We study the effects of mechanical ventilation on diaphragm structure and function, and the impact of those changes on outcomes for patients with acute respiratory failure. Diaphragm atrophy and dysfunction are common in mechanically ventilated patients and are related to diaphragm inactivity, or excessive loading during ventilation. This ventilator induced diaphragm dysfunction delays recovery and prevents patients from being able to breathe without assistance from the ventilator.
Monitoring diaphragm activity during mechanical ventilation is challenging and often requires invasive methods. However, point of care ultrasound offers a non-invasive and repeatable approach to visualize and measure diaphragm thickness and function, including changes in structure like atrophy. This protocol provides best practice guidelines for obtaining measures of diaphragm structure and function in healthy and critically ill populations.
In mechanically ventilated critical care patients, changes in diaphragm thickness from baseline are associated with diaphragm weakness and failure to wean from mechanical ventilation. Targeting optimal respiratory effort through therapeutic strategies such as respiratory effort targeted sedation and phrenic nerve stimulation may be able to mitigate ventilator induced diaphragm dysfunction and facilitate lung and diaphragm protective ventilation. The effect of these therapeutic strategies on diaphragm structure and function has yet to be evaluated.
To begin position the patient in a semi-recumbent position on their back. Cover the tip of the linear array transducer with ultrasound gel. After ensuring the ultrasound is in B-Mode for positioning, hold the probe by enclosing the tip of the probe with the thumb and index finger.
Palpate the chest wall surface to locate the right 8th, 9th, or 10th intercostal spaces between mid and anterior axillary lines. Then place the probe within the zone of apposition, making sure it's entirely situated between the ribs. If a rib appears in the image, adjust the angle of the probe by tilting it up and down until only the diaphragm is visible.
On the ultrasound monitor, identify two bright white parallel lines above the liver indicating the pleural and peritoneal membranes. Adjust the depth of the image by clicking on the increase or decrease depth button to maximize the size of the diaphragm. Ensure the diaphragm is visualized in the middle of the monitor.
This ensures maximum resolution of the pleural and peritoneal lines from surrounding structures. Next, to optimize image quality, open the ultrasound unit software. Then click the Gain button to adjust the image brightness.
Increase the gain by clicking the Increase button to brighten the image. Conversely, click the Decrease button to darken the image. Upon optimizing image quality, click the MMode button to place the ultrasound in the M-Mode.
Position the single vertical scan line between the sections where the pleural and peritoneal lines are the clearest. Run M-Mode over a full cycle of inspiration and expiration during tidal breathing. Then click on the Freeze to capture the actual state and Save to save the image.
Next, with a skin safe marker, mark the probe's location on the patient's body. Once the examination is complete, click the End Exam button. Next, for image analysis, start the DICOM viewer software and open the necessary DICOM files.
Then click on the Distance tool and draw a straight line from the inner edge of the pleural membrane to the inner edge of the peritoneal membrane at end expiration. Record this value as diaphragm thickness. Repeat similar measurements at peak inspiration of the same breath to obtain the diaphragm thickness at peak inspiration.
Analyze diaphragm thickness at end expiration and peak inspiration from the same breath to assess the diaphragm thickening fraction. Then, using the given equation, calculate the diaphragm thickening fraction for each breath. In healthy adults resting and expiratory diaphragm thickness ranges from 1.5 to 5.0 millimeters, while tidal diaphragm thickening fraction falls between 15 to 30%Position the patient in a semi-recumbent position on their back then using the B-Mode ultrasound, identify the diaphragm and optimize the image quality.
Next, click the MMode button to place the ultrasound into M-Mode. While in M-Mode, guide the participant to perform a maximum volitional inspiratory effort against a non-occluded airway. Freeze the recording and save the image.
Next, measure the diaphragm thickness fraction at end expiration and peak inspiration during a maximum inspiratory trial. Then, using the given equation, calculate the diaphragm thickening fraction max for each breath. The max thickening fraction is taken as the highest value from all three trials.
The maximal diaphragm thickening fraction in healthy adults ranges from 30 to 130%In critically ill patients diaphragm thickening fraction max less than 30%predicts a higher risk of failed weaning from mechanical ventilation.
