Question 9

College answer

Syllabus topic/section:

2.1.16 Populations requiring special considerations in Intensive Care / Obese: L1
2.1.5 Respiratory Intensive Care / Mechanical ventilatory support: L1

Discussion:  This question focused on respiratory physiology and invasive ventilation in the context of obesity. Candidates who were able to outline the effects of shunting and V:Q mismatch, negative effects of obesity on respiratory mechanics, aetiology of pulmonary hypertension and metabolic demands and oxygen consumption demonstrated that they had a sound grasp of respiratory physiology in its application to clinical practice.
Likewise detailing invasive ventilation strategies including specifics of protective ventilation, PEEP strategies, alterations to I:E ratios, recruitment manoeuvres, PIPs, plateau pressure adjustments, and alterations to proning techniques (amongst other possible headings) demonstrated familiarity with the acceptable standard of knowledge required of a transitional fellow for this topic.
Candidates would be advised to read the question and follow the instructions. Candidates who described airways, anatomical issues, intubation, tracheostomy, front of neck access, end of life planning and ECMO gained no marks as it did not address the question. Nominating ventilatory modes (e.g. APRV) without a rationale or justification of principles was not sufficient to pass.
The rubric is included to aid the candidate's future study.

Discussion

This has ended up in the "Mechanical Ventilation" section, even though the "special populations" section is so barren and empty, because it felt like a question mostly inspecting the management of the alarming ventilator. 

a) The effects of morbid obesity on respiratory physiology, for 3 marks, would not be as long as this, but the reader is invited to abbreviate it themselves:

• Increased chest wall weight and increased intrusion of abdominal contents into the chest cavity, which mean:
  • Expiratory reserve volume is decreased
  • FEV1 to FVC ratio is increased.
  • VC, TLC and FRC are decreased.
  • Respiratory compliance is decreased
  • Work of breathing is increased
  • Airway resistance is increased (until you increase the FRC with PEEP)
  • Thus, shunt and VQ mismatch, with increased FiO₂ requirements
• Disordered breathing
  • There could be sleep apnoea and/or obesity hypoventilation syndrome
    • Thus, CO₂ retention and pulmonary hypertension.
  • O₂  consumption and CO₂ production is increased (more tissue metabolising)
    • Thus ventilatory needs are greater

b) Strategies for the invasive ventilation of a morbidly obese patient with severe ARDS:

"With reference to the above effects", they said. So:

• Decreased respiratory compliance:
  • Increase PEEP: 
    • Use PEEP about 8 cmH₂O higher than what the ARDSNET tables would recommend;  or:
    • Oesophageal manometry: keep P_L -2 to +2 cmH₂O territory (aiming for zero)
    • Aim to restore FRC and reduce shunt
  • Ventilate with tidal volumes calculated for predicted body weight (PBW) instead of actual body weight
  • Tolerate higher plateau pressure and peak inspiratory pressure
  • I:E ratio with long I-time to improve recruitment of long time constant lung units
  • Use a pressure control mode with a square pressure waveform to maximise recruitment of these slow lung units
• Contribution of abdominal contents
  • Tilt the whole bed into reverse Trendelenburg
  • Use muscle relaxant to reduce the abdominal muscle contribution to pressure on the lung bases
• Increased metabolic demands on the respiratory system
  • Use a higher minute volume/resp rate
  • Tolerate hypercapnia
• Decreased ventilatory drive
  • Accept hypercapnia 
  • Adjust oxygenation expectations (aim lower)
• Use rescue strategies
  • Prone ventilation is not only safe and feasible, but the morbidly obese patients actually seems to benefit more than those with a normal weight
  • Recruitment manoeuvres are recommended to improve lung volumes after loss of pressure, eg. during airway disconnections or following intubation