Mechanical Ventilation
Case 10 Answers
A 57 year-old woman was intubated two weeks ago for respiratory failure resulting from ARDS due to urosepsis. About 5 days ago, her oxygen requirements declined such that she is now on an FIO2 of 0.4 with a PEEP of 5 cm H2O. She has been doing poorly, however, on her spontaneous breathing trials and has not been able to be separated from the ventilator. On her latest spontaneous breathing trial this morning, her tidal volumes were between 125 and 150 ml and her respiratory rates rose to 35 after only 10 minutes of spontaneous breathing.
What items should you consider on the differential diagnosis for a patient who cannot be liberated from the mechanical ventilator?
When a person is repeatedly failing spontaneous breathing trials it is not sufficient to simply put them back on the ventilator and repeat spontaneous breathing trials day after day. You must consider why they are failing these trials and search for reversible causes whose elimination might facilitate separation from the ventilator. The main reason that most patients fail spontaneous breathing trials is that their primary process has not improved sufficiently. Beyond this, there are several broad categories of other problems that contribute to persistent ventilatory failure and inability to separate from the ventilator. The first category includes neurologic issues such as an insufficient or absent respiratory drive, the lingering effects of sedative or narcotic medications due to accumulation in body stores and anxiety. The second broad category involves problems of neuromuscular competence. The problems that fall into this category leave the patient too weak to do the work of breathing on their own and include issues such as critical illness polyneuropathy/myopathy, insufficient nutritional status, electrolyte abnormalities (hypokalemia, hypophosphatemia, hypomagnesemia), hypothyroidism, and lingering effects of neuromuscular blocking agents. The final broad category relates to problems in which the demands being placed on the patient are too high. In other words, the patient is being asked to do too much work in order to pass their spontaneous breathing trials. This category includes items such as high airway resistance due to COPD or airway secretions, poor respiratory system compliance stemming from ARDS, pulmonary edema, auto-PEEP, large pleural effusions, or abdominal distention and, finally, high minute ventilation needs which may result from untreated infection, fevers, overfeeding, excessive dead-space or untreated metabolic acidosis.
What diagnostic steps can you consider to help you sort through this differential?
There are several simple measures that can be undertaken as part of the initial work-up including a physical exam to search for evidence of polyneuropathy, abdominal distention or volume overload, reviewing the patient’s medication history and fluid balances, and ordering pertinent laboratory studies (potassium, phosphate, magnesium, TSH). Review of the patient’s imaging studies is also useful as this may reveal the existence of persistent edema or large pleural effusions.
If your review of the pertinent patient information reveals that they are failing their spontaneous breathing trials because their minute ventilation needs are too high, you can consider ordering a study referred to as a “metabolic cart.” In this test, special equipment is brought to the bedside by a respiratory therapist who then collects exhaled gases and arterial blood gases from the patient. This information is used to determine the rate of CO2 production by the patient as well as to calculate the amount of their minute ventilation that goes to clearing their dead-space (dead-space fraction). Normal CO2 production is about 250 ml/min. Patients with higher rates than this will have to breathe more (i.e. higher minute ventilation) in order to clear all of this CO2 from their body and prevent rising arterial CO2 levels and worsening pH. A normal dead-space fraction is about 0.25-0.35. If the dead-space fraction is too high (eg. > 0.60-0.65) the patient will also have to maintain a high minute ventilation just to maintain sufficient alveolar ventilation, prevent hypercarbic respiratory failure and preserve their acid-base balance.
If you identify that the cause of their problems is excessive CO2 production, you can then search for the underlying cause of that problem. For example, the patient may be receiving too much nutritional support or may have an unrecognized metabolic acidosis. If you determine that the problem is excessive dead-space ventilation, you can search for causes of that problem such as pulmonary embolism or dynamic hyperinflation. However, in many cases, the high dead-space is due to the underlying lung disease (eg. slowly resolving ARDS) and there is not much you can do to fix the problem besides give the patient time to improve.
What can you do to help her get off the ventilator?
The most important thing to do in these cases is a systematic search for reversible causes of failure to separate from the ventilator and to treat those causes. For example, if your patient is volume-overloaded, they may benefit from diuresis. The patient with large pleural effusions may benefit from drainage of those effusions.
In the absence of easily reversible causes, patients usually need time for their underlying problem to resolve or for them to regain strength. These patients should receive daily spontaneous breathing trials using either T-piece of pressure support ventilation. SIMV should not be used for this purpose. Patients should be given one trial per day and in between trials should be put back on a sufficient level of respiratory support (either Assist Control or pressure support ventilation) to prevent tachypnea and maintain patient comfort. The goal is to allow the patient to rest and to avoid ventilatory muscle fatigue. A good way to think of this is as follows: Suppose a person just ran a marathon. You would not ask that person to then run a 10 km race as a follow-up. You would let them rest. Similarly, the patient who tires out during spontaneous breathing should not be asked to do any more work. Their ventilatory muscles need the rest.
Over time, you will gain a sense of whether the patient is close to getting off the ventilator or whether they will need a lot more time. For patients in whom it is clear they will need a significant period of time to get off the ventilator, consideration can be given to transferring the patient to a long-term acute care facility that specializes in the care of such patients.
At what point do you consider placing a tracheostomy tube in this patient?
There are many different indications for placing a tracheostomy tube in a patient. For example, some patients need them because they have lost the ability to protect their airway.
Patients who cannot be separated from the ventilator for prolonged periods of time eventually require tracheostomy placement in order to reduce the risk of complications from long-term use of an endotracheal tube. Tracheostomy tubes also offer the benefits of decreased sedation needs, increased patient comfort, increased chances for the patient to eat or speak, ease of patient transfer and ease of taking the patient on and off the ventilator without the need for reintubation and its associated risks if they fail a period of spontaneous breathing.
There is considerable debate as to when the tracheostomy tube should be placed. Some centers advocate placing the tracheostomy tube within the first several days of a severe illness such as ARDS while other centers prefer to wait longer periods of time (eg. 3 or 4 weeks or even longer) before deciding to pursue this option. At this time, there are no good data to help resolve this argument and the decision is very institution and physician-dependent.
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