Covid-19 -Respiratory Failure Management

EMcrit Project Recommendations



High Flow Nasal Cannula

 
safety of HFNC
  • There is widespread concern that using HFNC could increase the risk of viral transmission.  This doesn’t appear to be evidence-based.
  • Guidelines recommend HFNC
    • ANZICS guidelines on COVID-19 state the following:
      • “High flow nasal oxygen (HFNO) therapy (in ICU): HFNO is a recommended therapy for hypoxia associated with COVID-19 disease, as long as staff are wearing optimal airborne PPE.”
      • “The risk of airborne transmission to staff is low with well fitted newer HFNO systems when optimal PPE and other infection control precautions are being used. Negative pressure rooms are preferable for patients receiving HFNO therapy.”
    • Surviving Sepsis Guidelines state:  “For acute hypoxemic respiratory failure despite conventional oxygen therapy, we suggest using HFNC over conventional oxygen therapy (weak recommendation, low quality of evidence”
    • WHO guidelines on COVID-19 state that “Recent publications suggest that newer HFNC and NIV systems with good interface fitting do not create widespread dispersion of exhaled air and therefore should be associated with low risk of airborne transmission.”
  • Reasons that HFNC might not increase viral transmission are:
    • HFNC supplies gas at a rate of ~40-60 liters/minute, whereas a normal cough achieves flow rates of ~400 liters/minute (Mellies 2014).  Therefore, it’s doubtful that a patient on HFNC is more contagious than a patient on standard nasal cannula who is coughing.
    • HFNC typically requires less maintenance than invasive mechanical ventilation.  For example, a patient who is on HFNC watching television may be less likely to spread the virus compared to an intubated patient whose ventilator is alarming every 15 minutes, requiring active suctioning and multiple providers in the room.
    • The intubation procedure places healthcare workers at enormous risk of acquiring the virus, so intubation with a goal of reducing transmission is probably counterproductive (see figure above from Tran 2012).
      • 👁 Image of risk factors for nosocomial SARS transmission from Tran et al. here.
    • Existing evidence does not support the concept that HFNC increases pathogen dispersal substantially (although the evidence is extremely sparse).  This includes a small study of patients with bacterial pneumonia (Leung 2018) and an abstract regarding particulate dispersal by volunteers (Roberts 2015).
  • One possible compromise might be to use HFNC with a moderate rate of flow (e.g. 15-30 liters/minute, rather than 40-60 liters/minute).  Since 15-30 liters/minute flow is close to a baseline minute ventilation for a sick respiratory failure patient, adding this level of flow is unlikely to affect matters substantially.
evidentiary basis for HFNC
  • HFNC is generally a rational front-line approach to noninvasive support in patients with ARDS (based partially on the FLORALI trial).
  • One case series from China suggested that HFNC was associated with higher rates of survival than either noninvasive or invasive ventilation (of course, this could reflect its use in less sick patients)(Yang et al, see table 2).
  • A management strategy for COVID-19 by a French group used HFNC preferentially, instead of BiPAP (Bouadma et al.).

noninvasive ventilation (BiPAP & CPAP)

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traditional BiPAP probably isn’t useful for most patients
  • Reasons to avoid BiPAP:
    • In a multicenter cohort of 302 patients with MERS coronavirus, 92% of patients treated with BiPAP failed this modality and required intubation (Alraddadi 2019).
    • In the FLORALI trial of ARDS patients (with mostly pneumonia of various etiologies), patients randomized to BiPAP did worse compared to patients randomized to HFNC.
  • BiPAP could have a niche role in patients with combined syndromes (e.g. COPD plus COVID-19).  For more on the selection of BiPAP vs. HFNC, see this chapter on noninvasive respiratory support.
continuous positive airway pressure (CPAP) might be the best modality of noninvasive support ??
  • Atelectasis leading to hypoxemia seems to be a major problem among these patients.
    • 👁 Image of progressive alveolar collapse.
  • CPAP could have major advantages here:
    • CPAP can provide the greatest amount of mean airway pressure, and thus most effective recruitment.
      • 👁 Image comparing mean airway pressure due to CPAP vs. BiPAP.
    • CPAP doesn’t augment tidal volumes, so this could facilitate more lung-protective ventilation.
  • Possible approach to CPAP therapy in COVID-19:
    • Increase the CPAP pressure to 15-18 cm if tolerated.
    • Titrate FiO2 against oxygen saturation.  Falling FiO2 requirements indicate effective recruitment, whereas rising FiO2 requirements suggest CPAP failure.
    • Monitor tidal volumes and minute ventilation
      • 👁 Image illustrating how a noninvasive ventilator can be used as a monitor.
  • Further discussion of CPAP in COVID-19.
Indications for intubation?  
  • COVID-19 may cause hypoxemia with relatively little respiratory distress (“silent hypoxemia”).  For example, patients may be profoundly hypoxemic yet not be dyspneic – and such patients may “look” fine.  Therefore, work of breathing cannot be relied upon to detect patients who are failing HFNC.
  • There should probably be a lower threshold to intubate in COVID-19 than in most patients, for the following reasons:
    • Patients can develop worsening “silent” atelectasis and decline rather abruptly, without lots of symptoms.
    • Oxygenation techniques used to maintain saturation during intubation (e.g. mask ventilation) may increase virus aerosolization.  Thus, “pure” rapid sequence intubation without bagging is preferred.  This will be safer if the patient is starting out with more oxygenation reserve.
    • Intubation requires considerable preparation, so a semi-elective intubation is preferred to crash intubation.
  • Exactly when to intubate is always a clinical decision.  Potential indications would include:
    • Progressively rising FiO2 requirements.
    • Increasing work of breathing & clinical distress.
    • High absolute oxygen requirement.

Intubation Procedure

  • This represents a high risk for transmission to healthcare workers.
  • Airborne precautions are indicated (e.g. N95/FFP2 masks or positive air-purifying respirators, along with full face shields and full contact precautions).
  • Rapid sequence intubation with no bag-mask ventilation may avoid aerosolizing particles.  However, during the apneic period, a bag-valve mask with a PEEP valve could be passively held on the patient’s face to maintain positive pressure in the airway and thereby prevent de-recruitment.
  • Use of videolaryngoscopy may avoid placing the operator’s face close to the patient.
  • Attach a viral filter to the bag-valve mask before the procedure, if possible.  This should reduce the spread of viral particles out of the endotracheal tube following intubation (or during bag-mask ventilation if that is required)(Peng et al. 2/27).
  • Endotracheal tube confirmation with a stethoscope could pose a risk of transferring virus to the practitioner.  It could be safer to advance the endotracheal tube to a pre-determined depth, calculated based on the patient’s height (see MDCalc formula here).
  • 👁 Image of intubation protocol for COVID-19.

Invasive Mechanical Ventilation

pathophysiology:  COVID does not cause typical ARDS
  • COVID doesn’t appear to cause substantially reduced lung compliance (which is generally a hallmark finding of ARDS).
  • The predominant problem might be one or more of the following:
    • (i) Atelectasis (alveolar collapse).
    • (ii) Drowning of the alveoli by fluid.
  • If the predominant problem is atelectasis, then this will be relatively easy to manage.  Any strategy to increase the mean airway pressure will treat atelectasis (e.g. APRV or conventional ARDSnet ventilation using a high-PEEP strategy).
  • If drowning of the alveoli is a significant issue, this is a bit harder to manage.  Proning may facilitate drainage of secretions.  APRV may also be useful to facilitate airway clearance (rapid dumping breaths create expiratory airflow that can facilitate secretion clearance).
conventional ARDSnet ventilation
  • Tidal volumes should be targeted to a lung-protective range (6 cc/kg ideal body weight).
    • MDCalc can be used to calculate appropriate endotracheal tube depth & tidal volumes.
  • High PEEPs should be utilized (SSC guidelines).  An ARDSnet “high PEEP” table is shown below.  This table doesn’t need to be followed exactly, but it may be useful as a general guide.
    • 👁 Image of ARDSnet low-PEEP & high-PEEP tables here.
airway pressure release ventilation (APRV)
  • My opinion is that early APRV could be very useful for these patients (i.e. used as the initial ventilator mode, rather than a salvage mode).  APRV may be well suited to the pathophysiology of COVID, because it provides a high mean airway pressure and facilitates secretion clearance.
  • A practical guide to using APRV can be found here.  A reasonable starting place is generally:
    • P-high:  30-35 cm (higher if more profound hypoxemia)
    • P-low:  zero
    • T-high:  5 seconds
    • T-low:  0.5 seconds (titrate based on flow rates; consider reduction if tidal volume >8 cc/kg)
  • Improvement in oxygenation seen with APRV often takes several hours as lung tissue gradually recruits.
  • APRV initiation can cause hemodynamic shifts, so pay careful attention to blood pressure during initiation.
  • True failure to respond to APRV within 12-24 hours (e.g. with PaO2/FiO2 <100-150) would be a strong argument to move towards prone ventilation (discussed here).  However, when started early APRV may be more likely to succeed – thereby avoiding the need for proning.
permissive hypercapnia & optimization of metabolic acid/base status
  • Regardless of the ventilator mode, permissive hypercapnia may be useful.  The safe extent of permissive hypercapnia is unknown, but as long as hemodynamics are adequate, a pH above roughly ~7.15 may be tolerable (hypercapnia is preferred over lung-injurious ventilation).
  • A common error is to focus solely on respiratory parameters in order to improve the pH, while ignoring metabolic acid/base status.  For example:
    • ICU patients often have non-anion-gap metabolic acidosis (NAGMA).  Treatment of NAGMA with bicarbonate may be the safest way to address a low pH (rather than increasing the intensity of mechanical ventilation and thereby threatening the lung).
    • Even if the metabolic acid/base status is normal, IV bicarbonate may still be considered to improve pH, while simultaneously continuing lung-protective ventilation (discussed here).  Targeting a mildly elevated serum bicarbonate can facilitate safe ventilation with low tidal volumes (more on different forms of IV bicarbonate here).
proning
  • Prior to consideration of proning, optimization on the ventilator for 12-24 is generally preferable (discussed here).
  • For failure to respond to initial ventilator optimization (e.g. with persistent PaO2/FiO2 below 150 mm), prone ventilation should be considered.
  • Reports from Italy describe proning as extremely effective.
    • This makes sense, because proning is expected to be effective for basilar lung recruitment and secretion clearance (which seem to be the primary problems with these patients).
    • The question is whether the same effect could be achieved more easily using APRV.  Proning is very labor-intensive and will require consumption of lots of personal protective equipment (since multiple providers will need to the turn the patient repeatedly).  If the same effect can be achieved with APRV, that could be an easier solution (especially at centers which lack extensive experience with proning).

Disaster ventilation

 splitting ventilators
  • In a dire emergency, one ventilator can be used to support several patients.
  • Pressure-cycled ventilation should be used, with a driving pressure <13-15 cm (Aoyama et al. 2018).
  • Blog exploring the general strategy to setting the ventilators here.
  • Columbia Presbyterian protocol for splitting ventilators here.
  • Some additional ideas about how to hook everything up here.

Extubation

potential pitfalls 
  • Patients with COVID-19 often respond well to intubation and positive pressure ventilation (probably reflecting lung recruitment).  Unfortunately, they may continue to have a tendency to de-recruit their lungs.  Consequently, there may be an increased risk of deterioration after extubation.
  • According to a webinar with Italian intensivists, “Do not trust the first improvement,” because patients may have early relapses.
    • However, prolonged intubation also carries risk.
    • Can inflammatory markers predict who will relapse?  For example, if the patient is improving clinically but CRP, LDH, and lymphopenia are all worsening – perhaps this predicts clinical worsening and suggests that extubation should be deferred?
post-extubation support
  • ANZICS guidelines state that HFNC and/or noninvasive ventilation (with a well fitted facemask and separate inspiratory and expiratory limbs) can be considered as bridging therapy post-extubation, but must be provided with strict airborne PPE.
    • CPAP therapy or BiPAP (with high end-expiratory pressure) might be useful to prevent de-recruitment in these patients. (More on COVID-19 & CPAP here).
    • By the time of extubation, patients will often have been ill for well over a week.  It’s likely that their viral load will be decreasing at that point, so the risk of virus transmission may be lower (compared to the initial intubation).  More on transmission above.

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