Neonatal A & P RC 290

Neonate Compared to Adult  More compliant and flexible thorax  Large tongue  Large U-shaped epiglottis  Narrowest part of upper airway is at cricoid cartilage (where is an adult’s?)  Larynx is higher up  Carina is higher up  Both right and left main stem bronchi are at 45 degree angle to trachea  Neonate is an obligate nose breather

Neonatal PFTs (for a normal 3 kilogram infant)  Vt = ml  Respiratory rate =  VC = ml  Vd/Vt = 4.4 ml – 9.2 ml (25-40%)  FRC (24 hours after birth) = 80 ml

Thermal regulation Neonate prone to heat loss because of :  Increased surface area to mass ratio  Decreased amounts of sub-Q fat

Two Types of Heat Loss  Internal gradient – from body core to skin –Normally 1 degree C  External gradient – from skin surface to environment

Mechanisms of Heat Transfer

Methods of Heat Production  Voluntary Muscle Activity  Involuntary Muscle Activity, ie, Shivering  Non-shivering Thermogenesis –Brown Fat metabolism –Main method of heat production in the neonate

Brown Fat  2-6% of neonate’s body weight  Located around vital organs  Innervated by sympathetic nerves  Usually present until 2 months after birth

Causes of Moderate Cold Stress and Hypothermia The following impair thermogenesis. Either the infant can’t metabolize brown fat or he can’t metabolize it fast enough to keep up with the heat loss.  Asphyxia/Hypoxia  Sepsis  Intra-cranial pathology  Hypoglycemia  Environmental cooling (usually through the four mechanisms of heat transfer)

Effects of Moderate Cold Stress and Hypothermia  Increased O2 consumption and hypoxemia –Hyperthermia also increases O2 consumption –O2 consumption is lowest in a NTE (neutral thermal environment) as well as in severe hypothermia  Hypoglycemia  Metabolic acidosis  Inhibition of Surfactant production

Clinical Signs of Hypothermia (Moderate Cold Stress)  Lethargy  Cold skin  Bright red color  Slow shallow respirations  Bradycardia  Depressed CNS  Physiologic effects –Hypoxemia (due to increased O2 consumption) –Hypoglycemia –Metabolic acidosis –Lung dysfunction due to decreased surfactant

Hypothermia Treatment  Prevention is best – maintain NTE  Warm slowly otherwise O2 consumption may increase!  Heated and humidified O2  May need NaHCO3 for metabolic acidosis  Dextrose for hypoglycemia

Maintaining NTE Equipment

Isolettes  Maintain NTE by convection  Usually have double layered plexiglass walls to prevent heat loss due to radiation

Overhead Radiant Warmers  Warm by radiant heat  Used in L & D  Also used in NICU when neonate needs procedures performed outside of isolette

Mechanics of Neonatal Respiration Because of the flexible thorax, most of the resistance to breathing in the neonate is airway resistance

Inspiration Airway resistance is lowest because of negative pressure generated when intrathoracic pressure drops

Expiration Airway resistance is at it’s highest because of the increase in intrathoracic pressure. The increased airway resistance during expiration helps maintain FRC

Time Constant  TC = Cstat X Raw  Represents amount of time it takes for the proximal airway and alveolar pressure to equilibrate  TC is variable throughout the lung and during inspiration and expiration! –Expiratory TC is greater than inspiratory TC because airway resistance is highest during expiration  Normal averages: Adult =.66 seconds, neonate =.33 seconds  Knowing how pathology affects TC is crucial in ventilator management!

Decreased TC  Decreased Cstat or decreased Raw –Decreased Cstat more common in neonate RDS, atelectasis, pneumonia Decreased Cstat = increased elastic resistance  Proximal airway and alveolar pressure equilibrate more quickly when TC is decreased  May use high ventilator rates and narrow (or inverse) I:E ratios

Increased TC  Increased Raw or increased Cstat –Increased Raw more common in neonate Mucus plugs, aspirations, upper airway obstructions  Proximal airway and alveolar pressure take longer to equilibrate when TC is increased  Increased TC makes infant prone to gas-trapping so must use lower ventilator rates and wide I:E ratios to prevent lung damage and hemodynamic compromise

Neonatal Breathing Patterns Periodic Breathing  Apnea periods lasting less than 20 seconds  Heart rate and temperature not affected  Common in premature infants  Usually only need monitoring; possibly O2 therapy Apneic Breathing  Apnea periods last longer than 20 seconds  Heart rate and temperature decrease  Need monitoring, O2, and possibly ventilation –Sometimes methylxanthines help