Obtaining the MIP without a Manometer.

I always hate trying to find equipment, especially if your in the ICU & have to go to the basement where respiratory supplies are located. This quick trick is a very convenient way to obtain the Maximal Inspiratory Pressure at Low Volume, without a analogue pressure manometer.

Using Volumetric Carbon Dioxide Measurements to Optimize “P-High” During Airway Pressure Release Ventilation

There are many concerns when utilizing Airway Pressure Release Ventilation (aka. APRV, BiLevel, BiVent) in regards to lung injury.

 Inappropriate P-High settings may lead to large release-volumes resulting in over-distention, and volume induced lung injury [1].

Determinants and limits of the Draeger Narkomed Anesthesia machine in regards to ventilating the morbidly obese patient.

*Correspondence from 2007

Purpose

Determinants and limits of the Draeger Narkomed Anesthesia machine in regards to ventilating the morbidly obese patient.

A Problem with Plateau Pressures

When utilizing mechanical ventilation the plateau pressure is commonly evaluated to identify the potential for lung injury.  Many protocols (sepsis, ARDS) indicate targeting plateau pressures ≤ 30 cmH2O as a lung protective tactic.  

Unfortunately, in some patients the Plateau pressure may be falsely elevated and be relatively different then the transpulmonary/pleural pressure.

The MIP: a review of obtaining Maximal Inspiratory Pressure

“Breathe in as hard as you can”, a Respiratory Therapist yells to her ventilated patient. She is trying to coach the patient during a Maximal Inspiratory Pressure (MIP) maneuver, also known as the Negative Inspiratory Force (NIF).

This coaching is very common but is it necessary?

Can one obtain an adequate reading without coaching the patient or in a patient that is unresponsive?

First, think of the last time someone has had to yell at you, for you to initiate a breath?

It has probably never happen (not in less you overdosed on medication or illicit drugs)?

If the patient’s brainstem is intact and they have an adequate respiratory drive then a MIP can be obtained even if they are unresponsive.

So why do practitioners still yell at patients, & say the MIP is unobtainable?

Well it is most likely the result of poor technique.

So here are a few steps to guarantee that the technique is performed correctly, with measurable & reproducible results.

1. Patient criteria: the patient’s brainstem should be intact (no ischemic injuries).I don’t know why anyone would want to perform a MIP on a brain dead patient, when you should be performing an “Apneic Oxygenation Diffusion” test.

-One should caution performing the procedure in patients that are young, strong,

athletic, and have been mechanically ventilated for a short amount of time.  In these

stronger patients there have been incidences of rapid pulmonary edema secondary to

negative pressures created during inspiration against an occluded airway [1].

2. Respiratory Drive
   : the patient must have a strong inspiratory drive; the clinician must evaluate the common causes of a low respiratory drive (e.g. sedation, narcotics, hypocapnia, high levels of assist) and reverse/decrease these if they are present before the procedure.

3. Actively breathing: the patient must be actively breathing ideally in a spontaneous mode, or a partially assisted mode if needed. Make sure the ventilator is not auto-triggering.

4. Muscle Loading: the respiratory muscles should be loaded, by dropping support before the maneuver.

5. No muscle Fatigue: the patient should not be fatigue before the procedure, obtaining the MIP after a spontaneous trial will provide poor results.

Procedure

There are two main techniques used to perform the MIP. One is the Measurement at end-expiratory lung volume and the second is measurement at low lung volume.

Measurement at end-expiratory lung volume

This measurement is based on a total occlusion of the airway opening & easy to perform with the automated function of various newer mechanical ventilators.

The maneuver should be performed for at least 20 seconds or ten occluded efforts, and no longer than 25 seconds, making sure that the patients’ vital signs remain stable. 

Fig 1: MIP procedure performed on the Respironics Esprit Ventilator. 

Fig 1. Shows the maneuver for the MIP measurement obtained with the end-expiratory occlusion function of an Esprit ventilator. The maneuver starts at zero time and goes for 24 seconds. The first few efforts show negative deflations only in the range of -10 cmH2O. If one would end the procedure at this point one might assume that the patient is too weak. However, notice as the procedure is continued air hunger forces the patient to progressively increase his inspiratory efforts, which leads to a final measurement of -29.3.

The procedure should be repeated no more than twice.

Measurement at low lung volume

The second method is obtained below the function residual capacity (FRC); results are typically higher in this test since the respiratory muscles perform better under low lung volumes.

This test is performed with a simple t-piece device, & manometer. 

Example of a device used to obtain MIP at low lung volume

The test is performed by hooking up the patient per instructions, and occluding the ambient port at any time during the respiratory cycle.

Perform the test for the same amount of time, while assessing the patient for deleterious side effects.

Document the most negative number generated. 

RELATED POST

Obtaining the MIP without a Manometer

A Unique way to Obtain the NIF

Reference 

Lotti, G. & Braschi, A. (2009). Measurement of Respiratory Mechanics During Mechanical Ventilation. Rhazuns, Switzerland.

Negative Pressure Pulmonary Edema Reference

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