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What happens to the intensity of first heart sound (S1) in atrial fibrillation ?

Posted Jun 29 2010 10:10am

It is a well known  cardiac auscultatory  sign,   S 1  becomes  variable in intensity with the onset of atrial fibrillation.

In physiology , the intensity of S 1 is determined by many factors.

  1. The valve morphology(Thickness, Calcium , Rigidity )
  2. Valve mobility
  3. PR interval
  4. Force of LV contraction
  5. Preceding RR interval (4 and 5 are inter related)
  6. Insulation and auditory factors (Thick chest wall  etc)

How does atrial fibrillation modify the intensity of  S1 ?

It is to be noted , atrial fibrillation alters only one  of the above factors, namely the RR interval which becomes irregular.

The mix of short and long RR intervals  occurs at random  . A short RR interval, results in a relatively softer S1  and vice versa . The mechanism is directly attributable  to the degree of LV filling and subsequent change in force of contraction .

Many times , at fast ventricular  rates (Say >150) the distinction between short- long cycles is  negligible in terms of net cardiac cycle.

If  the RR interval , is too prolonged there can be an  inverse relationship  with s 1 intensity .It gets  muffled as the  mitral valve floats back  to it’s  orifice and a partial or even complete  closure occurs , making  force of LV contraction irrelevant in the genesis of S1 .

The vanishing act of PR interval  in atrial fibrillation.

It does not require great brains  to  understand ,  if P waves are absent ,  PR interval must also be absent !

If PR interval is absent ,  there  can be no  influence of it on the first heart sound. Logic demands  absence of PR interval must have some sort of  influence on the intensity of S1. As far as i know cardiology  literature has not answered this query.

What are the two types of S 1 variation ?

Experince  has shown us , the variation of S1 can be of two types*.

Sequence 1 : Varying between , Loud -Louder- Loudest -pounding

Sequence 2: Varying between , Loud -Normal – soft -Muffled

* Applicable only for those with shrewed ears !

S 1 intensity with reference to underlying pathology : Valvular vs Non valvular atrial fibrillation

It is obvious the impact of  varying RR interval on  the intensity of S1 will directly depend upon the underlying pathology. The  intensity of   S1  in  non valvular AF (Like , lone AF, Thyrotoxic AF, Hypoxic  AF ,Ischemic AF etc)  are  more vulnerable to  changing   RR interval .

In rheumatic heart disease , the influence of valve morphology , rigidness, calcification and presence  of MR  generally prevail over the  impact  of changing RR interval .So,  in a case of tight mitral stenosis  and AF  it  is expected the sequence 1 is more common .

In lone AF or AF due to CAD , sequence  2 is more likely *  Associated LV dysfunction , and ischemic   MR may further dampen the intensity of  S1 .

Clinical implication

Hearing  few occasional  loud  S1 in AF , is an indirect indication that underlying LV function is good,  as it reflects the force of  LV contraction .

Silent AF

Some hearts are notoriously silent even in the midst of AF. If  the silence is not  due to obesity  or  other insulation defects,  it suggests a sinister diagnosis ,  like severely  dysfunctional ventricle  like  DCM etc.

As a corollary, it is often noticed ,  palpitations* are , often not  felt  by patients with dysfunctional  ventricles  in spite of atrial fibrillation. (As loud S 1 is rare with dysfunctional ventricle)

*Palpitation is a symptom that equates to Dp/Dt of ventricles.

What  happens to mid diastolic murmur in AF ?

The murmur length  varies  linearly with reference  to RR interval. The pre systolic  accentuation disappears ,but   pre-systolic component may persist .

Final message

Simple, bed side auscultation during atrial fibrillation can give us vital clues about the etiology, and the  underlying LV function .  Let us not be ashamed to talk about clinical cardiology  . . .at least in the bed side !


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