Invasive Mechanical Ventilation (IMV)

Question 1

IMV modes

Answer 1

Common modes:
1) Assist/Control (AC Mode)
- mixed mandatory and assisted breaths, with consistent tidal volume. Any pt’s triggered breaths will be supported fully
- advantages: guarantees a minimum minute ventilation, low work of breathing
- disadvantages: auto PEEP and resp alkalosis in hyperventilating pt –> hypotension

2) Synchronised Intermittent Mandatory Ventilation (SIMV)
- ventilator will deliver a mandatory (set) number of breaths with a set volume while at the same time allowing spontaneous breaths (not supporting the spontaneous breaths)
- advantages: guarantees minimum minute ventilation, lowers mean airway pressure
- disadvantages: increased work of breathing for pt

Question 2

Setting of ventilator

Answer 2

1) Mode
2) FiO2
- improves oxygenation
- aim not more than 60% as it may cause O2 toxicity
- aim SpO2 88-95% or PaO2 55-80
- if requires FiO2 ≥60% to maintain adequate oxygenation, can
i) increase PEEP
ii) recruitment manouvre
iii) change mode

3) Tidal volume (Vt)
- improved CO2 clearance
- normally 6-8mls/kg (based on predicted body weight)
Calculate predicted body weight (PBW)
Males = 50 + 2.3 [height (inches) - 60]
Females = 45.5 + 2.3 [height (inches) -60]
- TV influence plateau pressure (Pplat):
- in lung protective strategy used in ARDS, use TV is 4-8mls/kg) with plateau pressure ≤30cmH20.
- If Pplat is >30, decrease Vt by 1ml/kg

4) RR
- improves CO2 clearance
- but too high may lead to auto PEEP

5) PEEP
- improves oxygenation
- use moderate PEEP (minimum 5cmH20)
- in ARDS consider use of FiO2/PEEP combination as per the ARDSnet 2011 guidelines
-
6) Pressure support
- improves CO2 clearance
= additional positive pressure provided during inspiration, particularly used in BPAP and SIMV. not applicable in A/C mode

7) I:E ratio
- being set indirectly via Vt and flow rate/pattern
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