The Pre-brief
A 45-year-old male with class-III obesity is intubated for severe ARDS. He is currently paralyzed and is being mechanically ventilated with the following settings: Volume control (VC-CMVs), TV = 6 cc/kg, RR = 28, PEEP = 24, FiO2 = 0.8. His oxygenation at these settings is acceptable. A routine turn is performed after placing the patient from sitting position to supine position. A few seconds after this, SpO2 drops to the 70s and the peak pressure alarm is going off intermittently. The RT disconnects the patient from the ventilator and starts manual bag-mask ventilation.
What are your thoughts about this practice? Is there an alternative approach?
Introduction
Strictly speaking, any device that provides ventilatory assist is a ventilator. By that definition, a self-inflating bag-valve mask (BVM) would qualify as a ventilator, albeit of the simplest type. BVM is undoubtedly an indispensable tool for providing emergent ventilatory assist. Typically, once the patient has been intubated, the task of providing ventilatory assist is taken over by much more sophisticated mechanical ventilators.
Yet, it is often taught that in an event of acute respiratory decompensation, one should disconnect the patient from the ventilator and perform manual bag-mask ventilation. The definition of ‘acute respiratory decompensation’ in this context may be rather subjective but generally involves acute hypoxemia and/or increased ventilatory pressures. So why is it that we opt to switch to a more primitive form of ventilatory assist in these acute scenarios? In this post, we will try to add some granularity to this discussion.
Potential benefits of switching from mechanical ventilator to BMV
Say you have two machines that can perform the same critical task except one is of far superior quality than the other. When would you ever switch from superior to the inferior machine? When there is a technical issue with the better machine to temporize things. It is underappreciated that the occurrence of a mechanical ventilator malfunction causing acute clinical deterioration is exceedingly rare. Some potential instances of device malfunction are listed below:
- Clogging of the heat-and-moisture exchanger (HME):
Probabilistically, this is perhaps the most likely “vent malfunction” you may encounter. Typically, this is a patient with a large amount of thick secretions that clogs up an HME that hasn’t been changed in a while. Clinically, this will appear as proximal airway occlusion (increased airflow resistance). Usually, this process would be gradual and hence picked up while examining the classic resistive pattern on the flow waveform. Nonetheless, I have seen at least one case where sudden clogging of an HME resulted in acute inability to ventilate. - Clogged expiratory filter or exhalation (PEEP) valve malfunction:
The expiratory filter may also get clogged if it is not changed in a while. Exhalation valve malfunction is an extremely rare aberrancy. Both these issues will also likely progress gradually and not cause a respiratory emergence. More importantly, these are rather easy to detect by paying attention to the pressure waveform (Figure 1 from a recent case discussed here).
Miscellaneous:
Some may say that dys(a)synchrony such as the phenomenon of auto (false) triggering can be included in this list (e.g. from leaks, cardiac oscillations, water in the circuit etc.). However, false triggering generally doesn’t cause catastrophic respiratory compromise. Furthermore, this is something that should be readily identified with a quick look of the ventilator and easily fixed by switching to a pressure trigger.
Finally, no machine is infallible and freak occurrences such as sudden loss of power has been reported.
The several pitfalls of switching to BMV
- Loss of diagnostic information: In an acute scenario, it is critical to get diagnostic information to understand what’s causing the problem. The ventilator provides a plethora of information that may help answer this question. E.g., if the problem is sudden increase in peak airway pressure, checking the plateau pressure will guide as to whether the main load is elastic or resistive. The importance of looking at the waveforms cannot be overstated. We lose all this advantage when we switch to BMV.
- Risk of decruitment: Switching from ventilator to BVM may result in sudden derecruitment in an ARDS patient. This may be avoided by clamping the endotracheal tube during the switch. A PEEP valve should also be attached to the BVM. Note that the maximum PEEP that can be provided with a PEEP valve is typically 20 cmH2O, which may not be enough in certain cases.
- Lack of tidal volume control: The typical BVM has a volume of ~1600cc. In a crunch scenario, it is not uncommon to provide dangerously large tidal volumes. This may be of special relevance in patients with ARDS and hemodynamic lability.
- Lack of respiratory rate control: By the same token, it is common to bag too rapidly. This may cause significant auto-PEEP and its several ill-effects in a patient with high airway resistance.
- Concern for aerosolization of infectious droplets: If an expiratory filter is not deployed on the BVM, bag-mask ventilation will lead to aerosolization of infectious particles. This is point is especially relevant in the current times.
General thoughts on this issue
The knee-jerk practice of switching from ventilator to BVM is often unnecessary and potentially harmful. The incidence of actual ventilator malfunction is very rare to justify this as a routine practice. Even if the switch is made and no immediate improvement occurs, the patient should be switched back to the ventilator due to the various benefits listed above.
A sample initial approach to acute respiratory decompensation in intubated patients is as follows –
- Take control of ventilation: sedate and/or paralyze
- If a sudden increase in peak inspiratory pressures is the chief concern and the circuit has an HME, remove the HME. It should be fine to ventilate without humidification for a few minutes.
- If there is any concern for false (auto) triggering, switch to pressure trigger.
- Look at the pressure waveform to see if there’s a problem in the expiratory limb of the circuit.
Once these steps have been performed and no device malfunction is identified, I can’t think of any reason why one would disconnect the ventilator bag manually. From here on, diagnostic information from the ventilator would help identify the actual problem. An important point worth mentioning here is that in conditions causing high peak airway pressure, the pressure alarm setting becomes very important. This is not just an alarm but once the set pressure threshold is reached, the breath is cycled off (pressure cycling) to avoid further increases in airway pressure. Hence, the pressure alarm setting may have to be liberalized (along with minimizing the delivered tidal volume) to allow continued ventilation from the mechanical ventilator.
A particular scenario where disconnection of the ventilator is considered therapeutic is a patient with critically high autoPEEP. This has been used as another reason to empirically disconnect the ventilator in respiratory emergencies. Firstly, this is certainly a potentially life saving maneuver and should be performed when critical autoPEEP is strongly suspected (more on this here). However, using this as an argument for unselected disconnection of the ventilator in all respiratory emergencies cannot be justified. A quick glance as the flow waveform would help identify patients that would benefit from this maneuver. (As an aside, setting the respiratory rate to the minimum level (e.g. 1/minute) will achieve the same effect as disconnecting the ventilator.)
Let’s discuss the case in the pre-brief. There is no evidence of device malfunction here and there is a good reason for why this patient dropped their SpO2. This patient is at high risk of derecruitment with altered positioning. Sitting or reverse trendelenburg positioning allow offloading of the chest wall in obese patients. A supine position will cause significant mass loading. Changing the position and increasing the PEEP (optionally preceded by a recruitment maneuver) would help in this scenario. As mentioned before, switching to BMV would risk further decruitment. Also, the maximum PEEP that can be provided with a BVM PEEP valve is typically 20, which may not be enough in this case.
The Debrief
- Disconnecting the ventilator and manual BMV should be performed in cases where device malfunction is the primary cause of decompensation. These instances are quite rare.
- Disadvantages of switching to BMV include loss of diagnostic information, risk of decruitment, lack of tidal volume and respiratory rate control.
- Attention to waveforms is crucial.
- A systematic initial approach to these scenarios would obviate the need for ventilator disconnection in most instances
References
- Passive heat and moisture exchangers. Blog: Deranged Phyiology https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%20507/passive-heat-and-moisture-exchangers
- Krishna Kumar B, Ravi M, Dinesh K, Nanda A. Ventilator malfunction. J Anaesthesiol Clin Pharmacol. 2011;27(4):576. doi:10.4103/0970-9185.86623
Is that also applied to patients who develop cardiac arrest while mechanically ventilated?
Thank you
Cardiac arrest is a different beast. There may be chaotic triggering with chest compressions and high PIP generated during the compression phase. Nonetheless, it’s possible to safely (& effectively) ventilate with the vent ( https://twitter.com/norfolk_tim/status/1371407408224997377…)
Although it’s not mainsteam