a temporary transvenous pacing wire may be required. A permanent pacemaker will be required in the longer term if the block is chronic and before contemplating elective surgery.

Bundle Branch Block

If the electrical impulse from the SA and AV nodes reaches the interventricular septum normally the PR interval will be normal. However if there is a subsequent delay in depolarisation of the right or left bundle branches, there will be a delay in depolarisation of part of the ventricular muscle and the QRS complex will be wide and abnormal.

A wide complex rhythm which is present at the start of surgery on initial attachment of the ECG monitor is usually due to bundle branch block (BBB), and is not an indication for cancelling the operation. However this does indicate the importance of attaching the ECG monitor before induction of anaesthesia, particularly where a pre- operative ECG is not available. Any changes on the ECG during anaesthesia and surgery can then easily be compared to the patients ' normal' ie pre-anaesthetic ECG tracing. The definition of which bundle is blocked can only be achieved by analysing a full 12 lead ECG. Two types of BBB are recognised.

• Right Bundle Branch Block (i). This may indicate problems with the right side of the heart, but a right bundle branch block type pattern with a normal axis and QRS duration is not uncommon in normal individuals.
• Left Bundle Branch Block (ii). This often indicates heart disease and makes further interpretation of the ECG other than rate and rhythm impossible.

Other forms of BBB

Bi-Fascicular Block (i and iii). This is a diagnosis which can only be made on a formal 12 lead ECG, and is included for completeness. It is the combination of right bundle branch block and block of the left anterior or posterior fascicle and appears on the ECG as a RBBB pattern with axis deviation. This progresses to complete heart block in a few patients.

Tri-Fascicular Block This is the term sometimes used to indicate the presence of a prolonged PR interval together with a bi-fascicular block. 

Pre - Operative Prophylactic Pacemaker Insertion

Where facilities allow, pacemakers are sometimes inserted prior to surgery in patients who are at risk of developing complete heart block perioperatively. Those at risk of this complication have recently been described by the American college of cardiology and the American heart association. A pacemaker should be considered for:

• 3rd degree AV block which is symptomatic or has a ventricular escape rate of less than 40 beats per minute. Where the rate is greater than 40, there is conflicting evidence of benefit but the weight of opinion is in favour of pacing.

• 2nd degree AV block of any type if there is symptomatic bradycardia.
• Asymptomatic 2nd degree heart block or first degree block with symptoms suggestive of sick sinus syndrome (intermittent tachycardia and bradycardia) plus documented relief of symptoms with a temporary pacing wire. In both of these cases the weight of current opinion favours pacing.
• Any type of bundle branch block with intermittent second or third degree block, or syncope should be paced.
• Bifascicular block is relatively common in the elderly and does not require pacing. 

Detection Of Myocardial Ischaemia

Cardiac events are the main cause of death following anaesthesia and surgery. Perioperative myocardial ischaemia is predictive of intra and post operative myocardial infarction. The likelihood of detection of ischaemia intraoperatively on the ECG is increased by the use of the CM5 lead as discussed above. This lead has the highest probability of detecting ischaemia, particularly in the lateral wall of the left ventricle which is the zone at greatest risk. Lead II is more likely to detect infero-posterior ischaemia and is therefore useful in those patients whose pre-operative ECG shows evidence of inferior or posterior ischaemia or infarction.

The ECG should always be recorded from before the start of the anaesthetic so that any subsequent changes can be observed. ST segment depression of 1mm or more below the isoelectric line with or without T wave changes indicates myocardial ischaemia. The magnitude of ST depression correlates with the severity but not the extent of the ischaemia. The ST segment depression moves progressively from up-sloping to horizontal to down-sloping as ischaemia worsens. Down-sloping ST segment depression may indicate transmural ischaemia (through the full wall thickness).

On a 12 lead ECG full thickness myocardial infarction results in ST segment elevation often with the subsequent development of pathological Q waves (greater than 1 mm thick and 2mm deep). In subendocardial infarction - typically there is deep symmetrical T wave inversion. In subepicardial infarction - there is loss of R wave amplitude without development of Q waves.

Management

• If ST segment depression develops during anaesthesia, 100% oxygen should be given, the volatile agent decreased and the blood pressure and heart rate normalised as far as possible. It is important to maintain diastolic blood pressure and systemic vascular resistance, in order to maintain coronary atery perfusion. In this situation methoxamine (if available) in 2mg iv increments titrated to effect may be useful.
• Postoperative management in a high care environment should be considered where possible, with oxygen therapy, adequate analgesia and correction of fluid and electrolyte balance being of great importance. Monitoring should be continued into the postoperative period as this is the time when further (often silent ) ischaemia and infarction may occur. Oxygen should be given to all high risk patients post operatively, ideally for at least 48 hours. 

Other Ecg Changes Seen In Theatre

Occasionally the ECG changes shape slightly with a change in position of the patient or during different phases of mechanical ventilation. This usually causes a slight change in the position of the heart and results in the ECG being recorded from a different angle. It is not usually of importance. 

Ecg Appearance Of Abnormal Potassium Concentrations.

The ECG trace may develop characteristic changes with alterations in the concentration of various electrolytes. It is rarely possible to diagnose these from the ECG alone but the reading may give arise to a suspicion which should be confirmed by the laboratory.

Hyperkalaemia

• Tall peaked T waves
• Reduced P waves with widened QRS complexes
• Ultimately a sine wave pattern - pre-cardiac arrest
• Cardiac arrest in diastole

Hypokalaemia

• Increased myocardial excitability - any arrhythmia may occur
• Prolonged PR interval
• Prominent U waves
• Enhancement of digitalis toxicity 

Further Reading And References

1. Ganong WF. A Review of Medical Physiology. Stamford: Appleton and Lange,1997.
2. Hutton P,Prys-Roberts C. Monitoring in Anaesthesia and Intensive Care. London: WB Saunders Company Ltd,1994.
3. Hinds CJ, Watson D. Intensive Care - A Concise Textbook.London: W.B Saunders Company Ltd,1996.
4. The 1998 ERC Guidelines for Adult Advanced Life Support. Resuscitation 1998;37:81-90
5. Hampton J. The ECG made easy. London: Churchill Livingstone, 1986.
6. Mangano DT. Perioperative Cardiac Morbidity. Anaesthesiology 1990; 72:153-84.
7. Nathanson MH, Gajraj NM. The Perioperative Management of Atrial Fibrillation. Anaesthesia 1998; 53: 665-76.
8. Gregoratos G et al. ACC/AHA guidelines for implantation of cardiac pacemakers and antiarrhythmia devices. Executive summary. Circulation 1998; 97: 1325-35. 

This article contained links to the following further information:

 Resuscitation from Cardiac Arrest

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