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Archive for the ‘Anatomy of heart’ Category

Heart is not like a rigid structure built with  bricks . . . . so , its too architectural mindset to describe cardiac chambers to be made up of walls. Rather , Its a four chambered muscle mass moulded together in a complex 3D interface with distinct surfaces rather than walls. It’s also important to realise, since the heart is positioned (rather hanging )delicately in the middle mediastinum resting on the diaphragm , its subjected to one more dynamism due to respiratory motion blurring the definition of surfaces as well. (Vertical vs Horizontal)

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Posterior surface is now referred to as infero-posterior

The posterior aspect of heart contains essentially the venous channels and the atrium (LA in particular)pulmonary veins and coronary sinus.  This happens right from 8 week heart open stage when venous end of lower straight heart tube folds up and posteriorly .

cardaic-looping-posterior-wall

It should be recalled only a small portion of lower aspect of posterior wall is alloted to left ventrilce.Instead the Infero diaphragmatic surface is formed by two-thirds  the LV and one-third Right ventricle.

anatomy-of-heart-posterior-wall-mi-lcx-rca-grays-grants-anatomy-netter

Image courtesy : From the great Netter

Nomenclature  issue 

The term posterior wall is now abandoned in most Echocardiography texts its replaced by inferior .The implication is more for Electrophysiologists with reference to accessory pathway localization

What is true posterior wall MI ?

As discussed before ,posterior surface of heart is different from posterior aspect of left and and right ventricle.

What does leads V7 V8 V9 record ?

It actually records electrical signals arising from posterior  aspect  of heart.  Left atrium,  pulmonary vein along with insulatory  effect of lungs dampens the potential . This makes the sensitivity of ST elevation  in posterior MI is low.

Blood supply of posterior surface

It’s highly variable.Both RCA and LCX arteries contribute with its  posterior left ventricular branches (PLV)

It can be inferred , LCX has more territorial rights than RCA in this unique zone of heart as the artery covers more posterior areas.

Read a related article

Back pain from anterior MI : Is it possible ?

 

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RCA is known for unusual variants.Here is a right main trunk which divides into two major branches, both taking a surprisingly similar course.Shall we refer it to as Dual RCA  or is it an early RV branch ? But how can both run posterior and parallel ?

PS: His LAD and LCX  were  normal.

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One casual question in my class led to this search for an anatomical mystery. When we were discussing  why left atrial oxygen saturation never reaches 100 % ? ,  it was attributed to desaturated  bronchial venous blood  draining  into pulmonary vein.

How does this bronchial vein enter pulmonary venous circulation ? How many bronchial veins are there ? What  anatomical plane it runs   ?

Surprisingly, even in this hi-tech era of academic excess, literature is sparse for this basic anatomical question. It is reported (In Greys anatomy ? ) Bronchial veins are two in number and both drain to Azygos and Hemiazygos veins (systemic) rather than pulmonary veins.

So is our assumption wrong ?

May not be.We realise these are only two visible and named bronchial veins .It is learnt they  probably carry only about 13 %  of bronchial  venous blood to systemic venous circuit.

bronchial venous drainage bronchial circulation

Image showing right and left bronchial veins draining to Azygos and hemiazygos veins.

 

It is assumed , remaining  87 % of  bronchial  venous blood drains to pulmonary  venous  circuit in an invisible  fashion (By unnamed twigs ?) desaturating  the LA blood by about one percent from 100 to 99 %. This is our current understanding. I haven’t come across any specific human  research that quantifies the bronchial venous channels and it saturation . It’s gratifying  to find one study specifically looked answer this question  in sheep study .(Charan H.B  et all Reference 1 )

 

where does bronchial vein drain drainage circulation pulmonary vein saturation

True physiological bronchial venous drainage seems to be  different from anatomical bronchial venous circuits .

 

Clinical implication of bronchial venous circulation.

In physiology it may not be important . However bronchial circulation (both arterial and venous)  can take many anatomical tracts when pulmonary micro vascular bed is structurally and functionally altered as in  COPD,   , pulmonary atresia  with aorto-pulmonary collaterals , congenital left to right shunts,post Fontan  circulation pulmonary AV malformations,lung tumors  etc .

Hemoptysis in acute pulmonary  venous hypertension is thought to be due to rupture of these bronchial veins as elevated pulmonary venous pressure reflect into bronchial veins  (As in mitral stenois and other conditions. )  This again would vouch for bronchial veins draining to pulmonary veins.

Final message

As on today , it can be concluded bronchial vein drainage goes both systemic and pulmonary venous circuit.Bulk of them appear to end in  pulmonary veins though clear anatomical evidence is lacking.

Post-ample

Exploring human anatomy appear a  grossly unfinished agenda even today, especially the micro and histo-anatomy. Teachers of basic sciences  should impress upon youngsters entering the medical school to pursue translational  research relevant to specific clinical  problems.

Students  may contact   <drsvenkatesans@yahoo.co.in> for specific areas of clinical cardiac anatomy topics that still requires answers.

Reference

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Car tyres warrant  replacement  every  20,000 km or so .Its batteries do require periodic attention. Human heart , which runs non stop from womb to tomb . . . deserves how much ? Unfortunately no one (What about your cardiologist?) can  provide  a flawless lifetime maintenance contract to this restless bio-mechanical pump ! Fortunately still, God has  created this wonder organ ,that can with stand the stress of life for nearly 10 decades . . . if we live a proper life !

However ,there are many areas in heart that are prone for mechanical stress even if it’s structurally normal .These are  the zones where the relatively fixed parts  rub against the dynamic zones .The joint between inflows ,outflows to the respective ventricle are at risk . Aortic root, along with the leaflets and annulus  formed by the fibrous skeleton is delicately close to the critical part of the conduction system namely the branching portion of AV node and His Purkinje.The process of degeneration is linked to age, valvular abnormality and genetic and  systemic metabolic profile also seem to matter in few .(Hypercalcemia and cardiac calcification ?)

Importance of the interface between Aortic valve and  AV conduction system in the current Era 

Degenerative aortic valve disease along with  the adjacent conduction system  of elderly is going to be a key cardiovascular disease in the future as the population is aging .We have made a big step forward in tackling this with innovative percutaneous aortic valve implantation  (TAVI) . However ,the artificial valve  is positioned vulnerablely  close to conduction system and incidence of complete heart block either during implantation or follow up is  high and a permanent pacemaker seems to be an integral part of this procedure for many.

aortic valve his bundle complete heart block 003

Heart seen from behind , Mitral annulus to left and tricuspid valve ring to the right .

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Right ventricle,being a venous chamber has distinct anatomical and physiological features to carry out this function.RV has a complex shape, its triangular in long axis and  crescent like in short axis , thin (<5mm)  more distendable  .Contraction of RV begins slightly early but ends later than LV  (30ms )

RV receives blood from RA and ejects in to PA in a sequential manner .The inflow, body and outflow contract somewhat like  intestinal peristalsis. This is facilitated  by the incremental  delay in the electrical depolarization of right ventricle.In physiological conditions, the later half of QRS  is  responsible for RV activity and RVOT is the last to contract. (This intrinsic electrical and mechanical  delay in RV contraction is a physiological inter ventricular  desynchrony . One  should be aware of this when planning cardiac resynchronisation therapy in cardiac failure.  )

Click over the image for an animation of RV contraction.

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Image courtesey Oxford spcialist hand book in cardiology :Echocardiography Paul Leeson , Second edition ,.Oxford university press 2012 Multi media .

Note:LV is  a fairly   elliptical and strongly  muscular pump and contracts in a  single go with maximum force.(dp/dt).

Final message

Though both right and  left ventricle originate from same  straight heart tube , developmentally the right ventricle evolves for a different form and function . Now,we realise there are lots of sharing of parental muscle fibers that engulfs and bonds both chambers.(Mind you ,This is the fundamental mechanism of ventricular interdependence.Of course ,IVS is a common wall shared lifelong by both chambers  without any (sibling related?)  hemo-dynamic dispute !

3D echocardiography and MR imaging has helped us to understand the RV morphology better and exciting articles written by pioneers are available  free for those who are interested.

Reference

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