3 Mechanical Ventilation, part 3 (ARDS)

Question 1

Remarks on ARDS

Answer 1

It is a restrictive lung pathology, where the compliance is low.
This may result in increased pressures.

Question 2

In ARDS, to minimize oxygen induced lung injury, the FiO2 should be maintained at _____

Answer 2

at the lowest concentration possible while maintaining an oxygen saturation of 88-94%
- this range of values is chosen because of the hemoglobin oxygen saturation curve
- even a relatively small increase in saturation above this range requires a marked increase in PaO2 and hence marked increase in FiO2
- below this range, a relatively small fall in PaO2 results in a marked fall in saturation

Question 3

remarks on oxygen saturation and oxygen tension

Answer 3

Oxygen content and hence oxygen delivery is much more dependent on oxygen saturation (SaO2) than oxygen tension (PaO2)

Question 4

Tidal volume in ARDS

Answer 4

Should, if possible, be restricted to 4-6 mL/kg predicted body weight
- It is important to calculate predicted boy weight from the patient’s measured height, as estimated body weight is often considerably higher.

Question 5

Low tidal volume and hypercarbia

Answer 5

Unsurprisingly, the use of a low tidal volume is often associated with hypercarbia.

In general, this is considered a price worth paying to minimize damage to the lung unless hypercarbia results in marked acidosis, or te patient has specific contra-indictaion to hypercarbia (e.g., raised intracranial pressure)

In order to allow time for renal compensation of the anticipated respiratory acidosis, the tidal volume should be reduced by 1 mL/kg over several hours.

Question 6

Indication for neuromuscular blockade in ARDS

Answer 6

Early, severe ARDS (PF ratio ≤150 mmHg, PEEP ≥5 cm H2O)

Question 7

Precautions in neuromuscular blockade in ARDS

Answer 7

1. Ensure optimal analgesia and sedative drugs prior and during neuromuscular blockade
2. Should be given only in the presence of individuals with appropriate airway and life support skills
3. Monitor degree of blockade clinically and consider peripheral nerve stimulation monitoring to prevent excessive blockade
4. Eye care including lubrication and eye covers to prevent corneal abrasions

Question 8

What is “Open lung ventilation?”

Answer 8

Combination of low tidal volumes (4-6 mL/kg), a recruitment maneuver to open the lungs, and the application of sufficient PEEP

Question 9

Remarks on recruitment maneuvers

Answer 9

1. Used in “Open lung ventilation”
2. While recruitment maneuvers make sense from a pathophysiological viewpoint, there are no robust outcome data to support their use.
3. The optimal method of recruitment has yet to be fully defined, although it is known that intermittent (short duration) high pressure is associated with fewer cardiovascular adverse effects than continuous high pressures

Question 10

Laplace law and recruitment maneuvers

Answer 10

1. It follows from Laplace’s law that the pressure required to re-expand a collapsed alveolus is greater than the pressure required to keep it open.
2. Thus while PEEP may prevent alveolar collapse, it is usually not sufficiently high to re-open alveoli.
3. To re-open alveoli, a recruitment maneuver (designed to produce a sufficiently high, but short duration increase in thoracic pressure) needs to be performed

Question 11

How to perform a recruitment maneuver

Answer 11

1. Deep sedation and paralysis
2. Increase FiO2 to 1.0
3. Increase airway pressure limit to 60 cm H2O and PEEP alarm limit to 40 cm H2O
4. Decrease trigger sensitivity to prevent autotriggering
5. Change to pressure control ventilation. Set pressure to 15 cm H2O above PEEP, I:E 1:1, RR 15
6. Increase PEEP to 35 cm H2O for 6 breaths
7. Reduce back to 20 cm H2O
8. Carry out decremental PEEP trial

(Another method: PEEP 20 - 25x10 - 20 - 30x10 - 20 - 35 x 10 - 20, then decremental PEEP trial)

Question 12

How to determine appropriate or sufficient PEEP?

Answer 12

1. to use the ARDSnet protocol in which PEEP is adjusted according to required FiO2
2. If a recruitment maneuver is done, a decremental PEEP trial should be carried out from a high level (e.g., 20-25 cm H2O) with monitoring of either the dynamic lung compliance or the PaO2

Question 13

How to perform decremental PEEP trial

Answer 13

1. Gradually reduce PEEP in steps of 1 cm H2O until PaO2 starts to fall - note the level of PEEP
2. Repeat recruitment maneuver
3. Set PEEP 2 cm H2O above previously noted value
4. Reset alarm limits, I:E ratio, RR, inflation pressure, mode as appropriate

Question 14

ARDS in the supine position

Answer 14

The dorsal part of the lung receives more perfusion than the non-dependent (ventral) parts for structural anatomical reasons and the gravitational effect
Ventilation is predominantly to the non-dependent, but less perfused alveolar units

Question 15

Indication of prone positioning

Answer 15

When initiated early (within 48-60 hours of intubaiton and MV), prone positioning has been shown to reduce 28 and 90-day mortatlity be nearly half in patients with severe ARDS.
Severe ARDS was defined as PF ratio ≤150 mmHg, PEEP ≥5 cm H2O, and FIO2 ≥0.6

Question 16

Procedure for proning

Answer 16

1. It is essential that during the procedure the lungs are are NOT allowed to collapse. Care should be taken, therefore, that the patient is not disconnected from the ventilator
2. Recommended duration is 16 hours consecutively, followed by at least 4 hours in the supine position.
3. Prone positioning can be repeated daily as requried, up to 28 days.

Question 17

Termination of prone positioning

Answer 17

1. Monitor oxygenation after 4 hours in the supine pisition, leave the patient in the supine position if
   PF ratio ≥150 mm Hg
   PEEP ≤10 cm H20
   FiO2 ≤0.6.
2. Also, deterioration in the prone position (decrease of the PF ratio ≥20%) should result in turning the patient back to the supine position