Volume-targeted ventilation is especially useful for small babies (<1250 g) who are more prone to the damaging effects of volutrauma and ventilator-induced lung injury.

Other similar, but by no means equivalent, types include: Targeted Tidal Volume mode on the SLE5000 (SLE systems, UK); and a number of different modes on the Stephanie paediatric ventilator (F. Stephan Biomedical, German), among others.

High-frequency ventilation (HFV)

CMV with large pressure gradients and large tidal volumes can cause ventilator-induced lung injury and its sequelae — worsening of acute lung disease, air leaks (pulmonary interstitial emphysema and pneumothorax) and chronic lung disease. To avoid these problems, the faster than normal respiratory rates used during HFV allow the use of small tidal volumes.

HFV aims to achieve adequate gas exchange (oxygenation and CO₂ removal) without the costs of barotrauma, atelectotrauma, and volutrauma.

HFV has two basic components:

1. A background continuous distending pressure (analogous to CPAP or PEEP), which is called the mean airway pressure (MAP). This produces continuous background inflation and recruitment of lung volume to achieve oxygenation.

2. ‘Oscillation’ of the airway pressure about the MAP at high frequencies. This is called the amplitude (or ΔP), and it determines the amount of alveolar ventilation and CO₂ removal (Fig 4.2).

Figure 4.2 Pressure wave form for high-frequency ventilation (HFV)

Optimal HFV is achieved by producing and maintaining optimal lung expansion. This is often referred to as a ‘highvolume strategy’. MAP is adjusted to optimise oxygenation. Increases in MAP lead to the recruitment of more alveoli and improved lung volume — this can be monitored by its effects on oxygenation, FiO₂ and CXRs. The aim is to achieve maximum alveolar recruitment without overdistending the lungs.

Ventilation is optimised by adjusting amplitude (ΔP) to achieve a desired PaCO₂. The amplitude of each HFV ‘breath’ appears large compared with the PIPs used during CMV. However, the pressures are attenuated as they are transmitted down the ETT and airways so that the pressure oscillation is quite small at the lung periphery (Fig 4.3). Higher amplitude will increase tidal volume and improve CO₂ clearance (i.e. decrease PaCO₂).

Figure 4.3 Attenuation of the HFV pressure wave form

CMV = conventional mechanical ventilation; HFV = high-frequency ventilation; MAP = mean airway pressure

As the ventilator frequency is increased, the lung impedance and airway resistance will increase and the tidal volumes decrease. Therefore increasing the frequency may decrease CO₂ clearance and increase PaCO₂. Conversely, decreasing the frequency often improves CO₂ clearance (although this will lead to less attenuation of the pressure oscillations and higher pressures transmitted to the alveoli).

Starting, changing and adjusting ventilation

Starting HFV

• A CXR before starting HFV is useful as a baseline measure of lung volume.

• Consider replacing the ETT with a larger one if there is significant leak around it.

• Where skin condition allows, transcutaneous O₂ and CO₂ monitoring should be commenced, as rapid changes can occur when changing to HFV, especially a rapid drop in PaCO₂.

• Oxygen saturation monitoring and an arterial line for ABGs and BP monitoring should already be established.

• Optimal pulmonary blood flow will require good blood pressure and perfusion. These should be optimised with volume replacement ±inotropes as necessary.

• Muscle relaxants are not indicated unless already in use.

• Sedation is used according to current unit policy.

• Low-compliance ventilator circuits are preferable.

• Use a low-volume humidifier chamber and keep it adequately filled — increasing compressible gas volume can affect amplitude.

Changing from CMV to HFV

• Leave the FiO₂ the same as on CMV.

• Set the frequency between 10 and 15 Hz (inclusive). The optimal frequency for HFV may be different in different disease states. Small infants with HMD may be managed at higher frequencies, and term infants may be best managed at lower frequencies. With very noncompliant or stiff lungs, even lower frequencies may be necessary.

• Decrease the conventional breath rate and increase MAP simultaneously over 10–30 seconds until you reach the desired MAP. The desired MAP will usually be 2–4 cmH₂O above the MAP used when the infant was on CMV. MAP is regulated by using the CPAP or PEEP control on some ventilators.

• Set the amplitude (ΔP) until there is a discernible chest wiggle.

• A background conventional breath rate may be set on some ventilators — this is not essential when changing over to HFV and can be omitted.

Fine-tuning or weaning HFV

Oxygenation and ventilation are usually considered separately; however, changing one will often affect the other. Always check all the settings after making any adjustment.

Ventilation

To change the PaCO₂ change the amplitude (or occasionally the frequency).

• To get rid of more CO₂ and therefore lower PaCO₂, ventilation may be increased by raising the amplitude.

• To decrease CO₂ removal and therefore increase PaCO₂, decrease the amplitude.

If adjustment of frequency is needed, decreasing the frequency increases CO₂ removal (the opposite to CMV). Always discuss this option with the consultant. (The response to a change in frequency may vary with different ventilators — the Infant Star has a fixed inspiratory time of 0.018 sec; the Dräger and SLE2000 have fixed I:E ratios so lowering the frequency will increase the IT.)

If there is still poor CO₂ clearance (i.e. high PaCO₂):

• Is the ETT blocked? Check for chest wiggle and breath sounds. Suck out or replace the ETT as necessary.

• Is there a pneumothorax? Transilluminate the chest, get a CXR.

• Increase the amplitude and see if the chest wiggle improves.

• Are the lungs under- or over-inflated? Get a CXR.

• If already at maximum amplitude, try reducing the frequency — lung impedance and airway resistance will fall, increasing tidal volume and CO₂ clearance.

Oxygenation

• Oxygenation is controlled by adjusting the MAP and FiO₂. The high-volume strategy dictates decreasing the FiO₂ first — therefore the MAP is usually not weaned until the FiO₂ is <0.4.

• The degree of lung inflation can be assessed by CXR. Arrange for a CXR when gases are stable to assess lung volume — usually 1–2 hours after starting HFV and every 12–24 hours after that.

• Normal inflation should allow the right hemidiaphragm to be at about the 10th rib (posteriorly). Once the baby is stable in an FiO₂ <0.4, the MAP should be cautiously reduced by 1 cmH₂O as allowed by the oxygenation. If the FiO₂ rises, this suggests you have dropped MAP too much.

If oxygenation remains poor (i.e. low oxygen saturations or PaO₂):

• Is the ETT blocked? Check for chest wiggle and breath sounds. Suck out or replace the ETT as necessary.

• Is there a pneumothorax? Transilluminate the chest, get a CXR.

• Are the lungs not inflated enough? Try increasing the MAP, get a CXR.

• Are the lungs over-inflated? Check for hypotension. Try decreasing the MAP — does oxygenation improve? Get a CXR.

Over-distension of the lungs with a MAP that is too high can impede venous return and/or pulmonary blood flow. This will decrease systemic perfusion and worsen oxygenation. If the baby is hypotensive or has a metabolic acidosis, the lungs may be over-inflated. Get a CXR. Try decreasing the MAP — does oxygenation improve?

Trigger ventilation modes

Modern neonatal ventilators have many different modes of triggered ventilation available. The main aim of these modes of ventilation is to synchronise the baby’s breath with that delivered by the ventilator. These modes rely on the baby’s breath being detected by the ventilator in sufficient time to allow the ventilator to begin a mechanical breath while the baby is inspiring.

There are a few different types of triggered ventilation, and they are often called different names by different ventilator manufacturers. Common types are:

• synchronised intermittent mandatory ventilation (SIMV) — the ventilator delivers a mandatory number of breaths (set by the operator) at a set PIP and IT, but it will attempt to synchronise these mandatory breaths with the baby’s inspiration.

— If the baby is apnoeic, the ventilator will deliver the set rate of breaths at the set PIP and IT.

— If the baby is spontaneously breathing, it will receive only the set number of mandatory ventilator breaths as well as its own spontaneous breaths.

• synchronised intermittent positive-pressure ventilation (SIPPV), also known as patient triggered ventilation (PTV) or assist control (A/C) — each spontaneous breath triggers a ventilator breath at a set PIP and IT.

— If the baby is apnoeic, a back-up rate (set by the operator) will be delivered at the set PIP and IT.

— If the baby is spontaneously breathing, it will receive a ventilator breath for each spontaneous breath (if breathing faster than the set back-up rate). If the baby breathes rapidly, you will need to decrease the IT.

• pressure support ventilation (PSV) — each spontaneous breath triggers a ventilator breath at a set PIP, but the IT is determined by the baby in that inspiration stops when the baby stops inspiring.

— If the baby is apnoeic, a back-up rate (set by the operator) will be delivered at the set PIP and a set IT.

— If the baby is spontaneously breathing, it will receive a ventilator breath for each spontaneous breath (if breathing faster than the set back-up rate).

Each of these modes can be combined with volume-targeted ventilation as described above.

Pneumothorax (PTX)

Needling the chest for a PTX

A procedure that is both diagnostic and therapeutic.

Site

Stay away from the heart, the internal mammary artery and the intercostal arteries — use either:

• 2nd intercostal space, mid-clavicular line — 2ICS, MCL; or

• 4th intercostal space, anterior axillary line — 4ICS, AAL.

Insert the needle as close as possible to the upper edge of the lower rib.

Figure 4.5 Site for ‘needling’ a PTX (pneumothorax)

Equipment

Attach a 23G or 25G butterfly needle to a 3-way tap attached to a 20 mL or 50 mL syringe. (See Fig 4.4.)

Figure 4.4 Equipment for ‘needling’ a PTX (pneumothorax)

Procedure

1. Prepare the skin with an alcohol wipe and let dry.

2. Insert the needle perpendicular to the chest wall (straight down) 1/2–1 cm in a small baby, 1–2 cm in a large baby.

3. Open the tap to the syringe and needle.

4. Aspirate — if the syringe fills with gas then there is a PTX. If you only get a few bubbles and some serous fluid it is not a PTX (yet!).

5. Open the syringe to the atmosphere (i.e. closed to the needle).

6. Empty syringe.

7. Repeat steps 3 to 6 until you are unable to aspirate any more gas. Note the amount of air aspirated from the pleural cavity (you can get up to 40–50 mL from a small baby and more than 200 mL in a large baby).

8. Remove needle — no dressing required.

Notes

• Be very careful that you do not empty the syringe back into the baby.

• If there is an ongoing air leak, you may need to leave the needle in situ and repeat the procedure until you can insert an intercostal catheter (ICC), especially if there is a tension PTX.

• After needling a chest for a PTX an ICC will usually need to be inserted.

Intercostal catheter (ICC) placement

The distance markings on the Argyle catheter are in centimetres from the most proximal side-hole, not from the tip. Insertion at the 1 to 2 cm mark is very adequate for a small baby, and at the 3 cm mark for a large baby.

General

• This is a sterile procedure.

• The best insertion site is in the mid to anterior axillary line, 5th–6th intercostal space, well away from the nipple.

• Use local anaesthetic if there is time.

Procedure

1. Make a deep hole down through the parietal pleura using first a No. 11 scalpel blade and then artery forceps. Once the hole is made you are ready to insert the catheter.

2. It is helpful to bend the trocar (1.5–2 cm from the tip) about 20–30 ° to aid in directing the ICC tip anteriorly.

3. Insert the catheter perpendicular to the skin. During insertion guide the tip of the catheter in the desired direction. This is facilitated by the bend in the catheter. Never use any force to insert the catheter through the chest wall.

4. The use of the trocar to puncture the hole through the pleura is extremely controversial. Either:

a the trocar is never used to make the hole between the skin and intra-pleural space, it is merely used to guide the direction of catheter placement (i.e. anteroinferiorly); i.e. the hole in the chest wall should be

Figure 4.6 Correct position of tip of intercostal catheter (ICC)

complete before the catheter and trocar go anywhere near the baby; OR

b use the trocar to make the hole through the pleura, but note that this is only safe if it is held with both hands, using both index fingers ∼1–1.5 cm from the tip, and all movement is from the elbows.

5. The ICC tip should be directed anteriorly (to lie in front of the lung), and infero-medially, towards the xiphisternum. It will then lie at the uppermost part of the pleural cavity when the patient is lying supine, i.e. at the antero-inferior rib margin. Free air will always rise and sit in the uppermost part of any cavity — this is where the catheter holes should be.

6. Always get a CXR after inserting an ICC.

Figure 4.7 Chest X-ray of correctly positioned intercostal catheter (ICC) tip

Notes

• It is normally not necessary to apply suction to an ICC. A Heimlich valve is quite adequate.

• Don’t suture the ICC in place: this produces a puckered scar.

• Taping with a transparent, bio-occlusive, polyurethane dressing (such as Bioclusive, Tegaderm, Opsite) is adequate: sandwich the catheter between 2 pieces of dressing (Fig 4.8) — make sure the skin is dry.

• Make sure the catheter still points in the right direction — i.e. antero-infero-medially. To facilitate this, the external part of the ICC should be placed under the ipsilateral arm, running up past the head.

Figure 4.8 Taping in an intercostal catheter (ICC)

Pocket Notes on Neonatology 2e

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