Immediate Post-operative Management of the Modified Blalock-Taussig Shunt (BTS) & central shunts, paediatrics (386)

History & Epidemiology

The Blalock- Taussig Shunt (BTS) was first described by Alfred Blalock & Helen Taussig in Baltimore in the 1940s. ²⁸ The “classical” BTS was a direct anastomosis of the subclavian artery to the pulmonary artery (PA). This developed into the “modified” BTS in the 1970’s: anastomosis between right or left subclavian artery and branch PA with a Gore-Tex vascular prosthesis. This type of surgical technique is currently used in our department.^1-3

The annual number of BTS being performed has fallen over the last 20 years. This is largely due to a change in surgical strategy, a general reluctance to perform them as a result of their significant complication rates, and the development of transcatheter    ductal stenting technique. BTS were originally used predominately in the management of tetralogy of Fallot (ToF), and although now most ToF patients benefit from primary complete repair.  BTS remains in use for infants who have anatomical considerations that prevent early repair or ductal stenting.^1-3

Most BTS are now carried out in patients with complex biventricular or single ventricle physiology. This has coincided with an increased length of stay in the ICU (mean length of stay for an uncomplicated BTS is 3 days). ⁴

Rationale for Use

A BTS may be placed in isolation or as part of a more complex operation such as the Stage 1 Norwood for Hypoplastic Left Heart Syndrome.^2,3

The BTS is usually inserted to increase blood to flow to the lungs. The size and length of the shunt in part determine the amount of blood flow to the lungs. If the shunt is too large, this may lead to excessive pulmonary blood flow and reduced systemic blood flow described as pulmonary over-circulation. Clinically, over-circulation manifests as pulmonary oedema, high output heart failure and poor peripheral perfusion, falling NIRS, low blood pressure, low mixed venous saturations and a rising base excess and lactate. ^1,2,3,5,5-7

Pitfalls & emergencies

Excess flow via the BTS may lead to difficulties when ventilation is weaned and occasionally require emergency re-intervention, if systemic and/or coronary perfusion is significantly compromised: BTS may need to be clipped (or very rarely taken down). If patent arterial duct (PDA) is present, it may need to be ligated.

If a shunt size is too small, inadequate pulmonary perfusion will lead to hypoxia (desaturation) and poor oxygen delivery to tissues^5-7. Pulmonary and systemic perfusion need to be carefully balanced. Staff need to remain aware that a functioning BTS results in a dramatic change in physiology from the pre-operative state.

In the postoperative period, close attention to detail is required as haemodynamics can be unstable while the cardiovascular system readjusts.⁵

The immediate post-operative period is the time when the incidence of shunt failure is highest. This can present acutely with precipitously dropping saturations. Acute shunt failure is usually secondary to the shunt becoming obstructed by a blood clot or kinking. This is an emergency and the management is discussed below^1,5,8. Always auscultate for the presence of a shunt murmur when the patient returns to PICUfrom theatre.

The reported rate  of shunt thrombosis is 12%.⁹ Some studies have suggested that aspirin may reduce rate of shunt thrombosis while others have failed to prove this.^9-11,11,12 A significant number of patients may also demonstrate resistance to aspirin. ^{26, 27} Shunts sited whilst the patient is supported on cardiopulmonary bypass as part of a combined procedure are potentially at a higher risk of clot following reversal of heparinisation at the end of bypass Competing sources of pulmonary blood flow, eg PDA, increase the risk of shunt thrombosis.

All shunts have an attrition rate; a study looking at the histopathology of shunts electively taken down found 21% had a 50% stenosis at a median age of 8 months. Smaller shunts were more likely to stenose.^3,7

Ongoing anti-coagulant therapy with intravenous heparin should be instituted soon after arrival from theatre if there are no bleeding concerns.

• Check with the Anaesthetist what the last ACT result in theatre was and if there were any issues with potential heparin resistance.
• The status of the PDA should be detailed in handover (open, ligated or clipped or already completely closed).
• Confirm with the Consultant Surgeon at handover that there are no specific reasons to avoid the following standard anticoagulation approach.

Anticoagulation therapy should be managed as noted in the Anticoagulation and antiplatelet therapy in paediatric cardiac patients guideline.

Arterial and central lines should be kept in place or replaced if not functioning, for at least the immediate 24 hours post-operatively to aid clinical management.

The standard post-operative cardiac patient pathway should be followed including monitoring and blood gas analysis - see RHC Cardiac patient pathway. Triple site NIRS should be utilised until at least 6 hours post-extubation and cessation of inotropic support.

Diagnosis

Consider in any patient who has a sustained desaturationwith a systemic to pulmonary artery shunt, or if shunt murmur is no longer audible. This will be followed by hypotension if the hypoxia is severe enough to cause acidosis.  If it blocks in a patient with an alternative source of pulmonary blood flow (e.g. patent arterial duct), the blood pressure may go up!

This can be differentiated from pulmonary over-circulation where a rise in the arterial oxygen saturations, fallings NIRS and evidence of reduced systemic cardiac output (poor capillary refill, reduced blood pressure, especially diastolic) will be present.

For guidance on managing pulmonary over-circulation see section 4.

Principle of management:

• Think shunt!
• Could shunt have blocked? Act quickly!
• Most likely to occur in a new shunt or in a dehydrated patient known to have a shunt.
• It is more likely if flow is competing with an open duct (PDA) but then it is unlikely to be an emergency until the duct closes.
• Check & exclude the respiratory reasons:
• Displaced ETT position
• Obstructed ETT
• Pneumothorax
• Equipment failure
• Auscultate: – is previous shunt murmur inaudible?
• Check arterial blood gas: - low PaO2,  rising lactate, fall in NIRS from baseline
• Review most recent APTT
• If below target range increased risk of blocked shunt

Management^5,6,15

• CALL FOR HELP
• Immediately call Cardiac Surgeon, Cardiac Theatre Team, Cardiac Physiologist and Cardiologist using “PICU cardiac emergency call out sheet”
• Resuscitate – A,B,C
• Request Urgent Echo but do not wait for Echo & surgical team to initiate treatment

Meanwhile:

• Ventilate in oxygen concentration 10% above baseline (consider hand ventilation via T-piece via air/oxygen blender)
• NB may need to adjust FiO2 as clinically indicated to reduce Pulmonary Vascular Resistance (PVR)
• Sedate & paralyse once intubated
• Bring chest re-opening trolley, suction and diathermy units to bedside
• Increase systemic vascular resistance (SVR) in a stepwise fashion
• 5ml/kg aliquots of 4% human albumin
• Phenylephrine (may be useful whilst noradrenaline infusion is being prepared)  
  1. Dilute 10mg of phenylephrine in 100ml 0.9% Saline 
  2. Draw up 1ml of this solution and dilute to 10ml with 0.9% saline 
  3. Administer 0.3-1ml/kg (3-10mcg/kg) as a slow IV injection to maintain adequate BP 
• Start or increase adrenaline infusion (max 0.05mcg/kg/min)
• Start noradrenaline infusion 0.1mcg/kg/min
• Reduce pulmonary vascular resistance (PVR)   
• Increase sedation and ensure ongoing paralysis
• Increase respiratory support (hand-ventilate if needed) with aim to decrease pCO₂ – aim for respiratory alkalosis. 
• Oxygenate
• Consider bolus magnesium sulphate (0.4mmoll/kg magnesium sulphate 50%)
• Increase anti-coagulation:
• Bolus heparin 100 units/kg IV
• Start heparin infusion at 20 units/kg/hr or increase rate by 10% if infusion is running.
• Restart Prostin in neonate if duct has not been surgically ligated
• Prostin may still be helpful when ductal constriction at the proximal left PA (i.e. LPA coarctation) could be an issue
• Consider second bolus of heparin 50 units/kg 15 minutes after the first dose
• These patients are candidates to be supported with ECMO pending intervention
• Ensure they are flagged as an “ECMO watcher”
• Emergency core group MDT to decide on ECMO vs cardiac catheter vs surgical re-intervention.**

**Cardiac catheterisation may be considered by the duty Cardiologist and core group MDT if Echocardiography does not provide sufficient diagnostic information. Angiography can aid establishing presence and nature of BTS occlusion. In addition, transcatheter intervention to re-establish occluded shunt patency can be considered in specific circumstances.^{16,19,22 -25} However, it should be noted that emergency cardiac catheterisation in a haemodynamically unstable patient carries significant risks, it is also is a less expedient method of re-establishing shunt patency than an emergency surgical revision. Therefore, ifthe patient continues to deteriorate despite first line medical interventions, return to theatre or chest reopening in PICU should not be delayed. Emergency ECMO may be needed to stabilise the patient prior to re-intervention.

Think of pulmonary over-circulation (excess pulmonary blood flow) in any BTS patient with high oxygen saturations in the lead up to the circulatory collapse with poor systemic perfusion, rising lactate, falling SvO2 and falling NIRS.

The management of pulmonary over-circulation can be complex and is dependent on the anatomy and physiology of the underlying lesion.

Consultant input is essential.

Pulmonary over-circulation may reveal itself in the early post-operative period or become more problematic when ventilation is weaned. Depending on shunt size, small babies weighing less than 3kg are particularly at risk. Over-circulation is more common if the patent arterial duct or other additional sources of pulmonary blood flow are present (aorto-pulmonary collaterals).

Diagnostic clues

• Relatively high saturations (>90%)
• Review NIRS – decreased renal and then cerebral saturations
• Rising lactate
• Often tachycardic
• May have relatively low mean blood pressure
• Widening toe-core temperature gap
• Low central venous saturations
• Increase in base deficit
• CXR- oedematous lungs

If diastolic blood pressure is low, look for signs of ischaemia on ECG – ST depression.

Treatment

Mild form may be treated simply with fluid restriction and diuretics.

As this becomes more problematic, manipulation of pulmonary and systemic vascular resistance (PVR and SVR) is required.

If high lactate (>2.5mmol/l) or ECG changes are present then this is an EMERGENCY.

Inform PICU Consultant & Consultant Cardiac Surgeon & Consultant Cardiologist & Cardiac Physiologist using the PICU Emergency Call-out sheet.

Repeat Echo and 12-lead ECG.

1. Reduce SVR
1. Consider reducing vasopressor therapy if patient is on this medication
2. Consider vasodilation – eg. Milrinone vs SNP (discuss with PICU consultant)
3. Overcirculation may also be present in conjunction with a low cardiac output state therefore inotropy may be required.
4. Avoid inadvertent cooling as this will increase SVR – aim to keep the patient warm
   
   
2. Increase PVR
   1. Increase PEEP
   2. Allow pCO₂ to rise gently (reduce minute ventilation)
      
      
3. Optimise oxygen delivery
   1. Target haematocrit 35 – 45%
   2. Optimise FiO2 to target SvO2

If haemodynamic instability and/or coronary ischaemia associated with low diastolic pressure persist, surgical re-intervention for PDA ligation and/or BTS clipping may be required. ^12,3,5,5-7. IV paracetamol for PDA closure in these situations has no use due to its limited effectiveness.

Prevention is the best strategy to minimise morbidities associated with pulmonary over-circulation: adopt strategies such as inodilatator, avoid excessive vasoconstrictors and avoid inadvertent body cooling.

The guidelines are constructed after consultation with standard textbooks, a Medline search (BTS, systemic to pulmonary shunt, blocked shunt, occluded shunt, shunt failure, Streptokinase and shunt, TPA and shunt, thrombolysis, aspirin, heparin, warfarin, anticoagulation and shunt, anticoagulation in children) and local expert opinion. The best available levels of evidence were used to construct these guidelines. Level 1 evidence is lacking in this area.