a wave and v wave roughly correspond to P wave and QRS complex in electrocardiogram respectively. a wave falls on S1 and v falls on S2. Hence, in sinus rhythm a wave is preceded by v wave . In 1st degree heart block, a wave (P) will occur much before S1 (onset of ventricular systole) and denotes a prolonged PR interval. And, with further prolongation, a wave may merge with previous v wave . In 2nd degree type 1 AV block, there will gradual prolongation of a-v interval in JVP ending with a wave which is not followed by v wave . In 2:1 AV block, there will be two a waves for each v wave . Alternate a wave will coincide with ventricular systole resulting in regular cannon a waves . In complete heart block, a-v interval is variable with a more than v waves along with irregular cannon a waves . Irregular cannon a waves will be seen whenever a wave coincides with ventricular systole because of AV dissociation in CHB. Irreg...
"All about Pearls and Essentials of Cardiology"