Applied anatomy and physiology of paediatric anaesthesia

APPLIED ANATOMY AND

PHYSIOLOGY OF

PAEDIATRIC

ANAESTHESIA

Dr. KHAIRUNNISA BINTI AZMAN

Anaethesiology department TGH

What Makes Pediatric Anesthesia Different?

Airway!! Airway!!! Airway!!!

• HYPOXIA is the most common cause of pediatric perioperative cardiac arrest. – INFANTS TURN BLUE FAST. UPPER AIRWAY OBSTRUCTION

during anesthesia (particularly at induction and emergence) is a fairly common phenomenon.

• Complete and/or Partial LARYNGOSPASM is a fairly common problem

• Infants and young children ARE NOT SMALL ADULTS. "One size fits all" DOES NOT APPLY.

Difference between adult VS Paediatric airway

Tongue Larger in propotion to the oral cavity than in adult

Epiglottis Narrower, U-Shaped, flops posteriorly

Larynx High & anterior. Level of C3-C4. (C5-C6 in adult)

Cricoid More conically shaped in infants, narrowest at cricoid ring whereas in adult it is at level of vocal cords

Trachea Deviated posteriorly & downwards Become anatomically similar to adult between 8-10 yo

Head large head, short neck & prominent occiput

Sniffing position will not help bag mask ventilation or to visualise the glottis Head needs to be in neutral position

AIRWAY & RESPIRATORY SYSTEM

• Neonates preferentially breathe through their nose – Narrow nasal passage easily blocked by

secretions & may be damaged by NG tube/ Nasal ETT

– 50% Airway resistance is from nasal passages

• The airway is funnel shaped & narrowest at level of cricoid cartilage – The epithelium is loosely bound to underlying

tissue

– Trauma to the airway easily results in oedema

– 1mm of oedema can narrow a baby’s airway by 60%

• Narrowest at cricoid rather than vocal cords

• Tube may be small enough to pass through cords but not cricoid

• Larynx is funnel shaped, so secretions accumulate in retropharangeal space

Ideal maneuvre is combination of jaw thrust & chin lift, keeping the mouth open

• It is suggested that a leak present around the ETT to prevent trauma resulting in subglottic oedema & subsequent post-extubation stridor

• Neonates& infant have limited respiratory reserve:

– Horizontal ribs prevent the “buckle-handle” action seen in adult breathing and limit an increase in TV

– Ventilation is primarily diaphragmatic

– Bulky abdominal organ/ stomach filled with gases from poor bag mask ventilation impinge chest content & splint diaghragm inadequate ventilation

• The ribs are cartilaginous & perpendicular relative to the vertebral column (Horizontal), reducing the movement of the rib cage

• The infant chest wall is remarkably compliant & compliance decrease with increasing age • Subsequently the functional residual capacity (FRC) is

relatively low. • FRC ↓ with apnoea & anaesthesia causing lung collapse

• The closing volume is larger than FRC until 6-8 yrs of age

– Increase tendency for airway closure at end of expiration

– Thus, neonates & infant generally need IPPV during anaesthesia & would benefit from a higher RR & the use of PEEP (High RR to maintain FRC)

– CPAP during spontaneus ventilation improves oxygenation & decreases the work of breathing

• Work of respiration may be 15% of O2 Consumption

• Muscle of ventilation are easily subject to fatigue d/t low percentage of Type I muscle fibres in diaphragm. – The num ↑ to adult level over 1st year of life

• The alveoli are thick walled at birth: – There is only 10% of the total number of alveoli

found in adults

– The alveoli clusters develop over the first 8 yrs of life

• Apneas are common post-operatively in premature infants – Significant if last longer than 15 sec & are

associated with desaturation & bradycardia.

NEONATAL CARDIAC PHYSIOLOGY • The transformation to neonatal circulation occurs with

the first few breaths, involes 2 major changes: • A marked increase in systemic resistance • A marked decrease in pulmonary resistance

• Remnats:Patent Foramen Ovale & Ductus Arteriosus

• The patent ductus contracts in the first few days of life & will fibrose within 2-4 wks

• Closure of foramen ovale is pressure dependant & closes in the 1st day of life but may reopen within the next 5 years

• Neonatal pulmonary vasculature reacts to the rise in PaO2 & pH & the fall in PaCO2 at birth

• However, alterations in pressure & in response to hypoxia & acidosis, reversion to the transitional circulation may occur in first few weeks after birth.

CARDIOVASCULAR SYSTEM

• In neonates Myocardium less contractile causing the ventricles to be less compliant & less able to generate tension during contraction

– Limits the size of stroke volume

– Cardiac output therefore rate dependant

– Infant behaves as with fixed cardiac output state

• Cardiac output

– 300-400 ml/kg/min at birth

– 200 ml/kg/min within few months

• Vagal parasymphathetic tone is most dominant which makes neonates & infants more prone to bradycardias

• Bradycardia:

– Assc with reduced cardiac output

– If assc with hypoxia, should be treated with O2 & Ventilation initially

– Cardiac compression will be required in neonate with HR 60 or less OR 60-80bpm with adequate ventilation

• sinus arrythmia is common in children, other irregular rhythm are abnormal

• BP is low at birth (approx. 80/50) secondary to a low SVR, d/t large propotion of vessels-rich tissue in children

• BP increases within the 1st month to approx. 90/60

• Reach adult levels at approx. 16 y.o • Neonates have a reactive pulmonary vasculature

– Reversion to transitional circulation may occur during 1st few weeks of life, precipitated by an increase in PVR (Eg: acidosis, hypoxia, hypercapnia) & decrease in SVR (eg: most anaesthetics)

CARDIOVASCULAR SYSTEM

Infant kidneys : Immature at birth, thus:

• ↓ GFR/ Renal blood flow

– Till 2yo, d/t high renal vascular resistance

• ↓ Concentrating capacity

– U/O 1-2mls/kg/hr

• ↓ Na reabsorption

– Tubular function is immature until 8 months, so infants are unable to excrete a large sodium load

• ↓HCO3/H exchange

RENAL SYSTEM

• Dehydration:

– Poorly tolerated

– Premature infants have increased insensible losses as they have large surface area relative to weight

– There is larger proportion of ECF in children (40% BW as compared to 20% in adult)

• Conclusion:

– Newborn kidneys has limited capacity to compensate for Volume EXCESS or Volume DEPLETION

HEPATIC SYSTEM

• Liver fx is initially immature with decreased function of hepatic enzymes

• Barbiturates & opiods for example have a longer duration of action d/t slower metabolism

GLUCOSE METABOLISM

• Hypoglycaemia is common in stressed neonate glucose level should be monitored regularly

• Glycogen stores are located in the liver & myocardium

• Neurological damage may result from hypoglycaemia – Prevention: IVI D10%

• Infants & older children maintain blood glucose better

• Hyperglycaemia is usually iatrogenic

HAEMATOLOGY

• At birth, 70-90% of Hb molecules are HbF. – Within 3 months, levels drop to around 5% & HbA predominates – HB in newbown ~ 18-20g/dL , HCT ~ 0.6 – 3-6 Mo : 9-12 g/dl as the increase in circulating volume

increases more Rpidly the bone marrow function

• HbF combines more rapidly with 02 but release less readily as there is less 2,3-DPG.

• O2 dissociation curve shifts to the right as the level of HbA & 2,3-DPG rise.

• Vit K dependant clotting factor (II, VII, IX, X) & PLT fx are deficient in first few months

• Transfusion recommended when 15% of the circulating volume has been lost.

Temperature control

• Poorly developed shivering, sweating & vasoconstriction mechanism d/t: – Large surface area to weight ratio

– Minimal subcutaneous fat

• Heat loss during anaesthesia d/t: – Conduction

– Convection & evaporation

• Optimal ambient temp to prevent heat loss: – Premature infant: 34⁰C

– Neonates: 32⁰C

– Adults: 28⁰C

• The fetus floats in warm amniotic fluid that is maintained at a temperature of approximately 98.6°F with very little fluctuation.

• Birth exposes newborns to a cooler environment in which they have to regulate their own body temperature.

• Newborns have a higher ratio of surface area to volume than adults. - This means that their body has less volume

throughout which to produce heat, and more surface area from which to lose heat.

- As a result, newborns produce heat more slowly and lose it more quickly.

• Effect of low body temp:

– Causes respiratory depression

– Acidosis

– Decreaswd cardiac output

– Increases duration of action of drugs

– Decrease platelet function

– Increases risk of infection

Temperature control

CENTRAL NERVOUS SYSTEM

• BBB is poorly formed – Drugs (barbiturates, opioids, antibiotics, bilirubin)

cross BBB easily cause prolong & variable duration of action

• Cerebral vessels in preterm infant are thin walled & fragile. – Prone to IVH

– Risk increased with hypoxia, hypercarbia, hypernatraemia, low HCT, Awake airway manipulation, rapid bicarb administration, & fluctuation in BP & CBF

Different Anatomy

Different Physiology Different Pharmacology

Different psychology ↓↓↓↓↓

Different approach & preparation

CONCLUSIONS ???

Be aware of:

• Sudden changes in hemodynamics

• Unexpected responses

• Unknown congenital problem

Most of the complications that arise are

attributable to a lack of understanding of these

special considerations prior to induction of

anesthesia

THANK YOU

PRACTICALITIES FOR ANAESTHETISING CHILDREN

PRE-OPERATIVE VISIT

• Evaluates: – Medical conditions of the child – The needs of planned surgical procedure – Physiological makeup of patient & family

• Weight; all drugs must be calculated according to weight • Investigations may occasionally be necessary:

– HB: Large expected blood loss, premature infant, systemic disorder, congenital heart disease

– Electrolytes: Renal or metabolic disease, IV Fluid, dehydration – CXR: Active respiratory disease, scoliosis, congenital heart

disease

• Discuss regarding post OP pain Mx – If suppository medications to be used Explain & get consent

PRE-OPERATIVE FASTING

Solids 6 Hours

Formula milk 4-6 hours

Breast milk 3-4 hours

Clear fluids 2 hours

PREMEDICATIONS

Sedations Analgesics

Midazolam Chloral hydrate

Ketamine Clonidine

Paracetamol Ibuprofen

Codeine phosphate EMLA Cream

BASIC SET UP

TABLE & WARMER

MACHINE/ CIRCUIT

MONITOR

SUCTION & AIRWAY

EQUIPMENT IV ACCESS DRUGS

Breathing Systems and Circuits

• The Pediatric Breathing Circuit is extendable up to 60 inches, and comes with a 1L (default) latex free reservoir bag.

• The Neonatal Breathing Circuit is a fixed length circuit which commands very low compliance loss volume.

• The neonatal circuit comes with a 0.5L (default) reservoir bag. The Y connector is designed to minimize anatomical dead space.

INTUBATION & INDUCTION

Greater alveolar to FRC ratio

High cardiac output to vessels

rich organ (eg: Brain)

Reduced tissue blood solubility

EFFECT OF FAST INDUCTION

INDUCTION: 1. IV Induction

2. Gas induction

FLUIDS MANAGEMENT

• Perioperative fluid management is divided into three phases

– Maintenance, deficit and replacement of losses.

• Administration: Volumetric chambers/ Microdrip/Infusion Pump

• Warm fluid/blood/blood product

• Include dextrose in maintainance hydration fluid if needed

– Risk of hypoglycaemia higher in premature babies

DAY 1 OL 50MLS/KG/HR D10%

DAY 2 OL 100MLS/KG/HR D10% ½ NS

> DAY 7 OL 150MLS/KG/HR D5-D10% ¼ NS

3RD SPACE FLUID LOSS

Intra-abdominal surgery 6-10 mls/kh/hr

Intra-thoracic surgery 4-7 mls/kg/hr

Eye surgery 1-2 mls/kg/hr

Neurosurgery

Superficial surgery

Fluid deficits: • Calculated and replaced based on duration of fasting, presence

of associated conditions like • Fever, • Vomiting,diahorrea, sweating • Particular disease state or surgical problem likely to affect

fluid status (bowel obstruction, peritonitis etc). INTRAOPERATIVE FLUID LOSS 3RD Space fluid loss & blood loss

ESTIMATED BLOOD VOLUME (EBV)

Premature Neonate 90-100mls/kg

Term neonate 80-90mls/kg

3mo – 1yr 75-80mls/kg

3-6 yrs 70-75mls/kg

>6 yrs 65-70 mls/kg

Allowable blood loss

ABL = WEIGHT x EBV X (H₀ - H₁)/Hₐ

H₀ = Starting Hematocrit

H₁ = Lowest acceptable hematocrit

Hₐ = average hematocrit

Intraoperative blood loss replacement is done with Ringer’s lactate 3 ml per 1ml of blood loss, 1 ml of colloid solution for each ml of blood loss and 0.5 ml of red cell concentrates for each ml of blood loss.

PAIN MANAGEMENT

• Regional:

– Caudal block

– Ilioinguinal block

• Local anaesthesia

• Post operative analgesia (Syp/Supp)

THANK YOU AGAIN

REFERENCES:

• Dr. M.N.Chidananda Swamy, Dr. D. Mallikarjun. Applied aspect of anatomy and physiology relevance to apaediatric anaesthesia. Indian J Anaesth 2004

• Sue clark. The differences of anaesthetic care in paediatrics compared to adult. The association of paediatric practice 2010

• F. Macfarlane. Paediatric anatomy & physiology and the basic of paediatric anaesthesia. Anaethesia tutorial of the week

• Paediatric anaesthesia digital book

• Ahmad A.L. Paediatric anaethesia basic and beyond presentation