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Chapter 17

Chapter 17. Mechanical Ventilators (plus transition to extrauterine life). Heads Paradoxical Reflex.

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Chapter 17

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  1. Chapter 17 Mechanical Ventilators (plus transition to extrauterine life)

  2. Heads Paradoxical Reflex • Mediated by rapidly adapting pulmonary stretch receptors (RARs) in the lungs, with properties quite distinct from those of the slowly adapting receptors (SARs) responsible for the Breuer-Hering inflation reflex. • Called Paradoxical since it has the ability to supersede the Hering reflex which normally limits large volume inspirations. • Thought to be responsible for a babies first breath

  3. What is the primary factor that initiates breathing in a newborn infant? • The fetus lives in a relatively hypoxic environment, with a pO2 of approximately 35. • This relative hypoxia is normal for the fetus and causes the pulmonary blood vessels to constrict. • This raises pulmonary blood pressure quite high, higher than the fetus's systemic blood pressure. • So, with each heart beat, most of the cardiac output follows the path of least resistance and flows to the fetus's body. Very little flows to the fetal lungs, due to the relatively high pulmonary blood pressure. This works out just fine in utero where the fetus isn't responsible for oxygenating it's own blood, but not so well after delivery when the placenta is no longer available to provide oxygen.

  4. What is the primary factor that initiates breathing in a newborn infant? • At the moment of birth, when the baby takes it's first breath, the pO2 within the baby's bloodstream begins to rise, causing the pulmonary blood vessels to begin to relax, lowering pulmonary blood pressure. • With subsequent breaths, the pO2 continues to rise, causing pulmonary vasodilation, which drops the pulmonary blood pressure lower than the systemic blood pressure (as it should be in adult circulation), and a greater portion of cardiac output begins to flow to the baby's lungs with each heartbeat, allowing the baby to sufficiently oxygenate its own blood.

  5. What is the primary factor that initiates breathing in a newborn infant? • As the pO2 begins to rise, the fetal shunts begin to close (functionally), including the patent foramen ovale, the patent ductusarteriosus and the ductusvenosus. This changes the pattern of blood flow thru the baby's heart and body to an adult pattern. If these shunts fail to close (functionally) at birth, or structurally within a few days-weeks of birth, then the baby may experience problems such as decreased oxygenation, murmur, CHF, poor feeding, poor weight gain, etc. • If the baby is deprived of oxygen at birth, due to complications such as birth asphyxia, meconium aspiration or pneumonia, these changes may not occur, and the baby may develop a life-threatening condition called persistent pulmonary hypertension of the newborn (PPHN).

  6. What is the primary factor that initiates breathing in a newborn infant? • Perfusing its body by breathing independently instead of utilizing placental oxygen is the first challenge of a newborn. At birth, the baby's lungs are filled with fetal lung fluid (which is not amniotic fluid) and are not inflated. • The newborn is expelled from the birth canal, its central nervous system reacts to the sudden change in temperature and environment. • This triggers it to take the first breath, within about 10 seconds after delivery • With the first breaths, there is a fall in pulmonary vascular resistance, and an increase in the surface area available for gas exchange. Over the next 30 seconds the pulmonary blood flow increases and is oxygenated as it flows through the alveoli of the lungs.

  7. What is the primary factor that initiates breathing in a newborn infant? • Oxygenated blood now reaches the left atrium and ventricle, and through the descending aorta reaches the umbilical arteries. • Oxygenated blood now stimulates constriction of the umbilical arteries resulting in a reduction in placental blood flow. • As the pulmonary circulation increases there is an equivalent reduction in the placental blood flow which normally ceases completely after about three minutes. • These two changes result in a rapid redirection of blood flow into the pulmonary vascular bed, from approximately 4% to 100% of cardiac output

  8. What is the primary factor that initiates breathing in a newborn infant? • The increase in pulmonary venous return results in left atrial pressure being slightly higher than right atrial pressure, which closes the foramen ovale. • The flow pattern changes results in a drop in blood flow across the ductus arteriosus and the higher blood oxygen content of blood within the aorta stimulates the constriction and ultimately the closure of this fetal circulatory shunt. • All of these cardiovascular system changes result in the adaptation from fetal circulation patterns to an adult circulation pattern. During this transition, some types of congenital heart disease that were not symptomatic in utero during fetal circulation will present with cyanosis or respiratory signs.

  9. What is the primary factor that initiates breathing in a newborn infant? • Following birth, the expression and re-uptake of surfactant, which begins to be produced by the fetus at 20 weeks gestation, is accelerated. • Expression of surfactant into the alveoli is necessary to prevent alveolar closure (atelectasis). At this point, rhythmic breathing movements also commence. If there are any problems with breathing, management can include stimulation, bag and mask ventilation, intubation and ventilation. • Cardiorespiratory monitoring is essential to keeping track of potential problems. Pharmacological therapy such as caffeine can also be given to treat apnea in premature newborns. A positive airway pressure should be maintained, and neonatal sepsis must be ruled out

  10. What is the primary factor that initiates breathing in a newborn infant? • Potential neonatal respiratory problems include apnea, transient tachypnea of the newborn (TTNB), respiratory distress syndrome (RDS), meconium aspiration syndrome (MAS), airway obstruction, PPHN and pneumonia/Sepsis. • PPHN can be a result of idiopathic means or as a result of persistent pulmonary vascular resistance. The treatment involves treatment of the underlying cause, surfactant delivery, PPV, Nitric Oxide, HFV, ECMO, Prostglandins, steroids and Oxygen

  11. Energy metabolism • Energy metabolism in the fetus must be converted from a continuous placental supply of glucose to intermittent feeding. • While the fetus is dependent on maternal glucose as the main source of energy, it can use lactate, free-fatty acids, and ketone bodies under some conditions. • Plasma glucose is maintained by glycogenolysis • Glycogen synthesis in the liver and muscle begins in the late second trimester of pregnancy, and storage is completed in the third trimester

  12. Energy metabolism • Glycogen stores are maximal at term, but even then, the fetus only has enough glycogen available to meet energy needs for 8–10 hours, which can be depleted even more quickly if demand is high. Newborns will then rely on gluconeogenesis for energy, which requires integration, and is normal at 2–4 days of life. • Fat stores are the largest storage source of energy. At 27 weeks gestation, only 1% of a fetus' body weight is fat. At 40 weeks, that number increases to 16%. • Inadequate available glucose substrate can lead to hypoglycemia, fetal growth restriction, preterm delivery, or other problems. Similarly, excess substrate can lead to problems, such as infant of a diabetic mother (IDM), hypothermia or neonatal sepsis.

  13. Energy metabolism • Anticipating potential problems is the key to managing most neonatal problems of energy metabolism. For example, early feeding in the delivery room or as soon as possible may prevent hypoglycemia. • If the blood glucose is still low, then an intravenous (IV) bolus of glucose may be delivered, with continuous infusion if necessary. Rarely, steroids or glucagon may have to be employed.

  14. Temperature Regulation • Newborns come from a warm environment to the cold and fluctuating temperatures of this world. • They are naked, wet, and have a large surface area to mass ratio, with variable amounts of insulation, limited metabolic reserves, and a decreased ability to shiver. • Physiologic mechanisms for preserving core temperature include vasoconstriction (decrease blood flow to the skin), maintaining the fetal position (decrease the surface area exposed to the environment), jittery large muscle activity (generate muscular heat), and "non-shivering thermogenesis".

  15. Temperature Regulation • "non-shivering thermogenesis "occurs in "brown fat“ which is specialized adipose tissue with a high concentration of mitochondria designed to rapidly oxidize fatty acids in order to generate metabolic heat. • The newborn capacity to maintain these mechanisms is limited, especially in premature infants. As such, it is not surprising that some newborns may have problems regulating their temperature. As early as the 1880s, infant incubators were used to help newborns maintain warmth, with humidified incubators being used as early as the 1930s.

  16. Temperature Regulation • Basic techniques for keeping newborns warm include keeping them dry, wrapping them in blankets, giving them hats and clothing, or increasing the ambient temperature. More advanced techniques include incubators (at 36.5°C), humidity, heat shields, thermal blankets, double-walled incubators, and radiant warmers while the use of skin-to-skin "kangaroo mother care" interventions for low birth-weight infants have started to spread world-wide after its use as a solution in developing countries.[

  17. Mechanical Ventilation • http://www.youtube.com/watch?v=dVqurKOJuD8 • http://www.youtube.com/watch?v=9gq34tkskkE • http://www.youtube.com/watch?v=EhQxO8pVy0A

  18. Support devices • Baby log • HFNC / NC • Transport • Neo Puff • Servo 1 • HFOV • Avea • HFV Jet

  19. RT Equipment in NICU • Nasal Cannula (0.25-2L), typically set below 1L, always with a blender and a bubble humidifier. Used for oxygenation issues only, or A’s and B’s, weaning off of PPV • Oxyhood: set 7-12 L, with a heated humidifier, blender, temperature probe and O2 analyzer. Used when high FIO2 required/pneumos for Nitrogen washout • HFNC: used when higher flows are required up to 8L, given with heated humidifier/circuit and special cannula. Always use with a blender; may be used when weaning from CPAP/vent • NCPAP: used for persistent grunting/retractions where surfatant is not required. PEEP set from 2-6, given through prongs/mask, through a stand alone machine or through the ventilator

  20. RT Equipment in NICU • Nasal SIMV: Used through a SiPAP machine or ventilator, essentially the same as SIMV-PC except given through the nose • Mask CPAP/PPV through a flow inflating bag, may also give through a T-piece/NeoPuff for short term relief • Invasive mechanical ventilation: Through a ETT, typically set in SIMV mode in PC, or a volume targeted mode, rates set between 15-30, Pressures are set anywhere from 10-25, FIO2 kept as low as possible, IT anywhere from 0.2-0.8 seconds • HFV: given as Jet ventilation or HFOV • Nitric Oxide: typically given in tandem with HFV • ECMO: again given in tandem with HFV or mechanical ventilation Other equipment: HHN (although far less common in NICU), CPT via mini massager, ABG/CBG supplies, suction equipment, airway supplies)

  21. RT Equipment • NOTE: • All oxygen delivering equipment in the NICU and PICU setting will utilize a humidifier. High flow devices will use a heated humidifier • The use of a blender is also common with most equipment as is a oxygen analyzer • Use of aerosols are uncommon due to the noise factor; except for HHN • Suction pressures are lower, and flows and FIO2 levels are lower, as are PEEP levels

  22. Introduction • The primary objective of Mechanical Ventilation is to support breathing until patient respiratory efforts are sufficient. • First mechanical ventilation for a neonate in 1959. • One of the most important breakthroughs in the history of neonatal care. • Mortality from RDS decreased markedly after MV. • New Morbidity developed, CLD (BPD)

  23. Indications • Apnea (prolonged or repetitive unresponsive apnea associated with bradycardia or cyanosis). • Respiratory failure in newborns:       PaO2 < 50 mmHg on FIO2 ≥ 0.6       PaCO2 > 60 - 65 mmHg (> 55 in infants < 1500 gm)       pH < 7.20 • Impending ventilatory failure (worsening oxygenation and/or respiratory distress    [↑ RR > 60 infants; > 40 children], retractions, grunting, nasal flaring even when    ABG values are within acceptable ranges) (anticipation of worsening lung pathology)

  24. Indications • There are no well defined criteria for when to initiate MV in infants and children. Many clinical factors come into play and must be individualized for each patient's problem.Early intubation and MV is recommended in many situations:•     Congenital anomalies affecting ventilatory function (diaphragmatic hernia)•     Infants with low Apgar scores and responding poorly to resuscitation efforts•     Infants with severe sepsis or compromised pulmonary blood flow (PPHN)•     Premature babies < 1000 gm•     Progressive atelectatic disease 

  25. Indications • Scheduled surgical procedure • Any acute or chronic cardiopulmonary insufficiency • May be due to problem with lung, cardiovascular system, CNS, or various metabolic disorders Clinical signs: • Repeated A-B spells

  26. Indications • The oxygenation index is a calculation used to assess FIO2/Pressure requirements to achieve a PaO2 • A lower oxygenation index is better - this can be inferred by the equation itself. As the oxygenation of a person improves, they will be able to achieve a higher PaO2 at a lower FiO2. This would be reflected on the formula as a decrease in the numerator or an increase in the denominator - thus lowering the OI. Typically an OI threshold is set for when a neonate should be placed on ECMO, for example >40,>30 HFV

  27. Blood Gas Scoring System For Assisted Ventilation  * A score of 3 or more indicates the need for CPAP or IMV.  Ambient O2 failure → CPAP  CPAP failure (10 cm H2O & FIO2 1.0) → IMV** May indicate the need for CPAP or IMV by itself, if cyanotic heart disease not present. 

  28. Contraindications • < 23 weeks gestation (?) or birth weight of less than 400 g (ref: NRP) • Congenital anomalies incompatible with survival (anacephaly, lethal genetic disorders) • Severe prolonged code with no reasonable chance of survival •  NOTE:  Parental involvement in the decision not to treat is vital. • Untreated/unvented pneumothorax (a contraindication for all PPV in all age populations)

  29. Neonatal Physiology Affecting Ventilation • Compliant chest wall and weak cartilaginous support of airways (excessive inspiratory efforts will collapse upper airway and lungs, increasing Raw and decreasing Vt) • Horizontal ribs and flatness of diaphragm reduce potential lung expansion and Vt • Peripheral Raw is 4x > than older children and adults • Distal airway growth lags behind proximal airway growth leading to increased peripheral Raw • Possible R-L shunting (PDA and/or foramen ovale) (L-R shunt through PDA increases the risk of pulmonary edema)

  30. Neonatal Physiology Affecting Ventilation • Increased risk of atelectasis and airway closure due to paucity of collateral ventilation between alveoli • Surfactant deficiency (↓ CL, ↓, FRC; may grunt and/or shorten Te to maintain FRC) • Postnatal clearance of lung liquid and ↑ pulmonary interstitial fluid • High metabolic rate • ↓ Muscle mass, ↓ oxidative capacity, ↓ Type 1 (slow twitch) muscle fiber

  31. Time Constant: An index of how rapidly the lungs can empty. • Time constant = Compliance X Resistance • In BPD time constant is long because of increased resistance. • In RDS time constant is short because of low compliance. • Normal = 0.12-0.15 sec

  32. Time Constant • Inspiratory time must be 3-5 X time constant • One time conststant = time for alveoli to discharge • 63% of its volume through the airway. • Two time constant = 84% of the volume leaves • Three time constant = 95% of volume leaves. • In RDS: require a longer I time because the lung will empty rapidly but require more time to fill. • In CLD: decrease vent rate, which allows to lengthen the I time and E time.

  33. Relationship to FRC

  34. Neonatal Ventilation • Time Cycled and Pressure Limited Ventilation with SIMV (most common type of conventional ventilation in the NICU) • Inspiration is stopped when the selected inspiratory time has been reached • PIP is the maximum amount of pressure exerted on the patient’s airway during the inspiration • Initial values = 16-20 cmH20 of PIP • Good chest rise and Good breath sounds

  35. Neonatal Ventilation Volume Controlled Ventilators: A preset volume of gas is delivered to the system after which inspiration is terminated. • When this gas has been delivered by the piston inspiration is terminated. • Tidal Volume • 4-6 ml/kg in low–birth-weight preemies • 5-8 ml/kg in term infants • 7-10 ml in pediatrics and adolescent patients • Volume losses by leaks from tubing system around the endotracheal tube. • Not common, unless using Volume targeted modes, such as with the Baby Log by Drager.

  36. Neonatal Ventilation • Peep = Positive pressure maintained in the patient’s airway during expiration; typically set between 3-5 cmH2O in most babies due to low FRC. Rarely do you go above 6 or below 3. • Prevents collapsed alveoli • Increases FRC • Improves compliance • Improves oxygenation • Decreases intrapulmonary shunting • Allows for lower PIPs to be used

  37. Mechanical Ventilation: Modes • All modes are available to the neonate • Time cycled IMV (with pressure limiting) • Newer neonatal vents may allow volume cycled IMV • Newer neonatal ventilators can do A/C volume cycle or pressure control

  38. Initial Setting on neonatal vent • Time cycled – Pressure Limited ventilator • PIP set 15 – 20 cm H20 • Achieve VT range of 4-6 ml/kg • Peep set 3 – 5 cm H2O (assess MAP, CXR 8-9 ribs expanded) • Rate set 20 – 40 bpm • Flow set 6 – 8 lpm • I time set .3 - .5 seconds for LBW and .5 - .8 seconds for larger infants • Keep alarms tight/set in SIMV mode, may use PSV 3-5

  39. Settings • PIP – good chest excursion, good lung aeration • Vt in pressure control = PIP – PEEP • Vt in pressure control changes with change in compliance and resistance • PIP set – change only with changes in compliance and resistance in 2 cm increments

  40. Inspiratory Time

  41. Positive End Expiratory Pressure

  42. Managing Ventilator Settings

  43. Inspiratory Trigger Mechanism • Time • Controlled MechanicalVentilation – NO patient interaction • Pressure • Ventilator senses a drop in pressure with patient effort • Flow • Ventilator senses a drop in flow with patient effort • Chest impedance / Abdominal movement • Ventilator senses respiratory/diaphragm or abdominal muscle movement • Diaphragmatic activity • NAVA- Neurally adjusted ventilatory assist • http://www.youtube.com/watch?v=fq2cna71G_o

  44. Target Values: MAP Mean Airway Pressure • Average pressure exerted on the airways from the start of one inspiration until the next • Is affected by IT, PIP, Rate, and PEEP • Baro/Volutrauma seen with values above 12 cmH2O • It is the most powerful influence on oxygenation!

  45. CPAP vs PEEP • Same distending alveolar pressure • PEEP is used in conjunction with ventilator rate • CPAP is used in spontaneously breathing patient, typically with use of nasal prongs or mask, not common with ETT in place, although PSV can be used along with CPAP

  46. Methods of administering CPAP • Endotracheal Tube • Patent airway, airway clearance • Disadvantage: plugging, malacia, infection • Nasal Prongs • Decrease infection, no malacia • Disadv. = plugging,pressure necrosis, gastric distention • Nasopharyngeal • Pressure necrosis, infection • Face Mask • Temporary measure prior to intubation or for apnea episode

  47. CPAP Indications: • Refractory Hypoxemia • PaO2 < 50 on an FIO2 of 60% or > • Many hospitals use 50% as the upper limit before changing to CPAP • Transitional therapy between simple O2 therapy and mechanical ventilation • Usually in the early stages of a disease or when recovery starts • Any disease that causes increased elastic resistance and alveolar instability

  48. CPAP: EFFECTS • Increased FRC , ie, back towards normal • Decreased shunt • Adequate PaO2 at minimal FIO2 • W.O.B. ? • By increasing FRC, CPAP should decrease the W.O.B. • However, it requires active exhalation which increases W.O.B. • To go on CPAP an infant needs to be breathing spontaneously and to have normal (or slightly lowered) PaCO2

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