Question 21

Created on Wed, 06/29/2016 - 18:45
Last updated on Sun, 06/10/2018 - 20:45
Pass rate: 34%
Highest mark: 5.75

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Critically evaluate the use of neuromuscular blocking agents in severe respiratory failure due to acute respiratory distress syndrome (ARDS).

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College Answer

Rationale & Theoretical Benefits

  • In general will reduce O2 consumption & CO2 production
  • Avoids patient ventilator dys-synchrony
  • Allows evaluation of respiratory mechanics
  • Generally required to perform a recruitment manoeuvre
  • In patients with pronounced spontaneous respiratory effort:
    • They may draw large spontaneous tidal volumes well in excess of appropriate ARDSnet values (pulmonary stress)
    • They may generate large spontaneous negative trans-pulmonary pressures during controlled breaths (pulmonary strain)
    • Spontaneous respiratory activity is difficult to otherwise supress (with sedation only), particularly in the setting of permissive hypercapnia
  • Observationally improves oxygenation in the short term
  • May reduce cytokine release and biotrauma
  • Facilitates prone positioning

Theoretical Problems

  • Contradicts strategies of maintaining spontaneous efforts and respiratory muscle strength.
  • May unmask poor compliance in patients who were generating very negative inspiratory pressures while breathing spontaneously, with consequent difficulties maintaining adequate tidal volume.
  • Potential for awareness during paralysis
  • Risks of ‘Critical Illness Weakness’ associated with use of NMBA. 
  • Risks of accumulation of NMBA drug in critical illness. 
  • Increased risk of positive fluid balance and thromboembolic risks.

Practical Issues

  • Timing / duration of paralysis not known – Papazian used 48 hr
  • Which NMB – non-steroid based may be better for reducing risk of weakness (Papazian used cisatracurium)
  • Train of four monitoring needed

Best Evidence                                                                                                         

  • Single French RCT (Papazian, NEnglJM Sept 2010) 
  • Severe ARDS, P/F < 150
  • Showed mortality improvement at 90 days with paralysis for 48 hrs in early severe ARDS hours (hazard ratio for death 0.68).  No increase in weakness.
  • Evidence for improved oxygenation beyond period of paralysis. 
  • Criticisms:
    • 25% of eligible patients enrolled, limits external validity
    • study used low levels of PEEP (lower than the control of several high vs. low PEEP
    • trials)

Practice statement

Anything reasonable – for example:

I do not use NMBs routinely in severe ARDS but in selected patients e.g. those difficult to ventilate / oxygenate I use cistracurium infusion with train of four monitoring

Discussion

Rationale for the use of neuromuscular blockade in ARDS

  • Improve chest wall compliance: The lung compliance is already poor enough; removing chest wall compliance from the equation helps to prevent absurdly high peak pressures
  • Improved patient-ventilator synchrony: Many of these patients end up ventilated with such perverse pressure/volume combinations that without paralysis there would be a constant and counterproductive battle of patient versus ventilator. True, this is something one could achieve with haemodynamically disastrous doses of sedation, but ... why would you?
  • Recruitment manoeuvres  generally require paralysis for the abovementioned reasons; if the patient tries to exhale or cough during one of these manoeuvres, a pneumothorax may occur.
  • Prone ventilation usually requires paralysis
  • Decrease skeletal muscle metabolism: this is a hidden benefit, which is frequently overlooked. Neuromuscular blockade prevents the skeletal muscle from performing anything but the very barest baseline of metabolic work; the ATP production in the muscle fibres drops to whatever is required to maintain ionic concentration gradients. This decreases the oxygen extraction ratio for a large percentage of your patients tissues. Oxygenated blood, formerly directed into muscles, is redistributed into splanchnic vascular beds. Indeed, in a study performed on ICU patients with respiratory failure, the oxygen extraction ratio was found to by decrease 5% (from 36% to 31%).
  • Improved assessment of respiratory mechanics: without the interference of respiratory muscles, the lung compliance can be assessed more correctly, and the pressure-volume loops are more meaningful

Disadvantages of sustained neuromuscular blockade

  • This strategy is contrary to the normal strategy of ventilating patients with the most spontaneous mode possible, to preserve their muscle strength and to increase their comfort.
  • Longer duration of ventilation and longer ICU stay (Arroliga et al, 2005)
  • Increased risk of critical illness polyneuromyopathy
  • Increased risk of pressure areas
  • Increased risk of DVTs
  • Accumulation of paralytic agents (unless you use something like cisatracurium)
  • Risk of paralysed awareness
  • Increased nursing care

Evidence for the efficacy of neuromuscular blockade in ARDS

  • Papazian et al, 2010 -  a 48 hour course of cisatracurium early in the progress of severe ARDS. The authors found a 10% absolute risk reduction of 90-day mortality (31.6% vs 40.7%) associated with the use of neuromuscular blockade, without any increase in the risk of ICU-acquired weakness. , This places cisatracurium in the rare category of things which have been supported by positive trials.  Alongside low tidal volume ventilation and low delta-P, this is one of the strategies which can be strongly recommended. Unfortunately, as the college mention, the study has several weak points:
    • Papazian et al only enrolled 25% of the eligible patients. 10% of patients excluded due to 'other reason', whatever that means.
    • 21% of the patients had PA catheters, which is totally contrary to modern Australian practice
    • 30% of the patients received nitric oxide, which has been thoroughly discredited in the literature and has fallen into disuse in Australia
    • Papazian et al used a surprisingly low PEEP, around 9 cmH2O on average.
    • The groups differed in their use of steroids (39% in treatment arm, 45% in placebo)
    • The mortality from ARDS was much lower than predicted, which underpowered the study.
  • Blanch et al, 2015 - a prospective study which observed that ventilator asynchrony is associated with a higher in-hospital mortality.