Ventilation

Ventilation

NOTE: THIS SKILL STATION IS NO LONGER PART OF THE CCAM COURSE, BUT FEEL FREE TO READ THIS SECTION ANYWAY!
This Skill Station covers:

-Planning for the ‘physiologically difficult’ airway

-Setting up the transport ventilator (Oxylog 3000/3000plus)

-Setting up a bed side ventilator (Drager Infinity V500)

-Ventilation in a patient with a restrictive problem

-Ventilation in a patient with an obstructive problem

The Equipment

Drager Oxylog 3000
Drager Oxylog 3000plus
Drager Infinity V500

Top Tips

Ventilating the patient with a restrictive problem (e.g. acute lung injury)1

-Oxygenation will be the main problem

-Increasing FIO2 is a quick fix but may be insufficient and high oxygen concentrations are harmful if delivered over long periods

-Tidal volumes should be restricted to 6mls/kg to avoid volutrauma (the injured lung has poor compliance and there is good evidence of harm at higher tidal volumes)

-The plateau pressure should normally not exceed 30cmH2O

-Arterial CO2 may rise and this should be tolerated unless pH is very low (<7.2). This is known as permissive hypercapnia

-Arterial oxygenation is dependent on alveolar pressure. As a result, pO2 is directly proportional to Mean Airways Pressure (MAP, the mean pressure during a respiratory cycle). The value for MAP can be found on all ventilators

-There are three determinants of MAP: inspiratory pressure, expiratory pressure and inspiratory time (a longer inspiratory time results in higher MAP).

-PEEP should be set depending on the FIO2 (see Table 1 below).

-At high levels of PEEP, beware of cardiovascular compromise due to impaired venous return

-When confronted with a hypoxic patient:

  • Increase the FIO2 to give yourself some time.
  • Check the MAP
  • Adjust PEEP, trying to follow the pairings in the table below
  • In pressure controlled ventilation, set the inspiratory pressure to create a tidal volume of 6mls/kg maximum
  • In volume controlled mode, set a tidal volume of 6 mls/kg, only if the peak pressure allows (it should be less than 30cmH2O)
  • Increase inspiratory time (and therefore I:E ratio)
  • Ensure there is still time for expiration by examining flow-time curve (see Figure 1 below)
  • Recheck the MAP to ensure it has risen appropriately, otherwise the next pO2 will be the same
  • Wean the FIO2 as oxygen saturations allow, aiming for 88-95%
Table 1: Peep and FIO2 settings from ARDSNet Study1
FIO2
0.3
0.4
0.4
0.5
0.5
0.6
0.7
0.7
0.7
0.8
0.9
0.9
0.9
1.0
PEEP
5
5
8
8
10
10
10
12
14
14
14
16
18
18-24
Figure 1: Gas trapping seen on flow : time curve

 Flow Time Curve Gas Trapping

Top Tips

Ventilating the patient with an obstructive problem (e.g. asthma)2,3

-Oxygenation is generally not the main problem

-Tidal volumes should be restricted to 6mls/kg to avoid volutrauma

-The plateau pressure should normally not exceed 30cmH2O

-Arterial CO2 may rise and this should be tolerated unless pH is very low (<7.2). This is known as permissive hypercapnia

-Flow rates should be high in asthma to allow for short inspiratory time and therefore longer expiratory times

-High intrathoracic pressures can occur in asthma due to gas trapping. This can cause cardiovascular compromise due to impaired venous return. Gas trapping is best monitored using Mean Airways Pressure (MAP). The value for MAP can be found on all ventilators

-When ventilating an asthmatic patient:

  • Increase the FIO2 to give yourself some time.
  • Drop the respiratory rate down to 6-8 breaths per minute
  • Ensure the I:E ratio is 1:4 or less
  • Decrease the PEEP to ZERO
  • In pressure controlled ventilation, set the inspiratory pressure to create a tidal volume of 6mls/kg maximum
  • In volume controlled mode, set a tidal volume of 6 mls/kg, only if the peak pressure allows (it should be less than 40cmH2O)
  • Note that volume controlled modes may be preferable as the tidal volume will remain constant if asthma should ‘break’, decreasing the risk of volutrauma
  • Ensure there is no gross gas trapping by examining flow-time curve (see Figure 1 above)
  • Check the MAP
  • Increase the respiratory rate, adjusting the inspiratory time to maintain the I:E ratio if possible
  • Depending on the mode you are in, you may need to increase the flow rate. Keep to a maximum of 60L.min-1 in constant flow, 80-90L.min-1 in decelerating flow
  • Recheck the MAP. As you add breaths, it will rise
  • Keep increasing the respiratory rate, wait a few breaths, then re-check the MAP. If the MAP starts to rise sharply, this indicates gas trapping: go back to the rate you had selected before the sharp rise in MAP
  • Aim to keep the MAP at 20cmH2O or below.
  • The MAP method (also known as the Totaro manoeuvre) is a better predictor of dangerous gas trapping than the flow-time curve, which may fail to return to zero and deter you from increasing the minute volume even when it might be safe to do so
  • Wean the FIO2 as oxygen saturations allow, aiming for 88-95%

Foam Resources

References

  1. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med. 2000;342(18):1301-1308.
  2. Holley AD, Boots RJ. Review article: management of acute severe and near-fatal asthma. Emergency medicine Australasia : EMA. 2009;21(4):259-268.
  3. Brenner B, Corbridge T, Kazzi A. Intubation and mechanical ventilation of the asthmatic patient in respiratory failure. J Emerg Med. 2009;37(2 Suppl):S23-34.