SA Block

Caused by the inability of the SAN to activate the atiia, leading to an absence of P waves on the ECG. These blocks can be classified as either complete or incomplete SA blocks. Incomplete blocks cause the occasional loss of beats. Complete SA block occurs when no impulses leave the SAN leading to Sinus Arrest , a complete lack of heart beats (no PQRS or T waves present). 

The intermittent absence of atrial activation causes a gap or pause on the ECG. This pause is either caused by a gradual reduction in the P to P interval (the distance between one P wave and the next). This results in a pause, the cycle then continues. This is also known as type I sinus exit block (Fig. 7.13). 

The other form of incomplete SA block, known as type II sinus exit block is determined due to its mathematical relationship with the conduction cycle. In this type of SA block there is no shortening of the P to P interval but instead an unexpected absence of a P wave and subsequent QRS complex. The pause is multiple of the P to P interval (Fig. 7.14). 

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Wu et al. (2000) showed the formation of self-assembled nanoparticles of P(SA -block-EG) in an aqueous environment and studied their degradation as a function of pH and temperature. Fu et al. (2002) repeated the synthesis of P(SA-Wock-EG) and studied the morphology and erosion kinetics of microspheres which they propose as vehicles for mucosal drug delivery. 

Poly(lactic acid) (PLA) has also been added to poly(SA) via melt polycondensation to produce the triblock copolymers poly(lactic acid-Wock-sebacic acid-Wock-lactic acid) (P(LA-block-SA-block-LA)) by Slivniak and Domb (2002). The PLA (d-, l-, and dl-) was incorporated by acetylation and addition to the PSA synthesis. They showed the formation of stable stereocomplexed particles with increased melting points and reduced solubility, and studied the degradation and drug release characteristics of the same (Slivniak and Domb, 2002). The stereocomplexes self-assemble as a consequence of the chirality in the PLA portions of the chains (Slivniak and Domb, 2002). 

Adverse effects include SA block or arrest, high grade AV block, ventricular tachycardia, arrhythmia or ventricular asystole, polymorphic ventricular tachyarrhythmia, hypotension (particularly when given IV), cinchonism, tinnitus, loss of hearing, gastrointestinal upset, severe headache, diplopia, photophobia, etc. 

Episodes of tachy-brady syndrome, atrial flutter, atrial fibrillation, SA block, or sinus arrest... 

Complete SA block otherwise known as sinus arrest has no such mathematical relationship. The pauses can last several seconds, and may cause patients to collapse (Fig. 7.15). The pause is normally terminated by an escape beat. As discussed earlier these escape beats originate further down the conduction system and are known as junctional or ventricular escape beats, depending upon their origin. These beats act as a safety net preventing Asystole. Treatment may require the use of drugs such as atropine or the insertion of an artificial pacemaker. 

To differentiate between nonconducted premature atrial contractions (PACs) and sinoatrial (SA) block, check the following ... 

If there s no P wave in the pause or T wave (see the shaded area in the bottom rhythm strip), then the rhythm is SA block. 

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