Ventilation Strategies

Fully Ventilated Mode Strategies:

Patients with ARDS consider using the ARDSNET Protocol

 

ARDSNET PROTOCOL:

 

1.     Does your patient meet ARDS criteria? Bilateral infiltrates on CXR consistent with pulmonary edema, no evidence of left atrial hypertension—if measured, PCWP  < 18 mmHg, and PaO₂/F_iO₂ ratio < 300.  ARDS must be acute in onset and patient must require mechanical ventilation.

 

2.     The ARDSNet protocol ventilator settings are very uncomfortable for the patient.  Therefore, be ready to give heavy sedation and possibly paralytics to avoid having the patient fight the ventilator.

 

3.     Initial ventilator settings:

• Calculate ideal body weight (IBW): for females, IBW = 45.5 + 2.3 (height in inches – 60); for males, IBW = 50 + 2.3 (height in inches – 60).
• Set mode to assist-control ventilation (ACV) and set initial tidal volume to 8 cc/kg (IBW).  Reduce to 7 ml/kg (IBW) after 1-2 hours and then to 6 ml/kg (IBW) after 1-2 hours.
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4.     The plateau pressure (P_PL) goal is < 30 cm H₂0.  Adjust the tidal volume to reach this goal:

• Ask RT to check P_PL with a 0.5 second inspiratory pause q4h and after each change in tidal volume or PEEP.
• If P_PL > 30, decrease tidal volume to 5 ml/kg IBW or even 4 ml/kg IBW if necessary.
• If P_PL < 25 and the tidal volume < 6 ml/kg IBW, increase tidal volume until P_PL > 25 or tidal volume = 6 ml/kg.
• If the patient is breath stacking or has severe dyspnea, tidal volume may be increased to 7 or 8 ml/kg IBW as long as the P_PL < 30.
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5.     Oxygenation goal = PaO₂ 55-80 mmHg or O₂ sat 88-95% in order to avoid oxygen-induced lung injury.  Basically, you’ll want to use a high level of PEEP for any given F_iO₂ setting:

 

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	20-24

 

6.     pH goal = 7.30 – 7.45:

• If pH 7.15 – 7.30, increase the set rate until pH > 7.30 or PaCO₂ < 25 (max rate = 35).  If the set rate = 35 and the pH is still < 7.30, consider giving NaHCO₃.
• If pH < 7.15, set rate to 35.  If the set rate = 35 and pH is still < 7.15, consider NaHCO₃.  In addition, increase tidal volume at 1 cc/kg IBW increments until pH > 7.15.  It is okay to go above the target P_PL at this point.
• If pH > 7.45, decrease set rate until patient’s respiratory rate < set vent rate.  Minimum set rate=6.

 

7. The goal I:E ratio is 1:1 – 1:3.  Adjust flow rate and inspiratory flow wave-form to achieve this goal.

8.     Conduct a weaning trial daily if the patient meets all of the following criteria:

• F_iO₂ < 0.4 and PEEP < 8 (as long as these values are < values from previous day).
• Patient can take spontaneous breaths (turn down the set vent rate and see).
• Patient is haemodynamically stable without use of high dose vasoconstrictors or inotropes.

 

9. Daily CPAP Trial:

• Set CPAP at 5cmH20 and FiO2 0.5 watch RR for 5 minutes
• If RR>35 failed CPAP trial go back to fully ventilated mode
• If RR 25-35 set PS at 20cmH2O and decrease by 5cmH2O every 5 minutes as long as RR<30 and TV>6mls/kg. Decrease PS to a minimum of 10cmH2O.
• If RR < 25 place patient on PS of 10cmH2O ensure RR< 30 and TV>6mls/kg.

 

Fully Ventilated Principles when not using the ARDSNET protocol:

 

1) Avoid damage from high FiO2 – reabsorption atelectasis and oxidant damage.

 

If FiO2 > 50%:

• Look for reversible causes for poor oxygenation and treat any reversible causes such as endobronchial intubation, sputum plugging, pneumothorax or fluid overload.
• Optimise PEEP
• Accept lower SpO2 88-94% as oxygen delivery depends on SpO2 more than PaO2.
• Consider recruitment maneuvers and repositioning patient.
• If patient requires > 70% O2 the consultant must be informed.

 

Optimising Peep:

It is difficult to know what PEEP is best. Increase PEEP and look at the effect on SpO2 and compliance. Optimal Peep will improve both oxygenation and compliance.

The mean arterial pressure must be watched while increasing PEEP as increased PEEP may decrease venous return.

Usually PEEP will need to be > 10cmH2O if FiO2>50%.

Compliance= Tidal volume/ Change in pressure. Look at the tidal volume obtained for the same change in pressure at different levels of PEEP.

 

Recruitment Manoeuvre: Do not do if patient has a high risk of developing an airleak such as patients with pneumocystitis or severe emphysema.

Care in patients with bronchiectasis.

Place patient on PCV mode, place the patient on FiO2 100% increase PEEP to a maximum of 30cmH2O with a inspiratory pressure above PEEP of 5cmH2O and a maximum peak airway pressure of 40cmH2O for 30 seconds. Make sure the mean arterial pressure is maintained. Then reset PEEP at estimated optimal level. Place back on original ventilation mode. If no improvement or worsening condition do not repeat manoeuvre.  This should only be done with medical officers present. Other recruitment manoeurves are described and can be performed as long as it is checked by the consultant.

 

2) Avoid damage from high tidal volumes and high airway pressures.

This is on the basis that in damaged lungs only a small percentage of the lung may take place in ventilation the so called “Baby Lung” and over inflation of lung units further damages the lungs and causes “barotrauma” or volutrauma” and may even cause a pneumothorax.

 

Set the tidal volume at 6mls/kg and aim for a plateau pressure <30cmH2O.

If the plateau pressure is >30cmH20 try 5mls/kg then to a minimum of 4mls/kg.

If the plateau pressure is still >30cmH2O call the consultant.

If the plateau pressure is <20 cmH2O then higher tidal volumes up to 8mls/kg are unlikely to be harmful.

 

3) Avoid development of Auto peep or breath stacking:

 

What is auto peep?

Auto peep is when expiratory flow has not completed before the next inspiratory breath. You can detect it on the lung flow curves the expiratory flow does not get to baseline. You can confirm this by performing an expiratory hold manouvre and measuring the intrinsic peep. If the intrinsic peep is greater than the delivered peep auto peep has developed.

 

What does auto peep do?

If the auto peep gets too high it will cause hypotension by reducing venous return to the heart, decrease compliance due to overinflation and at high levels will cause barotrauma.

 

a) In obstructed lungs

Patients with obstruction to expiratory flow are particularly likely to develop this such as acute severe asthma or in chronic airways limitation where long expiratory times are required. The I:E ratio is often 1:5 and in severe cases this but may even need to be up to1:10.

In these cases a high CO2 is often accepted you need to discuss with the consultant the levels of CO2 tolerated.

 

b) In normal stiff lungs

Expiratory time should always be set equal or longer than inspiratory time, ideally I:E ratio is 1:>1.5.

Respiratory rate should be set to allow an inspiratory time of about 0.8 seconds with an I:E ratio of at least 1:1.5. This usually limits the respiratory rate to less than 30. As long as the pH>7.2 and the CO2<55 keep within these parameters. Call the consultant if this is not possible.

 

 

 

 

 

Weaning to a spontaneous breathing mode:

Pressure Support mode is used to change the patient from a fully ventilated mode to a spontaneous mode a step in liberating the patient from the ventilator, these are the basic guidelines:

 

Spontaneous Modes of ventilation should be used when:

1)      The patient is heamodynamically stable meaning that low dose of vasoconstrictors or inotropes only are being used.

2)      The patient is not requiring high inspired oxygen concentrations or high ventilatory support.

3)      The patient has a neurological state that supports adequate spontaneous respiratory effort e.g. patients over sedated may have too low minute ventilation.

 

The parameters that suggest that changing from a fully ventilated mode to a spontaneous may be successful:

 

1)      FiO2 <45%, Peep <10 – this indicates the patient does not have too large a shunt.

2)      Plateau pressures of <20 – this indicates reasonable compliance

3)      A respiratory rate of <20 generating a CO2 < 45mmHg – this shows the minute ventilation requirements are not so high that the patient will tire.

4)      Able to take spontaneous breaths

 

The parameters that show that the patient may be tiring or decompensating on a spontaneous ventilation mode:

 

1)      Required FiO2 >60%, Peep>10

2)      Respiratory Rate(RR) >30 or RR<10

3)      Tidal volumes generated <6mls/kg on a maximum PS of 20.

 

Pressure Support (PS) may be increased by 5 if the tidal volume <6mls/kg or respiratory rate >30, to a maximum of 20. A minimum of 5-10 of PS should be used as this compensates the extra work of a patient breathing through an ETT. A respiratory rate may also be related to pain or agitation so these must be assessed. Failure to wean must be notified to medical staff and reversible causes looked for such as sputum plugs, fluid, wheeze etc.

 

 

Assessment for extubation:

Patients are likely to be extubated successfully if:

1)      A ratio of patient’s respiratory rate/tidal volume of <100 breaths per min/l on a PS ventilation of 5/5 or 5/10.

2)      They have adequate cough and minimal sputum.

3)      They have adequate muscle strength to sustain a head lift off the bed for 5 seconds or be able to lift their arms above their heads.

4)      Awake and co operative.

5)    On an FiO2 <40%