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Neonatal Ventilation

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Neonatal Ventilation Neonatal Ventilation Edward G. Shepherd History • Bourgeois, 1609: – “…give a small spoonfulof pure wine into the neonate’s mouth, …[to] help the infant to regain its spirits when being agitated by the labors, which sometimes make it so weak that it seems more dead than alive.” • Mauriceau, mid-1600’s: – “[The baby] should be rested on a warmed bed and brought near the fire, where the midwife having taken some wine into her mouth shall blow it into the infant’s mouth, which can be repeated several times if necessary … She should warm all the parts of the body to recall the blood and the spirits which retired during the weakness and endangered suffocation.” Neonatology 2008;94:144–149 History • Levret, 1766: – “There is one more means which sometimes works as by enchantment, it is to apply one’s mouth on that of the infant and to blow into it, taking care to pinch the tip of the nose simultaneously. This method is so effective that it is really rare that others are useful if it fails” Neonatology 2008;94:144–149 History • First ventilators: – Hunter, Chaussier, and Gorcy in the mid 1700’s • Bellows or gas attached to masks or tubes • 1800’s – Discovery of the breathing center – Determination of survival time in asphyxiated animals – First attempts at electrical resuscitation Neonatology 2008;94:144–149 History • By mid 1800’s ventilation fell out of favor – Fears of pnuemothorax, infection, and sudden death – Various positional techniques were developed • “Schultze swingings” • Arm extension and then chest compression – Stayed in practice until after WWII Neonatology 2008;94:144–149 History • 1869, Alexander Graham Bell • 1876, Woillez – Tank ventilators • 1872, Truehard Mobile Respirator Neonatology 2008;94:144–149 History • Modern ventilators – 1907, Draeger and Blume – “the Pulmotor” • Alternating positive and negative pressure – IMV followed – By 1960, 21 neonatal ventilators were produced Ventilation • “the exchange of air between the lung and the environment.” • Uptake of oxygen and elimination of carbon dioxide Ventilation • Many modalities – Non-invasive • NCPAP, NC, hood, etc – Invasive • CMV, HVOV, HVJV, liquid Ventilation • How do we choose? – Use the least invasive modality that is adequate for the job – Once intubated, use the modality most suited to the underlying physiology Ventilation • Pulmonary physiology – Compliance: the elasticity of the system • =change in volume/change in pressure • =ml/cmH 2O – Resistance: friction of gas moving through the system • =change in pressure/change in flow • =cmH 2O/ml/sec Ventilation • Pulmonary physiology • The product of resistance and compliance is called the Time Constant – “Time necessary for a step change in pressure or volume to equilibrate throughout the lungs” Inhalation Time Constant Exhalation Time Constant Introduction • Why do we need to ventilate neonates? – Lung disease – Apnea – Airways disease Introduction • Lung disease – Hyaline membrane disease – Transient tachypnea of the newborn – Persistent pulmonary hypertension of the newborn – Meconium aspiration syndrome – Pneumonia – Amniotic fluid aspiration – Etc. Introduction • Apnea – Immature respiratory center – Impaired respiratory center Introduction • Airways diseases – Choanal atresia – Pierre-robin – Laryngeal distortion – Sub-glottic stenosis – Etc Hyaline Membrane Disease Lung Disease • Hyaline membrane disease – Delayed or insufficient production of surfactant – Increased alveolar surface tension • LaPlace Law: pressure to make a sphere smaller = 2x tension / radius Normal Physiology Normal Inspiration Normal Expiration Hyaline Membrane Disease Abnormal Respiration Exogenous Surfactant Without Surfactant Ventilation • Hyaline membrane disease – Compliance or resistance disease? • Compliance may be decreased by factor of 10 • Resistance is essentially unchanged – Time constant long or short? • Do lungs fill and empty fast or slow? – Normal time constant ~ 0.1 s • If compliance is decreased and resistance stays the same then time constant decreases proportionately Hyaline Membrane Disease Ventilation • More math – Minute ventilation = the volume of gas ventilated in 1 minute – Minute ventilation = Rate x tidal volume – Normal? • 200-300 ml/kg/min So What? • If minute ventilation is fixed then changes in rate must lead to compensatory changes in tidal volume and vice versa – MV = R x TV – Or, TV = MV/R – If MV = 240 and rate = 60, then TV = ? • TV = 4 ml/kg – If MV = 240 and rate = 20, then TV = ? • TV = 12 ml/kg HMD • Low compliance lungs, prone to damage from overdistention and atelectasis – Low compliance means short time constant, which means it takes only a brief period to fill the lungs – Should we give high volumes or low volumes for each breath? Goal should be low volumes Back to math • If we set TV low, then what must we do with the rate? (MV = R x TV) – Has to be high • Rate = 20, TV = 12 • Rate = 30, TV = 8 • Rate = 40, TV = 6 • Rate = 60, TV = 4 • Rate = 80, TV = 3 – HFOV is the logical extreme Death or BPD Weaning in HMD • Keep the TV low – As the baby improves, compliance improves, TV will increase • Wean PIP first • Wean rate last – Typical initial settings • Rate 50 – 80 • PIP to move chest adequately • PEEP? Hyaline Membrane Disease PEEP • Adequate to avoid collapse (not too low) • Too much impairs venous return and potentially gas exchange (not too high) • Just enough to provide FRC (just right!) – Typically 4-6 Continued Support • Immediate goal, no matter the size of the baby – Extubate! – Caffeinate • Minimization of CMV is associated with – Reduced rates of BPD – Improved long-term neurodevelopmental outcomes – Reduced hospital stay – Etc. Continued Support • Subsequent modes should be directed at physiology – If chest wall is compliant (collapsible) or lung disease persists • NCPAP – If chest wall is more stable • NC • Hood • RA Hyaline Membrane Disease Continued Support • High flow nasal cannula – May provide some positive pressure – Provides supplemental oxygen – May allow better developmental care – Some caveats – More study needed BPD • Different physiological process – Normal compliance but high resistance, especially during exhalation – It takes a LONG time to empty the lungs • Behaves like asthma – Rate is determined by time of exhalation • If it takes 2.5 seconds to exhale and 0.5 to inhale, the fastest rate possible is 20 – Must have a slow rate or else breaths will stack • Hyperinflation may be the result, independent of PIP Figure 4. Pulmonary function in infants with severe BPD . The results of pulmonary function testing (percent predicted) are individually plotted (black dots) along with mean results (red “x”s). Overall, patients suffered moderate to severe obstruction with mild hyperinflation, but near-normal respiratory system compliance and TLC. Back to math • In resistance diseases rate must be slow or the lungs won’t empty – If rate = 20, TV = 12 – If rate = 16, TV = 16 – If rate = 12, TV = 20 • With a slow rate, TV has to be high to achieve survival Summary • Neonatal ventilation has been used for years to save lives – Dogma isn’t always correct • Respiratory failure occurs for numerous reasons, each of which has a predominant physiology • Ventilatory support should be tailored towards the underlying physiology – Short time constant diseases require high rate, low tidal volume strategies – Long time constant diseases require the reverse.
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