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Archive for December, 2019

Catheter based interventions in TOF  has caught the imagination of  Interventional cardiologists.decades ago. (Quereshi reported first in 1988 Royal Liverpool hospital ) .Somehow it could not develop into a full-fledged modality. The key issue in TOF  is,  RVOT obstruction is infundibular with some degree of valvular involvement. While the valvular component is amenable for easy correction by balloon, the infundibular stenosis requires some form of cutting or splitting. Embryologically,  the malalignment of IVS is the primary mechanism of obstruction. The balloon catheter is will find it difficult to tackle the alignment defect. .Obviously, surgeons can do a comprehensive RVOT reconstruction.

Things are beginning to change. Cutting balloons are available. Various dedicated VSD devices are being developed. Closure of large sub-aortic VSD  followed by  RVOT dilatation appears challenging task but distinctly possible in the near future.

Few cases of palliative RVOT dilatation with a balloon  in critical TOF  is been attempted We hope, in the coming decades at least simple forms of TOF are conquered by the interventional cardiologists!

Hardware: A small profile  coronary  cutting balloon  from Boston scientific .

What is in store for the future ?

3D printing of live heart and designer device or deployable patches for the malaligned VSD is possible. Currently, intracardiac ultrasound would assist the procedure.

RVOT reconstruction with RVOT stenting and percutaneous valves (Melody or Right sided TAVR equivalents) is already been done in post-ICR residual obstructions or late RVOT failure

Coronary cutting balloon flextome tof pulmonary valvuloplasty coronary hard ware

Flextome -Coronary cutting balloon

Balloon pulmonary valvotomy for tof tetrology of fallot

balloon angioplasty for TOF cutting balloon

pulmonary valvotomy in tof tetrology

pulmonary valvotomy in tof tetrology 3

 Other References

1.Boucek MM, Webster HE, Orsmond GS, Ruttenberg HD. Balloon pulmonary valvotomy: palliation for cyanotic heart disease. Am Heart J. 1988;115:318-322.

2.Qureschi SA, Kirk CR, Lamb RK, Arnold R, Wilkinson JL. Balloon dilatation of the pulmonary valve in the first year of life in patients with tetralogy of Fallot: a preliminary study. Br Heart J. 1988; 60:232-235.

 3.Parsons JM, Ladusans EJ, Qureshi SA. Growth of the pulmonary artery after neonatal balloon dilatation of the right ventricular outflow tract in an infant with tetralogy of Fallot and atrioventricular septal defect. Br Heart J. 1989;62:65-68.

4.De Geeter P, Weisburd P, Dillenseger P, Willard D. Valvuloplastie pulmonaire percutanée palliative dans les formes néonatales de tétralogie de Fallot. Arch Fr Pediatr. 1989;46:117-119.

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Exertional dyspnea disproportional to the effort is the most common (cardinal)symptom of heart disease. Whenever we discuss the mechanism of cardiac dyspnea , we primarily attribute it to left heart disease, elevated LVEDP and the resultant pulmonary congestion.Conventional teaching in the past (may be in the present too !) doesn’t implicate raised RVEDP in the genesis of dyspnea.

It’s good to recall , the sensation of dyspnea is felt at the peri -Amygdala nuclear zone after complex processing with various cortical and sub-cortical level .It is subjected to as many afferent triggers other than J receptors in pulmonary micro circulation. (Eg Exercising skeletal muscle). It is believed, mechanical stretch receptors exist within the walls of heart along  the sub-endocardial aspects of chamber.

(Muscle spindles which are the sensors of muscle tension are extensively noted in skeletal muscle that contribute to the origin of dyspnea .We are not yet accruing enough evidence  whether cardiac muscle do have the same muscle spindle or it’s equivalents to cause dyspnea when stretched. However, we clearly witness in the practice of clinical cardiology , isolated elevation of RVEDP ( also RVSP ) to cause significant dyspnea in specific clinical situations.

Potential causes for Isolated Right ventricular dyspnea

  • Pulmonary hypertension  (COPD included* where in it could be a combination of lung and cardiac dyspnea)
  • Acute pulmonary embolism
  • RV Infarction
  • Acute rupture of sinus of Valsalva aneurysm (RSOV) Here RVEDP is often > LVEDP and dyspnea is due to the acute stretch of RV
  • Isolated normal pressure TR(RVEDP is low still cause dyspnea  due to volume related RV triggers)
  • Any RVOT obstruction (Classically valvular pulmonary stenosis)
  • Does RV dilatation without elevated RVEDP cause dyspnea ?  Though right ventricle is developmentally and hemo-dynamically better suited to handle volume , still, it  struggles to manage sudden increase in volume .(Another clinical example is seen in patients who are on dialysis)

*RV diastolic dysfunction is still a Infantile hemo-dynamic concept .Whether it can raise RVEDP significantly during exercise and Independently contribute to dyspnea is at best a hypo-science.

Role of muscle spindle and mechno-receptors

 

Muscle spindle

structure of skeletal muscle spindle. Though we don’t have a highly developed spindles in smooth muscle and cardiac muscle we have evidence to suggest cardiac neural ending do have mechano-receptors with afferent connection through visceral neural plexus that can trigger both heart rate and respiratory centers Further reading : Neuroscience. 2nd edition. Show details Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Sunderland (MA): Sinauer Associates; 2001.

Bain-Bridge reflex: The hidden link in right heart dyspnea

Bain-Bridge reflex is a 100 year old concept. still helping us to understand the basics of right heart hemodynamics and how adjustments with acute volume loading take place.He proposed that  veno-atrial stretch receptors are located  primarily in great veins as it enter ,right atrium (RV as well).

This gets activated through vagus and stimulates  in brain-stem sympathetic system and increase the heart rate to handle the excess blood reaching the heart. How often we feel the symptom of palpitation  whether due to this reflex ( when it is operating) is not really tested. But, what we can infer is , the surge in sympathetic tone perceived can be perceived as  dyspnea.

*Clinical Relevance of the Bezold–Jarisch Reflex and its possible interactions with Bain Bridge reflex is a different topic.

It is interesting to note many of these reflexes cause hypo-tension, bradycardia and hypopnea (Even near Apnea.) The word dyspnea is surprisingly not used .It is highly plausible many of the unexplained dyspnea we see in otherwise healthy population is attributed to acute or chronic volume overloading or under-loading of right heart.

Role of PFO in right heart dyspnea

PFO is a natural decompressing orifice in the IAS guarded by a flip-flap safety valve which is a remnant of septum primum .Though it can flow either way , since the flap of the valve is larger in LA side,  it gets closed when  LA pressure raises but opens up , if RA pressure raises making it more often a right to left shunt at times of elevated RA mean pressure. In isolated right heat pathology , this communication shunts  right to left and  adds a new dimension to cardiac dyspnea (Now, It becomes a hypoxic /biochemical dyspnea over and above the right heart stretch related dyspnea )

Other mechanisms in right heart dyspnea

Pulmonary arterial stretch and altered QP : Role of ventilation perfusion mismatch should also be considered as a cause for dyspnea in isolated RV pathology. The term V/Q mismatch is a poorly understood term fro me. My Inference is, since RV contraction  provides the Q in the equation V/Q .Whenever Q falls V has to fall to maintain neutrality causing net hypoxia and dyspnea.

Final message

Dear fellows, never hesitate to attribute the origin of dyspnea,  to elevated RA mean pressure /RVEDP. It is due to RA/RV stretch secondary to volume and pressure overloading with a perfectly normal pulmonary capillary wedge pressure or LVEDP. As in the left heart ,this occurs both in pathological as well as perfectly exaggerated physiological times.

Reference 

1.Bainbridge FA. The influence of venous filling upon the rate of the heart. J Physiol. 1915 Dec 24;50(2):65–84. [PMC free article] [PubMed[]

2..A J Crisp, R Hainsworth, and S M Tutt  The absence of cardiovascular and respiratory responses to changes in right ventricular pressure in anaesthetized dogs. J Physiol. 1988 Dec; 407: 1–13(This paper actually undermines the importance of RV receptors. It is still perplexing to note both the inflow into RV (ie RA  and the out flow  pulmonary artery circuit has richly innervated by receptors , its difficult to accept why we  have failed to get much evidence for RV stretch receptors) Its potentially great area of research for cardiac physiologists. That will be a tribute to the greats like  Bain Bridge and Bazolds Jarich.)

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Why VTs have wide QRS complex?

Brief answer: VT  usually presents with wide QRS tachycardia because it originates in ventricular myocardium, travels muscle to muscle instead of the normal conduction system. However, VTs need not be wide always, if it captures the conduction system early and more proximally it can be as narrow as SVT.

Further reading: Only for cardiology fellows 

Two empirical statements are made here. (The scientific chances of both being reasonably correct are fair)

  1. 80 % of wide QRS tachycardia by default is VT. That means 20 % of wide QRS is not VT. We all accept that.
  2. 80 % of narrow QRS tachycardia is SVT. It may also mean, up to 20 % of VT can be narrow QRS.

It’s obvious, not all VTs are dramatically wide. When it is not wide, they test our knowledge and patience. Let us be clear about the factors that determine the QRS width during VT. Once we know this we can have our own inference.

What determines the width of QRS  in VT?

1.Origin of VT 

The focus of origin is extremely important. Pure myocardial focus distal to the conduction system is invariably very wide. We know VTs originating right over the fascicles are narrow.

2.His Purkinje breakthrough

The time taken to capture the normal septal conduction system is a critical determinant of QRS width during VT.This makes the VT from septal origin narrower.VT arising from the free walls obviously takes a longer time to engage the HIS Purkinje system. Imagine , If VT originates from the lateral mitral annulus,  how much time it may take to reach RV free wall and lastly RVOT. Here the VT will become bizarrely wide.

3.The structural integrity of His Purkinje

It is important to emphasize a fact , even if the VT captures HIS Purkinje early, if they are diseased , still the VT will be wider.(Example bundle branch reentry in DCM in which VT keeps going around the conduction system still, it’s wider)

4.Course 

Length of the re-entrant circuit. Macro reentry is expected to be wider. Focal or micro reentry will often be narrow, provided the distal circuit is not diseased.

5. Scars as barriers and boulders 

If the VT circuit is interrupted by random scars en-route (from origin to exit) the  VT width prolongs. (Evidence for scars is often visible in sinus rhythm ECG as notches /slurs or fragmentations in QRS )

6.Exit point of VT

This is a poorly understood term (at least for me) It is believed,  VT can exit only epicardially. The line joining the focus of origin and the exit point is expected to decide the QRS axis. The problem comes when VT breaks out multiple paths and possibly sub-endocardial as well.

7.LV dysfunction 

A severely dysfunctional ventricle can stretch the QRS irrespective of conduction system integrity.

8.The Ionic milieu of cells Interstitial resistance

We know,  biological current is nothing but Ions in motion. So, no surprise it can alter the QRS morphology. The classical example is hyperkalemia , that can make ECG a wide and blunt sine wave. Local acidosis, hypoxia also influence the QRS duration.

9.Drugs 

Any drug which has class 1C or 3 properties can slow the VT circuit velocity. Typically flecainide is well known to make QRS wider. Amiodarone may  reduce the ventricular rate. in VT instead of reverting it. Apart from this these drugs depress the ventricular myocardium severely and prolong the QRS width independent to its action on the conduction system.

10.Mechanism  of changing width 

VTs can have varying QRS width as reentrant circuits change or experience slow conduction due to autonomic influences. VT with downstream aberrancy is also possible as the VT rate by itself influences the conduction property distally.(Just lie SVT with aberrancy)

A paradox about the width of QRS in VT

A curious phenomenon is often seen, when VT occurs in patients with baseline ECG which is already wide (As in an ischemic dilated cardiomyopathy with LBBB/RBBB). Here, the VT  prematurely stimulates viable muscles distal to the diseased HIS  Purkinje system (Which they are deprived of early activation of till then) .They seem to relish the early arrival of electrical impulse by brisk activation that converts wide QRS complex to narrow one. (This  behavior is one of the principles of cardiac resynchronization therapy where we attempt to rewire the heart with multiple leads and shrink the QRS.)

*One more mechanism of wide QRS sinus rhythm becoming narrow during VT is due to a concept called source -sink relationship. The VT delivers enough energy overcoming His Purkinje resistance downstream. (This property is used in HIS bundle pacing )

Postamble

*Forget about wide vs narrow QRS debate. A significant chunk of VTs falls within intermediate width QRS(100-120ms) . Whether to label these as wide or narrow QRS  squarely lies on whims of the reader. (Should we take the widest QRS in 12 lead ECG?  Pre-cardial  vs limb lead  etc are not clear) Unfortunately, we don’t have a separate algorithm for this category. This issue demands a separate discussion.

 

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