Mechanical Ventilation
Case 1 Answers
A 55 year-old man with a history of COPD presents to the emergency room with a two day history of worsening shortness of breath which came on following a recent viral infection. In the emergency room, his oxygen saturation is 88% on room air. He is working hard to breathe and is only speaking in short sentences. On exam, he has diffuse wheezes and a prolonged expiratory phase. His chest x-ray reveals changes consistent with COPD but no new focal infiltrates. An arterial blood gas (ABG) is done and shows pH 7.17, PCO2 55, PO2 62, HCO3- 25.
What are the indications for starting a patient on mechanical ventilation?
There are several primary indications for initiating mechanical ventilation including: hypercarbic respiratory failure, hypoxemic respiratory failure, to prevent or reverse atelectasis, to prevent or reverse ventilatory muscle fatigue, to permit sedation and/or neuromuscular blockade (eg. surgery), to stabilize the chest wall (eg. massive flail chest) or to ensure airway protection (eg. a patient with altered mental status and a large upper gastrointestinal bleed).
One should be aware that with hypercarbic and hypoxemic respiratory failure, there are no specific thresholds that are used to determine when to initiate mechanical ventilation. For example, you do not automatically intubate a patient when their PCO2 rises above 60 mm Hg or their room air PO2 falls below 55 mm Hg. Instead, you must take into account the overall clinical situation and assess whether the degree of hypercarbia or hypoxemia is putting the patient’s life at risk. If they can be supported by other means, such as oxygen by face mask, you might hold off on initiating mechanical ventilation, whereas if their PO2 continues to fall despite high levels of supplemental oxygen, mechanical ventilation would be necessary. Similarly, if a patient is hemodynamically stable, you might try oxygen by facemask for hypoxemic respiratory failure but if the patient is showing signs of sepsis with hemodynamic instability and multiorgan dysfunction, you would move more quickly to intubate the patient and stabilize their respiratory status in order that you can focus on other important issues.
What do you think about the possibility of using non-invasive positive pressure ventilation (bi-level positive airway pressure) in this patient?
There are two forms of mechanical ventilation – invasive mechanical ventilation, in which an endotracheal tube is inserted in the patient’s airway, and non-invasive ventilation, in which the patient receives respiratory support through a tight fitting mask applied to their face. There are some situations in which invasive modes of mechanical ventilation are necessary and other situations in which patients can be supported by non-invasive means.
This particular patient falls into the latter category. Even though he is clearly struggling to breathe and has a rising PCO2 and a declining pH, it is legitimate to give him a trial of non-invasive ventilation. There is now a large amount of data showing that patients who present with COPD exacerbations and hypercarbia can be successfully managed with non-invasive ventilation; this strategy is associated with a decreased need for intubation and initiation of mechanical ventilation, improved survival and shorter ICU stays when compared to managing these patients without non-invasive ventilation (eg. oxygen by face mask alone). Given the data in this regard, this patient should be given a trial of non-invasive ventilation with close follow-up of his respiratory status. If he improves, he can remain on non-invasive ventilation but if his oxygenation or hypercarbia worsens despite optimal non-invasive ventilation, or if he becomes unresponsive or uncooperative, he will require intubation and invasive mechanical ventilation.
What is the difference between bi-level positive airway pressure (BiPAP) and continuous positive airway pressure (CPAP)? What are the indications for using these different modes of non-invasive mechanical ventilation?
In CPAP therapy, a constant level of pressure is applied to the airways throughout the respiratory cycle (inhalation and exhalation). This pressure serves to stent open the large airways and prevent alveoli from collapsing, thereby avoiding atelectasis and improving oxygenation. There is no additional pressure delivered during inhalation and, therefore, no ventilatory support. In bi-level positive airway pressure, also referred to as non-invasive positive pressure ventilation, the expiratory pressure applied to the airways during exhalation is the same as the pressure applied in CPAP therapy. During inhalation the device imparts additional pressure (pressure support or inspiratory assist) to the airways that serves to assist the ventilatory muscles in their efforts to generate inspiratory flow to the alveoli.
These differences in the way CPAP and bi-level positive airway pressure work have a large impact on the clinical situations in which they can be employed. Because of its effect on transmural pressure and its ability to prevent upper airway collapse, CPAP is indicated for management of obstructive sleep apnea. There are also data to support its use in patients with cardiogenic pulmonary edema; the applied pressure improves oxygenation by stenting open the alveoli. CPAP also improves hemodynamics in cardiogenic pulmonary edema by decreasing afterload and pre-load, thereby improving left ventricular function. Because of the added pressure during inhalation, bi-level positive airway pressure provides a means to support ventilation and the work of breathing. There is clear data to support its use in COPD exacerbations and to support patients with other forms of ventilatory failure such as amyotrophic lateral sclerosis and muscular dystrophy. There is less clear data supporting its use in asthma exacerbations, although many clinicians will often give asthma patients a trial of this therapy during an asthma exacerbation. There are also some data to suggest it has a role in treating oncologic patients with pneumonia. CPAP does not provide any ventilatory assistance and, therefore, should not be used in these situations.
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