Applied Resp – Other Physiology

Master answer

Definition

Respiratory Complications

1. Lung Volumes
   • ↓’d ERV (due to limitation of diaphragmatic excursion + Δ Compliance)
   • +/- ↑’d residual volume (2^nd to gas trapping or concurrent disease)
   • Δ ERV is usually greater than any ↑ in RV thus
     • ↓ FRC
       • Can fall to < Closing capacity → right-to-left shunting + arterial hypoxemia
     • ↓’d TLC
2. Lung Mechanics
   • ↑’d work of breathing due to
     • Altered pulmonary system compliance
       • ↓’d lung compliance
         • ↑ intrapulmonary blood volume
         • ↓FRC to < Closing Capacity → Small airway narrowing/closure
         • → ↑elastic work
       • ↓’d chest wall compliance
         • accumulation of fat tissue in and around the chest wall
     • ↑’d airway resistance
       • ↓ airway calibre
       • Elevation of diaphragm during erect/head up positioning
   • Result:
     • shallow rapid pattern of breathing
     • ↑ the work of breathing
   • ↓’d respiratory muscle efficiency
     • ↑ WOB → ↑RR → ↑ WOB → Capacity limitation
3. Gas Exchange
   • ↑’d metabolic activity of the excess fat + ↑’d workload on supportive tissues
     • ↑’d O2 consumption
     • ↑’d CO2 production
   • ↑’d ventilation/perfusion abnormalities (especially supine/under anaesthesia)
   • Limited maximum lung capacity (inability to increase ventilation in response to ↑’d demand)
   • ↑’d sensitivity to respiratory depressant drugs
4. Respiratory Control
   • Obstructive sleep apnea
     • defined arbitrarily: total cessation of airflow for 10 seconds or more despite continued respiratory efforts against an obstructed pharyngeal airway
     • Hypopnea: a 50% reduction in airflow or a reduction sufficient to lead to >4% reduction in arterial oxygen saturation
     • physiological responses to recurrent apnea
       • hypoxemia and hypercapnia
       • pulmonary and systemic vasoconstriction
       • Secondary polycythemia and ↑ pulmonary vascular resistance
       • Recurrent pulmonary vasoconstriction leads to right ventricular impairment.
   • Obesity hypoventilation syndrome (chronic OSA)
     • Loss of normal respiratory control during the day and daytime respiratory failure
       • Altered chemoreceptor sensitivity to carbon dioxide.
       • Reduction in respiratory drive leading to central apneic events (i.e. apnea without respiratory effort)
5. Airway Changes
   • Fatty infiltration of the soft tissues of the neck →
     • Difficulties maintaining airway
     • Requirement for higher than normal BV mask pressures → gastric inssuflatio → ↑ aspiration risk
     • Decreased FRC → ↓’d apoenic time on induction (compounded by ↑ O2 consumption)
   • High risk of airway and intubation difficulties at induction of anesthesia

Gladwin 2016

Answer by Gladwin