Hospital Medicine

Invasive Mechanical Ventilation

I. Problem/Challenge.

The most difficult challenge to face by a hospitalist in regards to invasive mechanical ventilation are when to intubate, what setting the ventilator should be set at and troubleshooting the ventilator.

II. Identify the Goal Behavior.

A hospitalist will need to identify when a patient needs intubation and then be able to successfully select the correct ventilator setting to best suit the needs of the patient and subsequently troubleshoot the ventilator when things go wrong.

III. Describe a Step-by-Step approach/method to this problem.

Decide to intubate:

A good rule of thumb is if you are considering intubation for a patient then you should probably intubate the patient.

Non-invasive ventilation (Bilevel Positive Airway Pressure or BiPAP) should only be implemented in patients that have immediately reversible conditions such as pulmonary edema or chronic obstructive pulmonary disease (COPD) exacerbation. Patients with respiratory failure that have conditions requiring more than a few hours to recover are better candidates for mechanical ventilation (e.g., Pneumonia requiring intravenous [IV] antibiotics).

One should consider intubation if any of the following criteria are met:

  • Impaired oxygenation: Hypoxemic respiratory failure

  • Impaired ventilation: Hypercapnic respiratory failure

  • Airway protection: increased work of breathing, facial trauma, severe alcohol withdrawal, upper airway obstruction, severe metabolic acidosis with inadequate respiratory compensation

Choose the mode of ventilation:

There are 3 most common modes of ventilation to choose from:

1) Assist control (AC)

-volume control ventilation: set volume given, patient determines pressure

-pressure control ventilation: set pressure given, patient determines volume

2) Synchronized Intermittent Mandatory Ventilation (SIMV)

3) Pressure Regulated Volume Control (PRVC): advanced volume mode that limits flow to reduce pressure

Initially one should start with either AC or SIMV. Volume control is a good mode for patients that are more sedated; especially for underlying pulmonary disease (high likelihood of fighting the vent in awake patients). Pressure control is good for patients that are more awake (most comfortable mode for the patient). SIMV is an excellent mode for younger patients with good respiratory function. It helps work the respiratory muscles as patients are able to initiate their own breaths but ventilator provides minimum mandatory breaths as well.

Choose the respiratory rate:

This may depend on why the patient needed invasive ventilation.

If it was for airway protection, then start with a normal respiratory rate.

If it was for impaired ventilation, consider a higher respiratory rate. If the patient has obstructive lung disease, then you may want to start with a lower respiratory rate to avoid Auto-positive end-expiratory pressure (PEEP).

Choose the tidal volume:

Most patients can be started at 6-8 milliliters/kilogram (ml/kg). For patients with Acute Respiratory Distress Syndrome (ARDS), initial tidal volume should be set at 4-6 ml/kg to maintain peak pressures < 30 millimeters mercury (mmHg).

If using SIMV then you must select a pressure for the pressure support breaths which should be started between 5-15 centimeters water (cm H2O). What you pick should be based on about how much tidal volume you would like the patient to receive on breaths they initiate outside of the pre-set breaths. While setting this consider the elasticity of the lungs and how much effort the patient will need to generate - weaker patients will need more pressure support.

Choose the inspired oxygen concentration(FiO2):

Inspired oxygen concentration should be set at 1.0 and lowered according to oxygen saturation measurements and arterial blood gas measurements. Goal oxygen saturation (SpO2) > 90% and partial pressure oxygen (PaO2) is > 55 mmHg.

Choose the PEEP:

PEEP helps open up the alveoli and improves oxygenation. Generally a PEEP is set at 5 cm H2O. ARDS patients require higher PEEP.

Ventilator guidelines for specific circumstances:

  • Acute Lung Injury (ALI)/Acute Respiratory Distress Syndrome

    • This condition causes a decrease in lung compliance and produces hypoxemic respiratory failure. Tables 1 and 2 summarize the Acute Respiratory Distress Syndrome network (ARDSnet) protocol for ventilating patients with ARDS/ALI (See Figure 1.)

Figure 1.

ARDSnet protocol for ventilating patients with ARDS/ALI

  • Obstructive Airway Disease

    • Mechanical ventilation may result in hyperinflation, auto-PEEP and resultant hypotension

    • Initial tidal volume should be set at 6-8 mL/kg and minute ventilation should be adjusted to keep pH at low normal. These patients often have compensated baseline hypercarbia.

    • Airway obstruction needs to be aggressively treated with bronchodilators

  • Asymmetric Lung Disease or injury (aspiration, contusion, localized pneumonia)

    • Remember that gas from the ventilator will follow the path of least resistance and therefore the non-diseased lung may get injured from overdistension and may actually worsen ventilation-perfusion ratios.

    • Positioning the patient with the less-involved lung down (dependent) may help improve blood flow

    • Expert consultation should be obtained

  • Neuromuscular Disease

    • Remember that these patients have normal lung tissue and therefore may require higher tidal volumes to avoid the sensation of dyspnea

Monitoring mechanical ventilatory support:

  • Obtain a chest x-ray post-intubation to assess tube placement

  • Obtain arterial blood gas 30-60 minutes post-intubation to assess effectiveness of the ventilator settings and intermittently as needed. Goal pH > 7.25

  • Frequently measure vital signs and directly observe the patient

  • Measure inspiratory plateau pressure as clinically appropriate

Troubleshooting:

After the initial settings are selected, one must be prepared to make changes and troubleshoot.

It is important that when making adjustments that the target is to correct pH and not partial pressure carbon dioxide (PaCO2).

The following are some common problems:

  • Dyssynchrony (fighting the ventilator)

    • Auto-PEEP

      • Results when patient physiology and ventilator setting result in an inadequate expiratory time and is common in patients with obstructive airway disease.

      • Treatment is to disconnect the ventilator and allow the patient to exhale, then reset the ventilator to increase the expiratory time either by changing the inspiration:expiration ratio or decreasing the frequency of ventilator breaths.

See Figure 2.

Figure 2.

Auto-PEEP

Breath trigger sensitivity:

  • If the trigger setting is too sensitive it may result in "autocycling" where patients get unintended breaths

  • Inspiratory flow rate

    • If the flow rate is very high it may cause the patient discomfort with inspiration

  • Hypotension associated with initiation of mechanical ventilation

    • Tension Pneumothorax

      • Look for absent breath sounds on one side and/or tracheal deviation

      • If these signs are present one should insert a large bore needle into the 2nd or 3rd intercostal space in the mid-clavicular line - Do not wait for radiographic confirmation

      • The insertion of a needle or catheter must be followed by a chest tube

    • Conversion from Negative to Positive Intrathoracic Pressure

      • With positive pressure ventilation, venous return to the right atrium is decreased which may cause left ventricular pre-load, stroke volume, cardiac output and blood pressure to decrease.

      • Treatment is volume resuscitation.

    • Auto-PEEP

      • Results when patient physiology and ventilator setting result in an inadequate expiratory time and is common in patients with obstructive airway disease.

      • Treatment is to disconnect the ventilator and allow the patient to exhale then reset the vent to increase with expiratory time either by changing the inspiration:expiration ratio or decreasing the frequency of ventilator breaths.

    • Acute Myocardial Ischemia/Infarction

      • Stress from the underlying cause of respiratory failure and/or the intubation itself may cause cardiac ischemia.

  • Persistent Hypoxemia

    • If the inspired oxygen is already set at 100%, then consider increasing the PEEP (can increase in increments of 2-3 up to 18 cm H2O)

  • High pressure alarm

    • If the peak pressure is elevated but plateau pressure is normal, consider a problem with the airway (i.e. bronchospasm, biting the endotracheal tube [ET] tube)

    • If the plateau pressure is high, then consider a problem with lung/chest wall compliance such as fibrosis, pulmonary edema, pneumothorax, or "fighting" the ventilator

See Figure 3.

Figure 3.

Pressure in the respiratory cycle

  • Hypercapnea

    • Generally speaking it is acceptable to have moderate hypercapnea (50-80mm Hg) especially if the patient has ARDS. Again, goal should be to maintain pH > 7.25.

IV. Common Pitfalls.

There is a tendency to wait too long for intubation. This is a dangerous pitfall since elective intubation carries far less risk than emergent intubation. It is important to get control of a patient's airway if they are unstable or heading towards respiratory failure.

It is also important to not cause injury with the mechanical ventilation. One important measure is to not hyperinflate the normal lung regions in patients who have acute respiratory distress syndrome. The way to prevent this is to set tidal volume at 4-6 mL/kg of ideal body weight.

V. National Standards, Core Indicators and Quality Measures.

No national standard/benchmarks established yet.

VI. What's the evidence?

Hill, LL,, Pearl, RG.. "Flow triggering, pressure triggering, and autotriggering during mechanical ventilation.". Crit Care Med. vol. 28. 2000. pp. 579.

Tobin, MJ.. "Mechanical ventilation.". N Engl J Med. vol. 330. 1994. pp. 1056.

Chiumello, D,, Pelosi, P,, Calvi, E,. "Different modes of assisted ventilation in patients with acute respiratory failure.". Eur Respir J. vol. 20. 2002. pp. 925.

Yang, SC,, Yang, SP.. "Effects of inspiratory flow waveforms on lung mechanics, gas exchange, and respiratory metabolism in COPD patients during mechanical ventilation.". Chest. vol. 122. 2002. pp. 2096.

Brower, RG,, Lanken, PN,, MacIntyre, N,. "Higher versus lower positive end-expiratory pressures in patients with the acute respiratory distress syndrome.". N Engl J Med. vol. 351. 2004. pp. 327.

Thille, AW,, Rodriguez, P,, Cabello, B,. "Patient-ventilator asynchrony during assisted mechanical ventilation.". Intensive Care Med. vol. 32. 2006. pp. 1515.

Georgopoulos, D,, Prinianakis, G,, Kondili, E.. "Bedside waveforms interpretation as a tool to identify patient-ventilator asynchronies.". Intensive Care Med. vol. 32. 2006. pp. 34,.

Nilsestuen, JO,, Hargett, KD.. "Using ventilator graphics to identify patient-ventilator asynchrony.". Respir Care. vol. 50. 2005. pp. 202.

Tobin, MJ,, Jubran, A,, Laghi, F.. "Patient-ventilator interaction.". Am J Respir Crit Care Med. vol. 163. 2001. pp. 1059.

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