Changes associated with obesity

Cardiovascular

• Physiology
  • ↑ Blood volume,
  • ↑ Systolic volume
  • ↑ Cardiac output
• Presents as
  • Ischaemic heart disease
    • obesity is an independent risk factor for ischaemic heart disease
  • Hypertension
    • ↑ Extracellular volume and cardiac output are characteristic of obesity‐induced hypertension.
  • Cardiac failure
    • Left ventricular hypertrophy and ↓compliance, combined with ↑ blood volume

Weighted adjusted metabolic, blood volume and cardiac output are decreased when per kg values are used

• Blood volume 50ml/kg vs 75ml/kg non obese
• Pharmacokinetics (PK)
  • Drugs with narrow therapeutic indices may cause adverse effects if administered according to their actual body weight
  • ↑ Initial drug distribution
  • ↑ Loading dose required
  • ↑ Maintenance requirements
  • ↑Renal and hepatic blood flow
  • Using linear per kg dosing schemes designed for non-obese patients overestimates volumes and clearances
    • ↑ Overdose adverse effects
  • Obesity-related cardiovascular compromise
    • ↓ Distribution
    • ↓ Clearance for drugs that have high intrinsic clearance and are perfusion limited (e.g. propofol)

Respiratory

• Difficult Airway
  • Incidence of difficult airway approximately 13%
  • Increased neck circumference
  • Fatty infiltration of the pharynx and peri glottic area
    • Increased risk of airway obstruction
  • Consideration to AFOI
• Obstructive sleep apnoea (OSA)
  • 5%
  • Screening Test: STOP BANG criteria
    • Snoring: Snore loudly
    • Tiredness: Day time somulence
    • Observed: Stop Breathing or Choking/Gasping during your sleep
    • Pressure: High Blood Pressure
    • BMI: Body Mass Index more than 35 kg/m2
    • Age: >50years
    • Neck Circumference: Shirt collar 16 inches / 40cm or larger
    • Gender: Male
  • Polysomnography in a sleep laboratory
• Obesity hypoventilation syndrome
  • The acid–base disturbance of OSA
  • Progressive desensitization of the respiratory centres
  • Ventilation reliance on hypoxic drive for ventilation
• Pickwickian syndrome: Hypoventilation syndrome in an obese persons – Diminished to absent ventilatory chemoresponsiveness, – Chronic hypoxia, – Hypercapnia – Polycythemia – Hypersomnolence
• Lung volume – Fatty deposits in the chest wall and increased intra-abdominal fat – Decrease thoracic compliance – Decrease lung volume – Functional residual capacity (FRC) – Rapid desaturation post-induction despite pre‐oxygenation. – Expiratory reserve volume (ERV)
• Gas exchange
  • PEEP improves the PaO2 but only at the expense of cardiac output and oxygen delivery
  • Increased V/Q mismatch