[6-24-2017 Whenever you have a severe asthmatic who may need intubation, remember to try Bi-Pap and see the resources referenced in Severe Asthma – Some Resources
Posted on December 31, 2016. That post has links to many of Dr. Weingart of EMcrit’s outstanding teaching podcasts and pdfs]
The source of the information of the post below is from Dr. Wall’s 2012 book (1).
1 to 3% of acute severe asthma attacks will require rapid sequence intubation (RSI) and mechanical ventilation even though mechanical ventilation is difficult to manage in these patients.
The management of a severe acute exacerbation of COPD is similar to that of status asthmaticus. However, because much of their airway obstruction is fixed, getting these patients off the mechanical ventilator can be very difficult. Therefore, if possible, a trial of Bi-PAP is indicated.
[1-4-2017: Dr. Weingart states that Bi-PAP is also the go to method for asthma patients who are tiring. See my post Severe Asthma – Some Resources. Posted on December 31, 2016
These patients (both asthmatics and COPDers) have been working hard to breath for hours and are physically exhausted. Often, the doctor will be “forced to act” meaning that a rapid sequence intubation is indicated even if evaluation suggests that the airway may be difficult (there may not be time for an awake intubation with the fiberoptic bronchoscope).
Diaphoresis is a particularly ominous sign, and the diaphoretic asthmatic patient who cannot speak full sentences, appears anxious, or is sitting upright and leaning forward to augment respiratory effort should not be left unattended until stabilized.(1)
If time permits, patients with asthma or COPD requiring intubation should receive 1.5 mg per kg of IV lidocaine three minutes before receiving the induction dose of the sedative (ketamine in asthma patients and etomidate in COPD patients who often have other medical problems that argue against ketamine because of its catecholamine release).
Summary for Initial Ventilator Settings (2)
1. Determine the patient’s ideal body weight.
2. Set a tidal volume of 6 to 8 ml per kg with FiO2 of 1.0 (100% oxygen)
3. Set a respiratory rate of 8 to 10 breaths per minute.
4. Set an I/E ratio of 1:4 to 1:5.* Pressure control is preferred. If using pressure control, the I/E ratio is adjusted directly by the I/E ratio parameter or by adjusting the inspiratory time parameter. If using volume control, the I/E ratio can be adjusted by increasing the peak flow rate, and the ramp inspiratory waveform should be selected. Peak IF can be as high as 80 to 100 L per minute.
5. Measure and maintain the plateau pressure (Pplat) at less than 30 cm H2O; try to keep the PIP at less than 50 cm H2O.***
6. Focus on the oxygenation and pulmonary pressures initially. If necessary, allow maintenance or gradual development of hypercapnea to avoid high plateau pressures and [resulting in] increasing auto-PEEP.
7. Ensure continuous sedation and analgesia with a benzodiazepine and a nonhistamine-releasing opiod, such as fentanyl, and consider paralysis with a nondepolarizing muscle relaxant if it is difficult to achieve ventilation goals.
8. Continue in-line beta 2-agonist therapy and additional pharmacologic adjunctive treatment based on the severity of the patient’s illness and objective response to treatment.
*The I/E ratio is the ratio of the inspiratory time to the expiratory time. Because patients with severe asthma [as well as those with COPD] have outflow obstruction, the expiratory time needs to be as long as possible to avoid auto-PEEP. Auto-PEEP is the development of unintended PEEP from incomplete exhalation of the delivered tidal volume. Auto-PEEP increases the risk of lung damage from mechanical ventilation (barotrauma).
**The plateau pressure (Pplat) is the pressure measured during a 0.2 to 0.4 second pause at end-inspiration. The goal is to keep Pplat at a level of less than 30 cm H2O because at that level of pressure is not usually associated with lung damage (barotrauma). Auto-PEEP is unlikely with a Pplat below 30.
***The peak inspiratory pressure (PIP) is the highest airway pressure measured during inspiration. It is not as predictive of barotrauma as the plateau pressure. However, it is a very important parameter to follow because: “A sudden rise in PIP should be interpreted as indicating tube blockage, mucous plugging, or pneumothorax until proven otherwise. A sudden dramatic fall in PIP may indicate extubation.” (3)
Monitoring the Mechanical Ventilation
“If initial ventilator settings disclose a Pplat of more than 30 cm H2O, consider lowering minute ventilation and increasing IF (inspiratory flow), both of which will prolong expiratory time and attenuate hyperinflation. If Pplat is unavailable, PIP may be used as a surrogate.” (4)
“The concept of controlled hypoventilation (permissive hypercapnia) promotes gradual development (over 3 to 4 hours) and maintenance of hypercapnia (PCO2 up to 90 mm H2O and acidemia (pH as low as 7.2).” (5) The purpose is to reduce the incidence of lung injury and hypotension from auto-PEEP. The way to do it is by reducing the minute ventilation and increasing the IF to 80 to 120 L per minute. This requires sedation and analgesia and, sometimes , neuromuscular blockade.
Hypotension in Mechanical Ventilation
Hypotension in mechanical ventilation can be due to tension pneumothorax or to relative or absolute volume depletion from decreased venous return due to increased intrathoracic pressure.
Tension pneumothorax is the least common cause. And now it can rapidly and reliably be diagnosed by bedside ultrasound exam as well as by chest x-ray.
If tension pneumothorax is ruled out, then empiric infusion of 1 to 2 liters of normal saline is indicated.
(1) Manual of Emergency Airway Management 4th edition, 2012, RM Walls and MF Murphy. pp. 368 – 376.
(2) – (5) Ibid.