Term 7 Week 4 (w/c 28/10/2013)
We’ve escaped the community and are back in theatre this week. You are the cardiac anaesthesia registrar, slated to cover Paeds Cardiac Surgery this week. Your first case tomorrow is an 8month old girl who is booked for a Blalock-Taussig Shunt.
Her diagnosis is listed as Tetralogy of Fallot.
What information would you like to elicit in order to help you manage this child peri-operatively?
Some interesting questions asked yesterday, thanks for the input. Most have concentrated on the degree and nature of the cyanosis, which is entirely appropriate.
You see the child. She is 8 months old, weighs 5.2kg and at rest, she is not cyanotic. The mother reports that she was born at term, was discharged routinely from hospital but soon after birth mom noticed that the child became discoloured during feeding and when she cries. She was seen and assessed at age 2 months as a tetralogy of Fallot, shunting was recommended but the pair were lost to followup until recently.
The cyanotic episodes are becoming more frequent and they are limiting the ability of the child to feed. The mother also reports that the child has “fainted” on one or two occasions. In terms of medical therapy, the baby is on a beta blocker (inderal). Mom reports that this has helped somewhat.
The child has had an evaluation by the paeds cardiology team who have demonstrated the typical findings of tetralogy of Fallot – RV hypertrophy, over-riding aorta (50%), VSD and infundibular narrowing (RVOT obstruction). During spelling the gradient across the infundibular obstruction was noted to be 50mmHg.
What are some of the relevant sequelae of ongoing cyanosis, wherever it’s clinical context? (child, adult)?
How should we manage these in this scenario?
Remember all input is welcome. yesterday was decidedly top heavy on the consultant side. In many countries, a thorough working knowledge of the management of this disease is expected of candidates for the specialist exit exam!
After assessing the child, you decide that you are happy to proceed. The Hb is 18g/DL and renal function is normal. The child is first on the theatre list for tomorrow. It is booked for left thoracotomy and modified BT shunt with a goretex graft from left subclavian to left pulmonary artery. What is the anaesthetic plan?
You have the child in theatre, induction has gone smoothly and the tube is in. As you are preparing for arterial line insertion, you notice a fall in ETCO2 and the saturation begins to drop.
What is your immediate plan of action?
A brief summary of the issues will follow tomorrow.
Thank you for the input this week. What follows is a brief summary of how I approach the child with a tetralogy of fallot. It represents nothing more than a personal approach, a rubric that works for me. Comments welcome.
Tetralogy of Fallot is a congenitally acquired cardiac condition characterised by right ventricular hypertrophy, a sub membranous VSD, overriding aorta and infundibular spasm. It can be categorised as a cyanotic defect, with low pulmonary blood flow.
Children usually present early with a history of cyanosis and hyper cyanotic spells, exacerbated with feeding and crying. Those who do not present early develop and are often seen to adopt the classical squatting pose to break the cyanotic spell.
Surgical management is usually directed at increasing pulmonary blood flow, initially with a palliative procedure, some variation of a Blalock Taussig shunt whereby pulmonary blood flow is augmented by draining the left subclavian artery into the left pulmonary artery. This is done to provide relief from the hyper cyanotic spells, and to “mature the pulmonary circulation” to a level where complete correction can be undertaken, usually at age 3-4y.
From an anaesthesiology point of view, these children provide a challenging opportunity. They are often underweight, with tenuous right ventricular function. The ongoing cyanosis is exacerbated by polycythaemia and many of the comments this week alluded to this. polycythaemia leads to increased viscosity of the blood with sluggish cerebral blood flow, worsened in states of dehydration. If the child is symptomatic or has had some sort of cerebral event preoperative venesection can be considered to reduce Hb to more manageable levels, although this is (in our setting) rarely done. The polycythaemia also has functional effects on coagulation, the hyper coagulability mentioned above is one issue. The other is that platelet function is suppressed and these children may exhibit increased surgical bleeding. The effects of the polycythaemia are exacerbated in the presence of dehydration so limit starvation time to 6hours for solids and 2 hours for liquids. The parents should be encouraged to give the child fluid right up to the 2h mark.
For those preparing for examinations, the controversial aspects in the management of Tetralogy patients coming for surgery are (a) whether or not to premeditate and (b) IV or gas induction.
Premedication presents a double edged sword. On one hand, the premeditated child is less likely to cry and struggle on induction which is advantageous as these may increase the likelihood of a hyper cyanotic spell. However, the heavy premedication required to prevent this may present the risk of respiratory depression, hypercarbia and hypoxemia. Premedication in the ward is not recommended. If your pre op holding area is monitored and access to pulseoximetry is easy, then premedication is reasonable. In my practice I typically do not premedicate. I have spoken to anaesthesiologists at major children’s hospitals and some recipes for premedication include Midazolam 0.5mg/kg PLUS Ketamine 7.5-10mg/kg po. I’m told the children are very compliant at this dose and tolerate IV insertion. Your mileage may vary but you’d need very well monitored holding areas for this technique, remembering that his cocktail can take up to 45minutes to work.
On choice of induction technique, the recommended approach is to establish IV access and do an IV induction. Again agents of choice are controversial. Ketamine probably represents the best option as these children generally are not in extremis and a conventional dose will maintain systemic vascular resistance, and augment contractility, which should limit right to left shunting. Induction with propofol is not recommended. A high dose opiate induction is also a reasonable approach. If IV access is available then a preload with 20ml/kg prior to induction will be helpful.
Typically, our practice is to perform a gas induction with Sevoflurane. The problem with this approach is that on gassing is dependant on adequate pulmonary blood flow which is obviously not present in this scenario. We take over ventilation early in order to prevent the breath holding that occurs during second stage (which feels like a very long time in these children) and the resultant hypercarbia.
Monitoring is with arterial and central access in the contralateral side. Surgery is done via a high left sided thoracotomy. The vessels are side clamped so there is no interruption in the limited pulmonary blood flow. On opening of the shunt there may be an element of reactive pulmonary hypertension due to the acute increase in flow. Analgesia is via opiates and intercostal nerve blocks. There is always potential for disaster (rupture/tearing of the vessels) and the bypass machine should be ready to run if not primed.
In terms of a broad approach to cardiac defects in children, a wise man once taught me this approach, a rubric, if you will.
Draw the anatomy – not in Frank Netter detail, but what we call a “box diagram”
Trace the path of the red blood cells, it sounds trivial, but draw it on your box diagram. It will give you a good working idea of the functional physiology of the defect.
Think. What is the effect of changes in downstream resistances? What will increasing SVR or PVR have? Decreasing?
What could go wrong? I.e what are the potential disasters
How will you manage these if they occur?
Now we know the anatomy, and the red cell path, we can look at 3, the effect of changes in downstream resistances.
SVR up leads to decreased R to L Shunt, the converse happens when SVR drops, like during an IV induction with Propofol or a careless gas induction
PVR increases worsen hypoxaemia. Decreases in PVR have little clinical effect because the RVOT obstruction usually limits flow.
Next, what can go wrong? In this case, a hyper cyanotic spell. How do we manage this? Physical maneuvers like flexing the hips or compressing the femorals increase SVR and limit right to left shunting. The same effect can be achieved with phenylephrine/neosynephrine. I usually draw up before hand, a few syringes with phenyl diluted to 1mg/kg/ml. I chase these with boluses of fluid (used to use colloids but now not so much) of 10-15ml/kg which seems to stent open the RVOT.
So this is a quick and dirty approach to TOF. The same rubric could be applied to any congenital cardiac condition.