DR ROWAN MOLNAR ANAESTHETICS STUDY GUIDE PART VI

Part VI: Sub-Specialty Anaesthesia

PART VI: SUB-SPECIALTY ANAESTHESIA

INCLUDES:1. Paediatrics 2. Obstetrics3. Cardiothoracic4. ENT/Head & neck5. Neurosurgery

SUBSPECIALTY ANAESTHETICS A: PAEDIATRIC

“They’re not just small adults”. . . But . . .“Nor are they all just big neonates, either”

CASE STUDY IVPaediatric Hypospadias Repair

HISTORY

4 year old boy (obviously!) Grade III hypospadias & chordee For EUA/repair Background: Mild asthma & ADHD

ISSUES Preop assessment & stabilisation Premedication Induction & IV insertion Prolonged surgery Postoperative analgesia Postoperative IV & IUDC

ADULT-PAEDIATRIC DIFFERENCES Psychosocial CNS Respiratory

AirwayOther

Cardiovascular

Renal/fluids

Gastrointestinal

Hepatic/metabolic

EndocrineHaematologicalImmunologicalMusculoskeletal Integument

THE PSYCHOSOCIAL DIMENSION There are (almost) always two patients – child and

parent(s). If you don’t keep the parents happy, or at least reassured, the child won’t be either – no matter how good the anaesthetic.

Children don’t understand that you are there to help – only that you are a stranger.

Children hate needles. Parents hate their children having needles. Even without this, cannulation can be difficult. Anything that ameliorates this is good: such as premedication, EMLA to cannula sites & inhalational inductions.

Parental presence at induction can be a good idea – as long as the parent is going to cope. If in doubt, a generous premed & a goodbye outside may be a better option.

ANATOMICAL DIFFERENCES 1

Body proportionsHead largerLimbs smallerIncreased surface

area to volume ratio

CNS differencesBrain & spinal

cord relatively larger

ANATOMICAL DIFFERENCES 2: AIRWAY

Head larger Nares (relatively)

larger Larynx higher

C3 in neonate -> C6 in adult

Epiglottis longer (& softer)

Cricoid ring narrowest part of airway

PAEDIATRIC RESPIRATORY PHYSIOLOGY

Chest wall mechanics & tracheobronchial tree “floppier”.

Tidal volume/dead space same as adults in mls/kg

Respiratory rate & minute volume higher FRC similar to adult in mls/kg, but vO2 higher,

so desaturate more quickly when apnoeic. Control of respiration immature till ~ 15/12

post conceptual age – up till then vulnerable to apnoeas – especially post GA &/or narcotics.

PAEDIATRIC CVS PHYSIOLOGY REFRESHER

Fetal circulation/Postnatal transition-predelivery: systemic & pulmonary circulations in

parallel, with oxygenation via placenta & high pressure/low flow on (R) side.

-Transition at birth to systemic & pulmonary circulations in series with fall in PVR & closure of shunts.

HaemodynamicsNeonates & infants have fixed stroke volumes: CO

dependant on HR – i.e. bradycardia = hypotension & shock.

Autonomic controlDifferent in neonates & children – response to hypoxia

is bradycardia (“Diving reflex”) rather than tachycardia.

BLOOD & BODY FLUIDS Blood volume 80-90 mls/kg (adult ~ 70) Birth Hb 180-200 g/L (adult 120-160)

Falls to ~ 110 @ 6/12 then rises. Fetal haemoglobin (HbF)

Different chainsLower p50 (Hb-O2 curve shifted

left)75% of Hb at birth minimal @

6/12. Body water 75-80% in neonate (adult

65%) ECF compartment larger than ICF

(crossover @ ~ 4/12)

PAEDIATRIC THERMODYNAMICS Infants at higher risk of hypothermia Higher surface area to volume ratio Remember the four modes of heat

loss:1. Conduction2. Convection3. Radiation4. Evaporation

All four occur more when the surface area to volume ratio is higher

HEAT PRODUCTION & REGULATION

Controlled in hypothalamus Balances heat loss & heat production Heat production

Shivering – poorly developed in neonate/infant

Metabolic thermogenesis (brown fat) Thermoneutral environment;

Point of minimum O2 consumtione.g. for unclothed term baby is ~ 33°C

SUBSPECIALTY ANAESTHETICS B: OBSTETRICS

Remember, once again you have two patients – but this time they are physically connected

CASE STUDY VCaesarian Section

HISTORY

38 yr old lady, P0G1 Booked LSCS IVF pregnancy Moderate PIH/pre-ecclampsia History of back pain Wants to be awake for delivery Needle phobic

ISSUES Preop consultation Investigations Premedication Choice of anaesthetic technique Choice of IV fluids Backup anaesthetic plan Postoperative monitoring Analgesia plan

PRINCIPLES Pregnancy is a normal, but vulnerable

condition. The prregnant patient is different Delivery is hazardous Operative intervention may be required Labour & delivery can be agonisingly painful Anaesthesia inevitably has (at least some)

foetal effects/implications.

DIFFERENCES IN PREGNANCY Psychosocial CNS Respiratory

AirwayOther

Cardiovascular

Renal/fluids

GastrointestinalHepatic/

metabolicEndocrineHaematological ImmunologicalMusculoskeletal Integument

DRUGS & THE PLACENTAGeneral rule: If it crosses the blood brain

barrier, it crosses the placenta!Placental transfer:

Narcotics/Sedatives/GA agents - HIGHMuscle relaxants -Essentially nilLocal anaesthetics – Significant (in freebase

form) . . . but peak maternal plasma levels usually post delivery