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Pediatric Ventilation: Physiology

Key Points

  • Ventilation of pediatric patients is challenging due to the age-related differences in pulmonary physiology from premature infants to young adults as well as the impact of underlying disease.
  • The risk of ventilatory-induced lung injury has fostered the use of strategies intended to prevent lung injury, but the optimal approach for any given patient remains to be defined.
  • Apparatus dead space can significantly impede effective ventilation and should be actively managed, especially in small patients.

Age-Related Respiratory Physiology

  • Newborns/infants have the fewest alveoli, leading to lower lung compliance.1 This age group also exhibits lower chest wall muscle tone resulting in less outward force of the thoracic wall. This creates an imbalance against the inward forces of lung recoil, which leads to a lower functional residual capacity (FRC).1 The lack of chest wall muscle tone also leads to a closing volume that is higher than FRC, leading to increased airway collapse.
  • Awake infants employ a higher respiratory rate, shorter expiratory time, and active vocal cord narrowing that reduces expiratory flow,1 all to optimize for FRC.
  • Fewer alveoli, lower FRC, and smaller airway diameters all contribute to a higher airway resistance in this age group.1
  • Lung physiology in older children is characterized by significantly more alveoli with continued multiplication even into adolescence with an accompanying decrease in airway resistance and an increase in lung compliance.2 By 2 years of age, increased chest wall musculature and rib ossification allow for an adequate counterbalance to lung recoil, providing passive maintenance of FRC.3

Lung Protective Ventilation in the Anesthetized Patient

  • Early studies of normal physiology documented mammalian tidal volumes in the range of 5 – 7mL/kg ideal body weight (IBW).4 Subsequent studies support the recommendation for similar tidal volumes to prevent ventilator-induced lung injury during surgery.5
  • Positive end-expiratory pressure (PEEP) is recommended as part of a lung protective ventilation strategy. General anesthesia results in a reduction in FRC by up to 20% in adults, 35% in children and adolescents, and 44% in infants.6 The addition of PEEP helps maintain FRC and thereby increase pulmonary compliance.
  • Strategies for titrating PEEP to optimize pulmonary compliance have been explored in adults. There is evidence that up to 12cm H2O of PEEP can be used without hemodynamic compromise in children7 and that a PEEP of 8 – 10cm H2O optimizes compliance in children younger than 7 years.8 PEEP is indicated to maintain lung volume for almost all patients. Exceptions may include patients with elevated intracranial pressure or patients susceptible to venous return impairment e.g., congenital heart disease or hypovolemia.
  • Clinical studies on the impact of lung protective ventilation on pediatric outcomes in the operating room are limited and not definitive. However, it is reasonable to ventilate pediatric patients intraoperatively with tidal volumes of 5 – 7mL/kg IBW plus PEEP. Individual ventilator settings should be guided by monitoring gas exchange with blood gas information when available or pulse oximetry using an FiO2 less than 0.3 and capnography.9

Dead Space Considerations

  • Dead space is any portion of the breathing circuit or lungs where there is bidirectional gas flow without gas exchange. In a circle system, apparatus dead space is between the circuit y-piece and the patient.
  • Apparatus dead space volume is increased by adding devices like heat and moisture exchangers (HME) and tubing extensions.
  • Increasing dead space causes an exponential increase in PaCO2 if minute ventilation is unchanged.
  • Efforts should be made to limit apparatus dead space volume to one-third of the tidal volume. For smaller patients, this effort should include using a compact HME or specialized connector for sampling respiratory gases and reducing the amount of circuit extension.10

References

  1. Stark AR, Cohlan BA, Waggener TB, et al. Regulation of end-expiratory lung volume during sleep in premature infants. J Appl Physiol. 1987; 62(3):1117-23. PubMed
  2. Gerhardt T, Hehre D, Feller R, et al. Pulmonary mechanics in normal infants and young children during the first five years of lige. Pediatr Pulmonol. 1987; 3(5): 309-16. PubMed
  3. Papastamelos C, Panitch HB, England SE, et al. Developmental changes in chest wall compliance in infancy and early childhood. J Appl Physiol. 1995: 78(1): 179-84. PubMed
  4. Stahl WR. Scaling of respiratory variables in mammals. J Appl Physiol. 1967; 22(3): 453-60. PubMed
  5. Futier E, Constantin JM, Paugam-Burtz C, et al. A trial of intraoperative low tidal volume ventilation in abdominal surgery. N Engl J Med. 2013; 369(5): 428-37. PubMed
  6. Von Ungern- sternberg BS, Hammer J, Schibler A, et al. Decrease of functional residual capacity and ventilation homogeneity after neuromuscular blockade in anesthetized young infants and preschool children. Anesthesiology. 2006; 105(4): 670-5. PubMed
  7. Ingaramo OA, Ngo T, Kehmani RG, et al. Impact of positive end-expiraroty pressure on cardiac index measured by ultrasound cardiac output monitor. Pediatr Crit Care Med. 2014; 15(1): 15-20. PubMed
  8. Lee JH, Kang P, Song IS, et al. Determining optimal positive end-expiratory pressure and tidal volume in children by intratidal compliance: a prospective observational study. Br J Anaesth. 2022; 128(1): 214-21. PubMed
  9. Feldman JM. Optimal ventilation of the anesthetized pediatric patient. Anesth Analg. 2015; 120(1): 165-75. PubMed
  10. King MR, Feldman JM. Optimal management of apparatus dead space in the anesthetized infant. Paediatr Anaesth. 2017; 27(12): 1185-92. PubMed