Pediatric Anesthesia Physiology

NEONATE - FIRST 4 WEEKS OF LIFE

INFANCY - FIRST YEAR TODDLER - 1 TO 3 YEARS PRESCHOOL CHILD - 3 TO 6 YEARS SCHOOL AGE CHILD - 6 TO 12 YEARS

SIZE : WEIGHT IS 1/20 th OF ADULT LENGTH IS 1/3 rd OF ADULT SURFACE AREA IS 1/9 th OF

ADULT. HEAD IS LARGER AND SCALP HAS LESS

HAIR COVER. SURFACE AREA TO VOLUME RATIO IS 70

TIMES THAT OF ADULT.

ONE OF THE MOST IMPORTANT DIFFERENCES BETWEEN PAEDIATRIC AND ADULT PATIENTS IS OXYGEN CONSUMPTION.

DEVELOPMENT 24 DAY- FETAL LUNG BEGINS TO

DEVELOP. 24 WEEKS- PRECURSOR OF

SURFACTANT APPEARS 28 WEEKS- SURFACTANT APPEARS ALVEOLAR MULTIPLICATION

CONTINUES UPTO 8 YEARS OF LIFE. LATER ON ALVEOLAR SIZE INCREASES.

SURFACTANT IS A LIPOPROTEIN SUBSTANCE THAT HELPS TO REDUCE SURFACE TENSION AT AIR- LIQUID INTERFACE AT ALVEOLI.

PRODUCED BY TYPE II CLARA CELLS IN PULMONARY ALVEOLI.

PEAK PRODUCTION OCCURS BETWEEN 28-32 WEEKS.

INDEPENDENT LIFE IS NOT COMPATIBLE BELOW 24 WEEKS OF GESTATION.

LECITHIN/ SPINGOMYELIN RATIO < 2 INDICATES HIGH RISK FOR RESPIRATORY MEMBRANE DISEASE.

SUPRAGLOTTIC: 1.LARGE HEAD 2.SHORT NECK 3.NARROW MOUTH OPENING 4.NARROW NARES 5.LARGE TONGUE 6.EDENTULOUS JAW7.SHORT, STUBBY, OMEGA SHAPED

EPIGLOTTIS, ANGLED OVER THE LARYNGEAL INLET

8.ANTERIOR AND CEPHALAD LARYNX

GLOTTIC: 1. ANGULATED VOCAL CORDS 2. NARROW AIRWAY SUBGLOTTTIC: 1. SUBGLOTTIC NARROWING AT CRICOID. 2. TRACHEA IS NARROW AND SHORT. 3. HIGH DIAPHRAGM. 4. HORIZONTAL RIBS. 5. HIGHLY COMPLIANT CHEST WALL. 6. LESS NUMBER OF ALVEOLI. 7. LESS PERCENTAGE OF TYPE I FIBRES IN

DIAPHRAGM (10-25%) AND INTERCOSTAL MUSCLES (19-46%).

OBLIGATORY NASAL BREATHING CONTINUES FOR 2-6 MONTHS OF LIFE.

HIGH AIRWAY RESISTANCE, HIGH WORK OF BREATHING COTINUES UPTO 5 YEARS OF AGE.

LESS EXPANSION OF ANTEROPOSTERIOR AND LATERAL DIAMETER OF CHEST WALL.

POOR MAINTAINANCE OF NEGATIVE INTRATHORACIC PRESSURE.

BASAL ATELECTASIS. HIGH RESPIRATORY RATE. EARLY FATIGABILITY. RAPID DESATURATION.

DEAD SPACE : 40% OF TOTAL TIDAL VOLUME. ALVEOLAR VENTILATION: 100-150 ML/ KG/MIN.

( ADULT- 50-60 ML/KG/MIN) LOW FRC; MV/FRC RATIO= 5:1 ( ADULT- 1.5:1) CLOSING VOLUME IS HIGH. HYPERVENTILATION RESPONSE TO HYPOXIA IS

ABOLISHED BY HYPOTHERMIA. INCOMPLETE DEVELOPMENT OF MEDULLARY

RESPIRATORY CENTRES IN NEWBORN RESULT IN PERIODIC BREATHING PATTERNS.

THUS, NEONATE IS ALMOST ON THE VERGE OF RESPIRATORY FAILURE.

VARIABLE NEWBORN

6 MON

12 MON

3 YEAR

5 YEAR

12 YEAR

ADULT

RESPIRATORY RATE 50 30 24 24 23 18 12

MINUTE VENTILATION (L/MIN)

1.05 1.35 1.78 2.46 5.5 6.3 6.4

ALVEOLAR VENTILATION (ML/MIN)

385 - 1245 1760 1800 3000 3100

NO. OF ALVEOLI 30 112 129 257 280 - 300

HIGH HEAMOGLOBIN LAVEL ( 16- 18 GM%)

PRESENCE OF FETAL HB ( HbF) GREATER BLOOD VOLUME ( 80- 100 ML/

KG) GREATER CARDIAC OUTPUT/ SURFACE

AREA RATIO. THUS, HYPOXIA DEPENDS MORE ON

CIRCULATORY FACTORS RATHER THAN RESPIRATORY FACTORS.

CHILDREN BETWEEN 4-7 YEARS OF AGE MAY PRESENT WITH UPPER AIRWAY OBSTRUCTION DUE TO ADENOIDS AND ENLARGED TONSILS.

SUBGLOTTIC NARROWING CAN BE SEEN UPTO AGE OF 8 YEARS.

MOST OF THE CONTRIBUTERS OF DIFFICULT AIRWAY ARE PRESENT DURING INFANCY.

AS CHILD GROWS OLDER, THE AIRWAY DIFFICULTY GOES DECREASING.

SALIENT FEATURES OF FETAL CIRCULATION PLACENTA – ORGAN OF OXYGENATION ANATOMICAL SHUNTS: DUCTUS ARTERIOSUS

FORAMEN OVALE PHYSIOLOGICAL SHUNTS : AREAS OF

ATELECTASIS PVR > SVR , LESS PULMONARY BLOOD FLOW.HENCE, NEWBORNS HAVE LOW PaO2 AND ARE

PRONE FOR HYPOXIA. FIRST BREATH OF LIFE CHANGES

EVERYTHING…

PLACENTA REMOVED FALL IN PORTAL PRESSURE CLOSURE OF DUCTUS VENOSUS.

EXPOSURE OF DUCTUS ARTERIOSUS TO OXYGENATED BLOOD FUNCTIONAL CLOSURE OF DA.

FALL IN PVR, RISE IN SVR. FUNCTIONAL CLOSURE OF FORAMEN

OVALE.

INCREASE IN PVR: - HYPOXEMIA - HYPERCAPNEA - PREMATURITY - PULMONARY INFECTION DECREASE IN SVR : - DRUGS CAUSING SYSTEMIC VASODILATION - DRUGS CAUSING CARDIAC DEPRESSION OPENING OF SHUNTS : - CANGENITAL CARDIAC DISEASES ANY OF ABOVE FACTOR CAN CAUSE SEVERE

REFRACTORY HYPOXEMIA.

LESS CONTRACTILE TISSUE IN MYOCARDIUM. POOR COMPLIANCE OF VENTRICLES IMMATURE SARCOPLASMIC RETICULUM IN

CARDIAC MUSCLE, REDUCED STORES OF CALCIUM.

SYMPATHETIC INNERVATION IS COMPLETE BY 18-20 WEEKS OF GESTATION, BUT CARDIAC STORES OF NOREPINEPHRINE ARE LOW.

VAGAL PREDOMINANCE. THE BARORECEPTOR REFLEX IS PRESENT,

BUT INCOMPLETELY DEVELOPED AT TERM IN HUMANS.

CARDIAC OUTPUT IS PRIMARILY HEART RATE DEPENDENT. (150-200 ML/ KG/ MIN IN NEONATE). AGE GROUP CARDIAC OUTPUT

BIRTH 350-400 ML/KG/MIN

ONE WEEK 100-150 ML/KG/MIN

ADULT 70 ML/KG/MIN

DRUGS THAT AFFECT CALCIUM CHANNELS ACTIVITY, CAN CAUSE SEVERE MYOCARDIAL DEPRESSION. Eg HALOTHANE.

PREDISPOSITION TO BRADYCARDIA. BRADYCARDIA IS SIGN OF HYPOXIA,

TREATED WITH VENTILATION PRIMARILY. BECAUSE OF THE HIGH GLYCOGEN STORES

AND THE ABILITY TO USE ANAEROBIC METABOLISM EFFICIENTLY, THE FETAL/NEWBORN HEART IS RELATIVELY RESISTANT TO HYPOXIA

ARTERIAL BLOOD PRESSURE IS LOW. (SBP=50-65 MM Hg AT BIRTH ).

AGE BLOOD PRESSURE ( MM OF Hg)

HEART RATE ( / MIN)

NEWBORN

50/25 100-180

2 KG 55/30 100-180

3 KG 60/35 100-180

1 MONTH 85/65 100-180

3 MONTHS

90/65 100-180

6 MONTHS

90/65 100-180

9 MONTHS

90/65 100-160

1 YEAR 90/65 80-160

BLOOD VOLUME IS HIGH.

AGE GROUP BLOOD VOLUME

NEWBORN 90-110 ML/ KG

1-2 YEARS 75-80 ML/ KG

2-16 YEARS 70-75 ML / KG

HIGH HAEMOGLOBIN LEVEL AT BIRTH (16-19 GM%).

HAEMOGLOBIN IS PRIMARILY HbF DURING FIRST 3 MONTHS.

PHYSIOLOGICAL ANEMIA CAN OCCUR BY 3 MONTHS OF LIFE DUE TO-

1.TRANSITION FROM HbF TO HbA 2.DECREASED ERYTHROPOETIN ACTIVITY 3.DECREASED RBC SURVIVAL TIME ( 70

DAYS).

DIMINISHED RENAL FUNCTION DUE TO 1.DECREASED RENAL PERFUSION

PRESSURE 2.DECREASED GFR AND TUBULAR

FUNCTION

AGE SERUM CREATININE

1 YEAR 0.41

12 YEARS 0.61

18 YEARS 0.91

AGE GFR ( ML/ MIN/ SQ. METER)

2-8 DAYS 39 (17-60)

6-12 MONTHS 103 (49-157)

2-12 YEARS 127 (89-165)

COMPLETE MATURATION OF RENAL SYSTEM OCCURS BY 2 YEARS.

DRUGS EXCRETED BY GLOMERULAR FILTRATION HAVE PROLONGED HALF LIFE eg. ANTIBIOTICS.

INABILITY TO HANDLE FREE WATER AND SOLUTE LOAD CAN CONTINUE UPTO 20 WEEKS OF LIFE. NEONATES ARE LIABLE FOR DEHYDRATION.

NEONATES ARE OBLIGATE SALT LOOSERS.

DEVELOPMENT: - 3 WEEKS OF GESTATION: LIVER DEVELOPMENT

BEGINS - 10-12 WEEKS OF GESTATION: FETAL LIVER IS

INVOLVED IN GLUCOSE REGULATION, PROTEIN SYNTHESIS, AND LIPID SYNTHESIS AND IS CAPABLE OF SOME DRUG METABOLISM.

THE FETAL LIVER CONTAINS APPROXIMATELY THREE TIMES THE AMOUNT OF GLYCOGEN AS THE ADULT LIVER, BUT THE GLYCOGEN IS NEARLY COMPLETELY RELEASED WITHIN HOURS OF BIRTH TO COMPENSATE FOR INTERRUPTION OF THE PLACENTAL SUPPLY OF NUTRIENTS.

HEPATIC BLOOD FLOW IS LOW. ENZYME SYSTEMS FOR DRUG

METABOLISMS ARE NOT FULLY ACTIVE. DRUG EXCRETION BY BILE IS DELAYED

eg. MORPHINE HALF LIFE IS PROLONGED.

PLASMA PROTEINS ARE LOW. GLYCOGEN STORES ARE LOW,

TENDENCY FOR HYPOGLYCEMIA IN PRETERM NEONATES.

NEONATES ARE PREDISPOSED TO HYPOTHERMIA DUE TO-

1.LARGE SURFACE AREA TO VOLUME RATIO 2.DECREASED INSULATING TISSUE 3.MINIMUM ABILITY TO SHRIVER IN FIRST 3

MONTHS INFANTS ARE PRIMARILY DEPENDENT ON

NON SHRIVERING THERMOGENESIS BY BROWN FAT METABOLISM.

NEUTRAL TEMPARATURE FOR TERM NEONATE IS 32DEGREE CELCIUS.

COMPRISES OF 2-6% OF TERM INFANT’S BODY WEIGHT.

LOCATED AROUND SCAPULAE, MEDIASTINUM, KIDNEYS AND ADRENAL GLANDS.

CONTAIN MANY MITOCHONDRIA, RICH BLOOD AND AUTONOMIC BLOOD SUPPLY.

NORADRENALIN MEDIATED HYDROLYSIS OF TRIGLYCERIDES LEADS TO PRODUCTION OF HEAT.

BROWN FAT METABOLISM IS LOST UNDER ANESTHESIA.

BROWN FATS DEPOSITS DECLINE DURING FIRST FEW WEEKS OF LIFE.

HYPOTHERMIA PREDISPOSES TO – METABOLIC ACIDOSIS INCREASED OXYGEN UPTAKE RIGHT TO LEFT SHUNTING RESPIRATORY DEPRESSION AND

HYPOVENTILATION DELAYED RECOVERY FROM ANESTHESIA DELAYED METABOLISM OF DRUGS DYSARRHYTHMIA AND CARDIAC

DEPRESSION.

ECF COMPARTMENT OF NEONATE IS HIGH ( 35% IN NEONATE AS COMPARED TO 20% IN ADULT).

FLUID REQUIREMENT FOR A TERM NEONATE

DAY 1- 70 ML/ KG DAY 3- 80 ML/KG DAY 5 – 90 ML/KG DAY 7 – 120 ML/KG GOAL OF FLUID THERAPY IS TO MEET THE

METABOLIC NEEDS AND AVOID HYPOGLYCEMIA.

CALORIE REQUIREMENT IN INFANTS - 0-10 KG- 100 KCAL/ KG/ DAY 10-20 KG- 1000 KCAL+ 50 KCAL PER EACH KG

BETWEEN 10-20 KG 20 KG AND MORE- 1500 KCAL+20 KCAL/KG FOR

EACH KG OVER 20 KG. 100 ML OF WATER IS REQUIRED FOR EACH

100 CALORIES OF EXPENDED ENERGY. ACCORDINGLY, 0-10 KG- 4 ML/KG/HR 10-20 KG- 40 ML+ 2 ML/KG/HR >20 KG- 60 ML + 1 ML/ KG/ HR.

DAILY SODIUM REQUIREMENT: FOR A PRETERM NEONATE- 4-5

MEQ/KG/DAY FOR A TERM NEONATE- 2-3

MEQ/KG/DAY. DAILY POTASSIUM REQUIREMENT : 2-3

MEQ/KG/DAY HYPOGLYCEMIA- BSL< 40 MG% HYPERGLYCEMIA- BSL> 125-140 MG% HYPOGLYCEMIA IS MORE DANGEROUS

THAN HYPERGLYCEMIA.

FOR DEFICITS AND 3RD SPACE LOSS- RL FOR MAINTAINANCE- 5% DEXTROSE IN

0.45% NORMAL SALINE FOR MOST CHILDREN, LACTATED

RINGER'S SOLUTION IS THE ONLY FLUID REQUIRED.