Normal Sinus Rhythm

Atrioventricular Blocks Atrioventricular blocks (otherwise known as heart blocks) can be divided into three degrees. First-degree heart blocks are characterized by P to R intervals longer than 0.20 seconds. Second-degree heart blocks are characterized by some P waves being blocked at the AV node. This results in some P waves occurring without following QRS complexes. Third-degree heart block is characterized by a complete dissociation between P waves and QRS complexes. Nursing Tip: A hint for separating the heart blocks into degrees is that first- and third- degree blocks usually have regular QRS rates. Feature Differences Between First-Degree, Second-Degree, And Complete Heart Block Type of heart block First degree Second degree, Mobitz type 1 (Wenckebach) Second degree, Mobitz type 2 Complete or third degree Rate Generally at intrinsic rate of 60/min to 100/min May be slower Atrial rate generally at intrinsic rate of 60/min to 100/min. Ventricular rate depends on number of impulses conducted through atrioventricular node; will be slower than atrial rate Atrial rate and the ventricular rate are always different; atrial is faster than ventricular. Regularity of atrial rhythm Regularity of ventricular rhythm Regular Regular Atrial rate generally at intrinsic rate of 60/min to 100/min. Ventricular rate depends on number of impulses conducted through atrioventricular node: may intermittently be the same as atrial rate when impulses are being conducted. Ventricular rate will be slower than atrial rate when impulses are blocked Regular Regular Regularly irregular Regularly irregular or irregularly irregular P wave P to P wave is constant P to P wave is constant P to P wave is constant P:QRS ratio 1:1 1:1 until block occurs, then a P wave with a pause absent of a QRS complex, then cycle restarts with a P wave and associated QRS complex PR interval (the primary clue to the degree of heart block) Constant PR interval but prolonged >0.20 s or 200 ms QRS width Normal <0.12 s Variable: PR interval progressively lengthens until a QRS complex is dropped A pause follows the dropped QRS complex, then PR interval resets to shorter interval and the cycle repeats, very cyclic Normal <0.12 s 1:1 until block occurs, then 2 or more P waves that are followed by a pause without QRS complexes. 1:1 conduction may recur or may progress to ventricular asystole or complete heart block PR interval remains constant until a block occurs and the QRS complex is dropped, resulting in a pause. When or if conduction resumes, the PR interval remains constant Regular with ventricular rate slower than atrial Rate. May become irregular as heart becomes ischemic P to P wave is constant but P waves may be difficult to see if buried in the QRS complex or T wave Variable, more P waves than QRS complexes Dropped QRS complexes/pauses None Electrocardiographic appearanceb Yes: pattern of progressive lengthening of the interval from QRS to QRS until a QRS is dropped after a P wave; then a pause occurs, and theinterval resets Normal if block is at level of bundle of His Wide (0.12 s) if block in bundle branches. Yes: QRS-QRS rate is constant until a QRS complex is dropped and a pause occurs. Regular PR interval is not consistent. Impulses are not conducted from the atria to the ventricles. Visually the PR interval is not constant and does not progressively lengthen, “no pattern” Normal or wide. Normal (<0.12 s) if block at level of atrioventricular node or bundle of His. Wide (0.12 s) if block in bundle of His None