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RESPIRATORY MECHANICS

  • Developmental considerations

    • Basal metabolic rate is higher in children than in adults, resulting in decreased metabolic reserve in the face of increased oxygen consumption during critical illness

    • More compliant chest wall with decreased elastic recoil

    • Incomplete alveolarization and lack of collateral ventilation (pores of Kohn and canals of Lambert develop at ages 3–4)

    • Airways in children have increased resistance and lack rigid cartilage

      • More susceptible to dynamic compression and airway obstruction

  • Compliance: Distensibility of the lung; ease of expansion of the lungs and thorax, determined by pulmonary volume and elasticity; measure of the ratio of change in tidal volume and the pressure it produces:

    C=V/P
    • High compliance = healthy lung, neuromuscular weakness or paralysis

    • Low compliance = acute respiratory distress syndrome (ARDS), pneumonia

  • Resistance (Poiseuille's law): Resistance to flow is inversely related to the fourth power of the radius; therefore, if the radius is halved, the resistance is increased 16-fold.

    • Results in profound decrease in flow as laminar flow transitions to turbulent flow (see Figure 18-1)

  • Lung volumes (see Figure 18-2)

    • Tidal volume (VT): volume of air moved during quiet breathing.

    • Vital capacity (VC): maximal volume of air that can be forcibly exhaled after a maximal inspiration.

    • Residual volume (RV): volume of air remaining in lungs after a maximal expiration. It cannot be expired no matter how vigorous or long the effort.

    • Total lung capacity (TLC): volume of air in the lungs at the end of a maximal inspiration.

    • Functional residual capacity (FRC): volume of air remaining in the lungs at the end of a normal expiration.

      • FRC is reduced by supine positioning, abdominal distension, restrictive lung disease, and sedation.

  • Normal gas exchange requires:

    • Transport of oxygen (O2) to the alveolus

    • Diffusion of O2 across the alveolar-capillary membrane

    • O2 transfer from the blood to the organs

    • Carbon dioxide (CO2) removal from blood into the alveolus

  • Ventilation: Refers to CO2 exchange at the alveolar level.

    • Determined by minute ventilation and anatomic dead space.

      AlveolarVentilation=[VTDeadSpace (Vd)] ×RespiratoryRate(RR)
      MinuteVentilation=VT×RR
      DeadSpaceVolume(Vd)=Anatomic+Physiologic

    • Dead space = the sum of gas volume within the conducting airways that does not reach the alveoli (anatomic) and volume of gas that doesn't participate in gas exchange secondary to inadequate perfusion (physiologic).

    • Physiologic causes of hypercapnia:

      • Increased CO2 production (fever, burns, overfeeding)

      • Decreased alveolar ventilation

        • Decreased VT

          • Decreased central nervous system (CNS) drive (sedatives)

          • Neuromuscular weakness

          • Flail chest

        • Increased Vd

          • Pulmonary embolus

          • Hyperinflation (asthma, bronchiolitis, cystic fibrosis [CF], excessive positive end-expiratory pressure [PEEP])

          • Decreased cardiac output (dehydration, dysrhythmia, cardiomyopathy, post-cardiopulmonary bypass)

          • Increased pulmonary vascular resistance

  • Oxygenation

    • Oxygen content of the blood (CaO2)

      CaO2=(...

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