Vent Practicalities

This is Part 5 of a 5 part series on fundamental concepts in ventilator management. See Part 1 here.

Everything up to this point has been to provide a foundation to understanding ventilator management.  This section will demonstrate how theory is translated into practice.  This section is also structured to be used as a quick reference on the basics of vent management.

General Algorithm:

1. Prevent Hypoxia:
   2. Initial management of Hypoxia is to increase the FiO₂.
   3. When the FiO₂ approaches 60%, start to raise the P_EEP. Raising the P_EEP above 10 cmH₂O requires severe underlying pulmonary dysfunction to be present.
   4. Increase tidal volume or respiratory rate only as a last resort.
2. Protect the Lungs:
   1. Target a V_t of 6mL/kg_IBW, but stay under 8ml/kg_IBW.  CO₂
   2. Keep FiO₂ under 60%.
   3. Keep P_IP under 40 cmH₂O, or P_plat under 35 cmH₂O (if available).
3. Maintain eucapnia (note: COPD patients will have an elevated baseline):
   1. Increase V_t.
   2. Increase Respiratory Rate.

The general algorithm is formatted in order of priority: the techniques and goals listed in “Protect the Lungs” can only be used if they do not conflict with the techniques and goals of prevent hypoxia.  A few things to note: the “Prevent Hypoxia” requirement is satisfied once the patient is above 88%, however targeting 92% provides a safety margin from sudden desaturation.  Eucapnia is the lowest priority unless the patient has a severe underlying metabolic acidosis.  These patients will enter cardiac arrest if their compensatory respiratory alkalosis is normalized prior to correcting the underlying metabolic acidosis.  Do not attempt to force the end tidal back into the normal range, it will return to normal once the metabolic acidosis is corrected.

Patient population specific strategies:

1. The ventilator patient with normal lungs:

The majority of patients in this class will have a neurological pathology that impairs their respiratory control.  The focus of managing these patients is on maintaining respiratory control while avoiding secondary harm from Ventilator Induced Lung injury.  A good starting point is SIMV-Volume Control, as this avoids both hypo- and hyperventilation, while also ensuring consistent V_t regardless of patient activity.  These patients generally have adequate respiratory muscle strength, and so require less Pressure Support than most.  If the patient demonstrates intact respiratory control, they can be transitioned to a more comfortable mode.  If the patient displays aberrant respiratory control they should be sedated to allow the vent to maintain proper control of respiration.

2. The patient with moderately reduced lung compliance:

Classically, this is the patient with pneumonia or mild to moderate ARDS.  These patients will require a higher P_IP in order to deliver an adequate V_t.  These patients will often benefit from a slightly higher P_EEP in order to keep the FiO₂ down.  Assist/Control offers a good balance of patient comfort and taking over work of breathing.  Initially put these patients on the vent in Volume Control at the desired V_t and monitor the P_IP.  Switch to Pressure control if the patient does not tolerate Volume control or if the P_IP is excessively high.  Keep in mind that the P_IP will generally need to be reduced in order to maintain an appropriate V_t.

3. The patient with mild to moderate obstructive lung disease:

This is the patient who is suspected to have increased airway resistance, but the capnograph hasn’t yet shown evidence of flow restriction (shark fins).  Managing air hunger in these patients is very important, and a simple way to do this is to use Pressure Controlled ventilation.  Pressure control allows for large gas flows at the start of the breath, which helps reduce the sensation of air hunger.  Pressure control also has the additional advantage of limiting intrathoracic pressure to the P_IP, which reduces the potential impact of unrecognized breath stacking.  Finally, connecting a metered dose inhaler port in-line with the vent circuit simplifies the management of the patient if they begin to show signs of flow restriction on the capnograph.

4. The Long term ventilated patient:

The long-term vent patient represents an unusual case because these patients are often fully alert and oriented.  If the patient is neurologically intact, ask if they have any preferred settings, as this drastically shortens the process of adjusting the vent to the patient.   These patients generally do well in BIPAP mode, as this allows them maximal control over their ventilation.  Assist/Control – Pressure Control is also tolerated well.  An important clinical pearl is to learn how the patient indicates they are distressed while adjusting their vent settings.  This facilitates rapid titration of the vent to patient comfort.

5. The Heart failure/Pulmonary Edema patient:

Heart failure patients require positive pressure to push lung edema back into the vasculature.  The goal for these patients is to avoid negative intrathoracic pressures and to reduce work of breathing as much as possible.  Assist/Control – Pressure Control addresses both of these needs while also being one of the more comfortable vent modes available.  Patient comfort is very important in this case, because it simplifies sedation management while also reducing excess demand on the circulatory system from psychomotor agitation.

These patients can be broadly divided into two categories: those with and those without hypotension.  The management strategies for both are broadly the same, however hypotensive patients should be allowed more time to accommodate to vent setting changes relative to normo- or hypertensive patients.

6. The patient with severe obstructive lung disease:

Severe restrictive lung disease can be identified through persistent flow restriction on capnography in spite of inhaled bronchodilator therapy.  Classically, this is the patient in status asthmaticus.  Medical management of these patients is beyond the scope of this tutorial, however it must be noted that optimal drug therapy simplifies vent management.  These patients benefit from slow respiratory rates, short I-times, low (or absent) P_EEP, and tidal volumes towards the upper end of the 6-8 mL/kg_IBW.  High FiO₂ ventilation combined with permissive hypercapnia can be used as a temporizing measure if appropriate resources to manage this challenging situation are not immediately available.  Prior to proceeding down this diagnostic path, however, ensure that there is an unrestricted path from the ventilator to the trachea: verify that none of the hoses are kinked, and suction the patient if indicated.

7. The patient with severely reduced lung compliance:

Critically reduced lung compliance is most commonly encountered in the setting of severe ARDS.  These are some of the most challenging ventilator patients to manage, and many exotic technologies are currently being applied to the stabilization of these patients.  It must be noted, however, that ARDS does not have a sudden onset (i.e. over the course of minutes).  Patients with rapidly deteriorating lung compliance must first be evaluated for the presence of tension pneumothorax, breath stacking, or a mainstem intubation.  Once these are ruled out, further evaluation and care should be performed by providers skilled in the management of ARDS.

8. The patient who does not tolerate the vent:

Getting a patient to tolerate mechanical ventilation involves a delicate balance between appropriate sedation and optimal ventilator management.  The only way to reliably determine if a particular strategy will work for a patient is to implement the strategy and observe how the patient responds.  Some patients will be too agitated to permit carefully tailoring the vent to the patient.  Agitation often results in the patient’s respiratory efforts not synchronizing with the breaths delivered by the ventilator.  This results in low tidal volumes and high peak pressures, further exacerbating the patient’s agitation.

Sedatives should always be quickly available to control agitation during initial vent setup.  Fentanyl is an ideal agent for suppressing acute agitation in ventilated patients.  Dosing above the standard analgesic range is often required, as the primary goal is suppression of the patient’s respiratory effort.  Adjust the vent to fit the patient’s needs as their respiratory drive gradually returns.

A good sedation strategy keeps the patient comfortable while the priorities listed above are accomplished.  A good clinical pearl is to take care of any sedation needs prior to placing the patient on the transport vent, since a comfortable patient will stay synchronous with the vent while any necessary adjustments are made.