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Reflections on neonatal mechanical ventilation.

Outcomes from neonatal intensive care have seen remarkable improvements over the last 15 to 20 years. In a report from the University of Chicago published in the journal Pediatrics in 2004 investigators reported a 57% improvement in survival rates in infants born < 1000 grams and an 85% improvement in survival in infants < 700 grams between 1990 and 2001. These, and other remarkable improvements have been attributed to the advent of surfactant therapy, improvements in monitoring, nursing care, ventilatory techniques and prenatal care, although none of these assertions have been proven. As a respiratory therapist, I can certainly testify that ventilation of low birth weight infants has changed considerably in the last 25 years.

I can remember cruising around the neonatal intensive care unit listening to the Knack singing My Sharona (c.1979), while dutifully adjusting all inspiratory times to 1.0 seconds. We didn't really get nervous about peak inspiratory pressures until they were over 30 cm H2O. We targeted PaCO2's of 35 to 45 mmHg, and cheerfully cranked up the pressures and or the rate until the ventilation improved, by God. Initially we had no transcutaneous monitors and the pulse oximeter was still on the drawing boards. Very few low birth weight patients had arterial lines, so we did a lot of capillary blood gases. Measurement of tidal volume at the bedside in a 700 gram patient was still a dream.

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The first transcutaneous monitor I used was a Litton device and worked very well. But we only had one instrument for 12 beds. All ventilators were time-cycled, pressure-limited, time-triggered, continuous flow ventilators like the Baby Bird or the Bear Cub. We were all trained that peak inspiratory pressures were the guilty party when neonates developed lung injury. There were lots of patients with Bronchopulmonary Dysplasia and Pulmonary Interstitial Emphysema.

I am overcome with a warm fondness when I ponder all the ingenious techniques for managing the frequent apnea-bradycardia spells of a typical very low birth weight infant. These included putting a rubber bladder under the iddy-biddy mattress and periodically inflating it with a Baby Bird so that it gently "bounced" the patient every 5 seconds. Another was the "swift-kick" technique in which these spells were treated with a gentle kick to the bottom of the incubator by a passing nurse or RT when the heart rate dropped. I have this vivid memory of a nurse with both hands in an incubator, when the bradycardia alarm went off on the patient immediately behind her. She calmly reached back with her leg and thumped the bottom of the incubator behind her and "presto", the Bradycardia resolved. (My attorney insists that I insert the following disclaimer: I no longer recommend any of the abovementioned therapies.)

So what has really changed in our techniques of neonatal mechanical ventilation since then? One important change is that permissive hypercapnea is now widely practiced. Most of us are no longer fixated on obtaining normal PaCO2's, as long as acceptable pH's are obtained (usually >7.25). We also now are much more focused on avoiding hyperventilation as there appears to be a relationship between low PaCO2's and periventricular leukomalacia. We also tend to use much shorter inspiratory times.

Another major advancement is the development of non-invasive monitoring of oxygenation and ventilation. These include transcutaneous monitoring, end-tidal gas monitoring and pulse oximetry. The advent of these technologies alone may not have improved outcomes, but when good technology is combined with standardized clinical processes, outcomes have changed. This was the case when Chow et al reported a large drop in their retinopathy of prematurity rates after the acquisition of better pulse oximeter technology and the application of a rigorous oxygen management protocol.

Mercifully, the monocular focus on airway pressures as the cause of lung injury is finally starting to change. It is my belief that the most frequent cause of lung injury in neonates is lung over distention, e.g. tidal volume. Lung volumes are certainly related to airway pressures, but you can have significant changes in lung distention without any changes in peak inspiratory pressure. Note what happens when surfactant is administered. The patient's lung compliance suddenly improves. If you don't drop the ventilating pressures rapidly, the lung will over distend and may get injured. So what caused the injury? Not the pressure, it remained the same. What changed was the tidal volume. Of course, differing pressures in adjacent parts of the lung that have different time constants may also cause injuries, due to the development of large shearing forces between these adjacent lung tissues.

Thus, there is a deep and fundamental shift in ventilator management theory that has and is taking place in many NICU's. The focus in more advanced units is on managing tidal volume, which is being kept at 5-8 mL/kg. The ability to measure tidal volumes at the bedside has made this kind of an approach possible. There are a number of small, easy to use computerized differential pneumotachometers that allow for easy and inexpensive measurement of tidal volume.

Many ventilators now used in NICU's also have built in capabilities to display "measured" tidal volume. However, care must be taken here. Some ventilators that were heavily marketed as "neonatal," were not able to accurately measure tidal volumes, even after adjustment for volume lost due to tubing compliance. Many believe this is because these ventilators depend on flow measurements made back inside the ventilator, proximal to the exhalation valve, instead of at the proximal airway. Some of the errors are large and have led some researchers to recommend the use of proximal airway sensors for tidal volume measurement in neonates. It is always a good practice for clinicians to view the claims of the manufacturers with a healthy skepticism.

There are now a number of ventilators that place a flow sensor at the proximal airway, that are suitable for use with neonates. Some of these have been show to have excellent accuracy and precision. In a series of neonatal bench tests, we found that the ventilators with proximal flow sensors displayed more accurate tidal volumes than those that measured volumes distally. These ventilators use hot-wire anemometers, as well as differential pneumotachometers to measure flow at the proximal airway. One investigator reported that accurate neonatal tidal volumes can be measured distally, with a sensor proximal to the exhalation valve. This finding seems to be at variance with most of the other reported data.

Putting flow sensors proximal to the neonatal airway has a number of associated problems. The anemometers in particular sometimes require a lot of "fiddling" with the sensors to keep their readings from being affected by the build up of secretions and humidity in the sensor. They have to be replaced frequently and allowed to dry. However, a diligent RT can manage these. Another issue is the weight of the sensors. This increased weight pulling on the endotracheal tube may increase the risk for accidental extubation. However, again, a diligent bedside team can manage these issues. To me, the value of the accurate measures of tidal volume seems to outweigh the potential problems or risks.

The changes in how neonates are ventilated are quite profound, especially when you consider the advent of high frequency ventilation. I will discuss some controversies surrounding the use of high frequency ventilation in neonates in my next column.

by John Salyer RRT-NPS, MBA, FAARC
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Title Annotation:NEONATAL RESPIRATORY CARE
Author:Salyer, John
Publication:FOCUS: Journal for Respiratory Care & Sleep Medicine
Date:Jan 1, 2006
Words:1229
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