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MIP/NIF in weaning from mechanical ventilation: strength and load

June 28, 2013

Original post written by Giuseppe Natalini on 28th June 2013

As promised to some of ventilab’s friends, here is the post on Maximum Inspiratory Pressure (MIP) (also called  Negative Inspiratory Force, NIF). What is it? What's its usefulness? In order to answer these questions, let’s analyze Piero's case together.

 

Piero is a 78 year old man who has been admitted to the ICU for a postoperative COPD exacerbation. He has a tracheostomy, and weaning from mechanical ventilation has been difficult for him. Just a few hours after we disconnect him from the mechanical ventilator, he becomes dyspneic and his breathing becomes rapid and shallow. This is something that happens relatively often in our patients. Why can’t Piero be weaned from mechanical ventilation?

 

We know that dependency from mechanical ventilation can be due, mainly, to two reasons: a high load or a reduction in the strength of the respiratory muscles.

 

 

If we can understand which one of these is Piero’s main problem, we can direct our efforts towards its real solution.  

 

In order to examine the strength of the respiratory muscles, we can measure MIP. MIP is the measurement of the variation in pressure generated by the respiratory muscles during a maximal inspiration against a closed airway. In intubated or tracheostomized patients (like Piero), we must occlude the airway at the end of expiration and ask the patient to inhale as deeply as possible. The variation in airway pressure is our MIP. Some mechanical ventilators have the option to measure it automatically, otherwise it is possible to occlude the airways an instant before the start of maximum inspiration, freeze the airway pressure trace during the maneuver and analyze it. We must explain to the patient what he has to do, and while he performs the maneuver he must be guided and encouraged to do it well. Repeating the maneuver 3-5 times and using the greatest variation in pressure as the MIP value is recommended. 

In patients who are not compliant, it has been suggested to occlude the airways (with a unidirectional valve) for 20-25 consecutive seconds and to detect the maximum airway pressure variation.


We measured Piero’s MIP, as we often do in patients for whom weaning from mechanical ventilation is especially challenging. We explained the maneuver to him and we asked him to repeat it 3 times. We put Piero in CPAP with 0 cmH2O (i.e. we left him connected to the ventilator without any applied positive pressure) and we occluded the airways at the end of expiration. At the same time, we recorded airway pressure. Here is the result:

Piero was able to reduce his airway pressure to a maximum of 30 cmH2O during airway occlusion. What does a MIP of 30 cmH2O mean? Piero seems to be a relatively weak patient, but this does not appear to be the only cause of his unsuccessful weaning. Indeed, Piero’s  “normal” MIP  should be much greater (between 50 and 95 cmH2O, see note at the end of the post), but usually muscle weakness in itself becomes the cause of weaning failure when MIP is below 20 cmH2O.

Therefore, MIP gave us some useful information which is nonetheless not enough to fully understand the problem. Now we must understand how much is the load for his respiratory muscles, in other words how much it “costs” him to take a breath. Hence we have measured esophageal pressure, and you can see the answer below:

During a normal respiratory cycle esophageal pressure decreases, on average, 16 cmH2O for each inspiration. In the image above we can see the esophageal pressure trace for three consecutive inspirations: each reduction in pressure corresponds to the pressure generated by the respiratory muscles during normal respiratory activity. 

Now we know both of the factors at play: Piero’s respiratory muscles’ load and strength. The load is around 16 cmH2O and strength is around 30 cmH2O. During each inspiration, Piero must use more than half of the maximum force he can develop: an unsustainable effort. Indeed it is believed that spontaneous breathing cannot be maintained for long when the ratio between load (=esophageal pressure variation during normal respiration) and strength (=MIP) is greater than 0.2, i.e. when more than 20% of the maximum force is used for each breath. 

We can therefore say that Piero, even if he is not extremely weak (MIP>20 cmH2O), must sustain a work which is too much for his own strength. We measured Piero’s intrinsic PEEP and found it is 8 cmH2O: for each breath, half of his effort is used to overcome intrinsic PEEP.

Now we have all the elements we need to consciously guide our efforts in order to wean Piero from mechanical ventilation. First, we must try to reduce the intrinsic PEEP that is generated during spontaneous respiration (in this case we are not interested in fighting PEEPi during mechanical ventilation):

1) we must maximize bronchodilation


2) we must reduce spontaneous respiratory rate and minute ventilation: in order to do so we must reduce CO2 production (with a correct nutritional intake and control of hyperthermia), eventually occasionally using opioids during phases of tachypnea which do not resolve rapidly


3) reduce flow limitation by keeping the patient sitting up.


It is also important to increase the strength of Piero’s respiratory muscles. The most important factor in doing so is, in my opinion, the correct modulation of ventilatory assistance, with the objective of avoiding both constant fatigue and the reduced use of respiratory muscles (excessive assistance and, worse, autocycling). Other factors are a correct nutritional intake (calories, proteins, calcium, phosphate), and probability daily breathing exercises against a threshold load. Additionally, the maintenance of a sitting position: the diaphragm needs footholds, too.

We certainly haven’t resolved Piero’s problems yet, but surely we know so much more than the simple observation of his failed weaning, and we have created a personalized treatment plan.

In conclusion:

  1. MIP/NIF can be useful in clinical practice in case of difficult weaning;

  2. If we find a MIP<20 cmH2O, the main problem is respiratory muscles weakness: we must act mainly to solve this problem (as much as possible);

  3. If MIP is, as often happens, between 20 and 50 cmH2O, it is useful to evaluate the load, i.e. the variation of esophageal pressure during normal ventilation. A ratio between normal esophageal pressure variation) and MIP >0.2 means that the load is too high for the patient’s abilities.

A smile for all of ventilab’s friends (hoping good weather returns…)

 

Note. The normal range for MIP can be calculated as follows: for males 126 - 1.028*age + 0.343*body weight in kg+ 22.4; in females 171 - 0.694*age+ 0.861*body weight in kg- 0.743*height in cm + 18.5

 

References

 

- ATS/ERS Statement on Respiratory Muscle Testing. Am J Respir Crit Care Med 2002; 166: 518-624

- Cader SA et al. Inspiratory muscle training improves maximal inspiratory pressure and may assist weaning in older intubated patients: a randomised trial. J Physiother 2010; 56:171-7

- Harik-Khan RI et al. Determinants of maximal inspiratory pressure: the Baltimore Longitudinal Study of Aging. Am J Respir Crit Care Med 1998; 158:1459-64

- Martin AD et al. Inspiratory muscle strength training improves weaning outcome in failure to wean patients: a randomized trial. Crit Care 2011; 15:R84

- Moxham J et al. Assessment of respiratory muscle strength in the Intensive Care Unit. Eur Respir J 1994; 7: 2057-61

- Truwit JD at al. Validation of a technique to assess maximal inspiratory pressure in poorly cooperative patients. Chest 1992; 102;1216-9


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