• Bryan Winchell

Pediatric Dead Space in Mechanical Ventilation


When transporting pediatric or neonatal patients on a ventilator, are you eliminating all possible dead space? Do you know to use the the capnography waveform to tip you off to CO2 rebreathing?

CASE 1:

12 kg, 2 year old trauma, with worsening hypercapnia despite increasing rate and tidal volume. What clues does the capnography waveform give you?

Notice evidence of “ETCO2 tailing” that indicates CO2 is being pushed back into the patient during the inhalation phase. This is caused by too much mechanical dead space (in relation to the tidal volume). The first part of the breath includes a mix of CO2 and oxygen that tapers off to all oxygen as the dead space is evacuated.

The solution to this is to reduce dead space found between the circuit wye and ETT. This is where we stack up ETCO2 detectors, an HME, flex tubing, swivel elbows, inline suction, neb adapters, etc. Slim it down!

CASE 2:

4.2 kg, 25 day old, RSV, pneumonia/respiratory failure. Difficulty controlling hypercapnia (ETCO2s 65-70) despite increasing Pressure Control and rate: hypercapnia and abnormal waveform resolves with BVM. Is there anything abnormal in the ETCO2 waveform?

At first glance, the ETCO2 waveform appears unremarkable, until you notice where the baseline is supposed to be. This is a remarkably lifted baseline, indicating a significant amount of CO2 entering the patient during inhalation. In fact, the entire breath is a mixture of CO2 and oxygen, indicating the dead space is significantly larger than the actual tidal volumes.

This can be resolved by eliminating the HME and using circuits and equipment designed for neonatal transport. A way to deduce this is to place a BVM right on the ETT and see if this corrects the waveform.

CASE 3:

10 kg, 1 year old, intubated for status epilepticus. The flow waveform appears to have prolonged expiratory segment, however there was no clinical suspicion for obstructive pulmonary process. What else could be prolonging the expiratory flow waveform?

The patient was hypercapnic (ETCO2 48-52) despite tidal volumes in excess of 10ml/kg: notice the abnormal ETCO2 waveforms below.

Once the sending agency's HME was removed, the ETCO2 returned to baseline, and hypercapnia resolved. In addition, the flow waveform rapidly returned to baseline once the HME was removed, indicating the resistance noted in the first photo was induced by the saturated HME.

CASE 4:

35 kg, special-needs adult with ARDS, critical hypercapnia and hypoxia. Tidal volumes unable to be increased above 170 ml due to high pressures. Notice “tailing” on ETCO2 waveform.

Eliminated in-line suction and HME, ETCO2 reduced from 75-80 down to 50. ETCO2 tails not as notable. Ultimately able to decrease ETCO2 from 80 to 40, and increased SpO2 to 87% by lengthening inspiratory time (TI) in transport. (Optimized pMean or mean airway pressure).

The inline suction and HME were a total of 80 ml of dead space. This is a routine setup installed on nearly all ICU patients, that accommodates nebulizers, suctioning, and the HME. In most adult patients this dead space would be negligible, but in this case the patient had something more like "pediatric tidal volumes" due to severe ARDS and a lung-protective strategy. When you consider the amount dead space in terms of the tidal volume, that was almost half of the delivered Vt.

EQUIPMENT REVIEW:

It is critically important to use the right equipment when transporting pediatric and neonatal patients. What do you carry on your aircraft or ambulance? Conventional HMEs and flex tubing cause an accumulation of dead space. “Pediatric” circuits are not necessarily suitable for infants or neonates.

A neonatal ETCO2 detector should be used for ETT sizes 4.5 and smaller. This markedly reduces dead space, and more accurately measures ETCO2.

You may want to take a look at what HMEs, elbows and flex tubings you are stocking. Each have pros and cons, and should be added with careful thought.

Stacking a lot of hard plastic pieces end-to-end and then attaching to a pediatric ETT can be called a “Tower of Terror”. This is a recipe for inadvertent dislodgment or ETT kinking.

Below is our preferred method on infants. It is probably best to use a commercial device like the NeoBar and Neo-fit to secure the ETT instead of tape. We have tried both and settled with using the Neo-fit and COMFIT devices for pediatric and adult ETT securement. Traditional commercial devices can impinge on the internal diameter of the ETT when tightening the screw clamp on small ETTS, and allow for easier kinking on ETTs of all sizes.

You can use a towel roll on the upper chest to stabilize the circuit and ETT. Active (in circuit) humidification is probably the best bet for patients <10kg, so that you don’t have to deal with the dead space caused by an HME (and more satisfactory warming/humidification).

Either way, if the HME is causing CO2 rebreathing or increased vent pressures (wet HMEs are classic for increased resistance), you should consider removing the HME. This method totals up to 4 mL dead space from the wye to the ETT.

For children 10kg and larger, a task-specific HME could be added between the ETT and ETCO2 detector, like the Humid-Vent 1 or Thermovent 600. Both of these add 10-11 mL dead space.

Just as any area with critical care transport, you are called to be a content expert. If you are transporting pediatric patients, particularly ones less than 10kg, it is imperative to bring the right equipment to the bedside and know how to build a mechanical ventilation system that is tailored for a very small patient.

Do everything in your power to reduce dead space between the wye and ETT. Use the correct size ETCO2 adapter, critically appraise the need for an HME vs dead space it costs, and get rid of excess elbows, adapters, and flex tubing.

Finally, reduce the effect of the “Tower of Terror” by limiting what accoutrements you attach to the ETT, using a towel roll to support it, and a task-specific ETT securement device. Use your blankets and seat belts to secure the circuit in place.

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FoamFrat or the author has no conflicts of interest, or benefit to recommending specific products or equipment.