Obesity hypoventilation syndrome

Aka Pickwickian syndrome

Definition

• Obesity and daytime hypoventilation in the absence of pulmonary disease, neuromuscular disorders or chest wall disorders
• BMI >30, PaCO2 >45 while awake

Pathophysiology

• Progression from obesity to OHS is multifactorial
• Lung mechanics
  • Reduction in FRC and expiratory reserve volume (ERV)
  • Central pattern of morbid obesity
    • Highlighted by higher waist:hip ratios and larger neck circumferences
    • Central adiposity correlates with lower FBC and FEV1
  • High pleural pressures
    • Reduced respiratory system compliance primarily from lung compliance rather than chest wall
    • Respiratory system compliance approximately 60% lower than normal subjects
    • Results in tidal breathing near residual volume
    • Low lung volumes further affect respiratory mechanics unfavourably
      • Airways prone to closure and lungs are less compliant
      • Results in air trapping and iPEEP
  • Supine positioning
    • Further compounds adverse lung mechanics with reduced FRC, compliance and airflow limitation
    • Abdominal fat pushes against diaphragm
    • Work of breathing increased
    • Gas exchange further impaired by low ERV and higher dead space
  • Respiratory muscle fatigue in face of increased WOB
    • Maximal voluntary ventilation is reduced with inadequacy of compensation
  • Sleep-disordered breathing
    • Most patients with OHS have OSA
    • Compensatory rise in bicarbonate overnight in setting of OSA has to be unloaded during the daytime to avoid blunting the effects on change in pH produced by changes in PaCO2
    • Sustained hypoxia overnight contributes to pathogenesis of OHS also
  • Ventilatory drive
    • Higher O2 consumption (VO2) and CO2 production (VCO2) resulting in higher minute ventilation and higher neural drive to breathe
    • Patients with OHS are unable to increase their neural drive, permitting daytime hypercapnoea
    • Blunted response to hypoxia and hypercarboea

Clinical presentation

• Initial presentation in critical illness is acute on chronic hypercapnoea.
• Severely obese, severe OSA, drowsy, cor pulmonale, loud P2, facial plethora, raised JVP, enlarged neck circumference, rapid shallow respirations
• Serum bicarb >27 is 75% sensitive and specific
• Secondary erythrocytosis
• Malignant OHS – Critically unwell with hypercapnoeic respiratory failure and multiorgan dysfunction related to obesity

Treatment

• Most studies undertaken in sleep labs on ambulatory patients
• BiPAP
  • Set EPAP to sleep study pressure required to maintain patent airway (if known)
    • If unknown, start at 10 and raise 2-3cmH20 every 5 minutes depending on airway patency, patient comfort and subjective compliance
  • Maximum IPAP 20-30cmH20 for adults generally
  • Set pressure support at least 8cmH20 higher and raise in increments to achieve target Vt
  • Can set rate to an apnoeic backup rate or a mandatory rate (particularly useful if obtunded but not for intubation)
  • V60 user guide
    • Ineffective triggering is usually remedied by raising the EPAP to ensure airway patency
      • V60 uses Auto-Trak Sensitivity to adjust triggering sensitivity automatically
    • Pressure rise time
      • Defines the times required for inspiratory pressure to rise to the set (target) pressure
      • 1-5 (1 is fastest)
    • Triggering
      • Flow triggered in all modes
      • Either certain volume of gas accumulates above baseline flow (volume method) or inspiratory effort from patient distorts the expiratory flow waveform sufficiently
      • Auto-Trak Sensitivity does it all (see below)
    • Cycling to exhalation
      • Patient expiratory effort distorts the inspiratory flow waveform sufficiently OR
      • Patient flow reaches the spontaneous exhalation threshold (SET) OR
      • After 3 seconds at the IPAP level (timed backup safety mechanism) OR
      • When a flow reversal occurs (mask leak)
    • Mask leak
      • This can be normal or abnormal
      • Each breath, the V60 measures the baseline flow and recalibrates to increase flow accordingly to achieve targeted flow to patient allowing for leak
    • AutoTrak+ (under menu on screen)
      • Optional adjustment to sensitivity from Normal to higher or lesser sensitivity while maintaining automatic sensitivity adjustments of algorithm
      • Can alter Trigger and E-Cycle
      • Trigger
        • Normal, +1 to +7
        • Higher the number, the more easily the ventilator is triggered
      • E-cycle
        • At lowest setting (-2), inspiration terminates later (allowing less time for exhalation)
        • At highest setting (+6), inspiration terminates earlier and exhalation is longer
    • CPAP mode
      • C-Flex feature reduces the pressure at the beginning of exhalation to improve comfort
      • Ramp allows for gradual introduction of CPAP over x minutes
    • S/T mode (BiPAP)
      • If patient fails to trigger a breath in the defined cycle time, then a mandatory breath is delivered at the IPAP and for the I-Time set
    • Ramp
      • Sets time taken to reach IPAP (5-45 minutes) to improve comfort

Last Updated on September 27, 2021 by Andrew Crofton