Respiratory support

Published on 03/06/2015 by admin

Filed under Neonatal - Perinatal Medicine

Last modified 03/06/2015

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CHAPTER 4 Respiratory support

Mark Davies, David Cartwright, Luke Jardine

Respiratory distress

Respiratory distress, and the need for respiratory support, is one of the most common problems seen in neonatal units. Respiratory distress is characterised by one or more of the following:

• tachypnoea (>60 breaths/min)
• expiratory grunt
• recession (of any soft tissue around the thorax — e.g. intercostal)
• nasal flaring
• cyanosis.

The most common cause is hyaline membrane disease (HMD), also known as infant respiratory distress syndrome. The most important cause is infection (see pages 107–12).

Other causes soon after birth include: retained fetal lung fluid (also known as transient tachypnoea of the newborn or ‘wet lung’); meconium aspiration syndrome; other aspiration syndromes (blood or liquor); air leak (pneumothorax or pulmonary interstitial emphysema); pulmonary hypoplasia (usually due to a space-occupying lesion in the chest, such as in congenital diaphragmatic hernia, or oligohydramnios); pleural fluid; and airway obstruction.

Other late-onset causes include: infection (viral or bacterial); chronic lung disease; chylothorax; and heart failure secondary to a left-to-right shunt.

The general principles of management include:

• put the baby somewhere you can watch it — admit to nursery
• monitor respirations, heart rate, oxygen saturation
• give just enough oxygen and no more (reasonable oxygen saturation targets are shown in the table on page 39)
• take a blood culture and start antibiotics
• give intravenous fluids
• chest X-ray (CXR) — exclude causes that may alter management, such as air leak, diaphragmatic hernia or pleural fluid
• consider respiratory support.

Continuous positive airway pressure (CPAP)

CPAP is a method of respiratory support used in the care of preterm and term infants. Indications for CPAP include:

• respiratory distress
• apnoea of prematurity
• airway obstruction
• post-extubation in very low birth weight infants.

Delivery methods vary according to institution, and include single prong (long or short), bi-nasal prongs (long or short), or mask.

The aim of CPAP is to hold the alveoli and airways open and prevent them collapsing during expiration. It therefore protects functional residual capacity, allowing the lungs to operate at maximal efficiency (by optimising their position on the pressure–volume curve). CPAP also stabilises the ribcage, reduces chest wall distortion during inspiration, and increases the efficiency of the diaphragm. It also regulates the respiratory rate (because of stimulation of the Hering–Breuer reflex) and results in increased inspiratory time and tidal volume. When given via the nose (nasal CPAP or NCPAP) it dilates the upper airway, which may explain its benefit in mixed or obstructive apnoea.

The benefits of CPAP include: a reduction in the need for intermittent positive-pressure ventilation (IPPV); decreased rate and severity of apnoea; increased chance of successful extubation; and decreased respiratory acidosis and oxygen requirements post-extubation.

The risks of CPAP include a potential for an increased incidence of intraventricular haemorrhage, nasal trauma and pneumothorax, as well as increased nursing care and the cost of consumables.

Starting CPAP: try a starting CPAP of 7 cmH2O (range 5–10). This may be decreased or increased depending on the level of oxygenation and severity of apnoea.

Nasal intermittent positivepressure ventilation (NIPPV)

NIPPV is a simple, effective mode of respiratory support where the infant is given CPAP plus a background ventilator rate with a set PIP (peak inspiratory pressure). A synchronised ventilator setting should be used.

It is more effective than CPAP alone in facilitating successful extubation and may also be useful in preventing intubation in apnoea of prematurity. Concerns have been expressed over a possible increased rate of gastrointestinal perforation, but this has not been supported in systematic reviews and may be minimised by using synchronised ventilation.

To place the infant on NIPPV

1. Choose the ventilator mode of synchronised intermittent mandatory ventilation (SIMV).
2. Set the PEEP (positive end expiratory pressure).
3. Set the PIP (peak inspiratory pressure) — use a PIP 2–4 cmH2O above that used pre-extubation, usually 14–20 cmH2O.
4. Set the rate to 10 breaths per minute (range 10–25).

These settings may be increased or decreased depending on the level of patient oxygenation and severity of apnoea.

Surfactant

• Intra-tracheal surfactant is usually given to intubated infants with HMD — the earlier the better.
• Natural, animal-derived surfactants are usually used.

We reccomend Survanta (Beractant, Abbott Pharmaceuticals) — the dose is 4 mL/kg, repeated if necessary after 6 hours.

Other types are:

— Curosurf (poractant alfa, Genepharm Pty Ltd) — 2.5 mL/kg
— bLES (bovine Lipid Extract Surfactant, bLES Biochemicals, Inc.) — 5 mL/kg

• Give surfactant by 2 hours if fraction of inspired oxygen FiO2 > 0.3. Repeat at 6 hours if FiO2 > 0.25.
• For babies <1250 g, give surfactant immediately if FiO2 > 0.3 or PIP > 18 cmH2O (before lines and X-ray).
• It is reasonable to give immediately to all babies <30 weeks gestational age who are intubated at birth.

Surfactant administration

It is not clear which is the best method of giving surfactant; there are variations in the methods of administration between and within different neonatal units. Administration of the total surfactant dose into four equally divided aliquots is the method recommended by the manufacturer of Survanta. Some administer it as a slow instillation via an endotracheal tube (ETT) over 5–10 minutes. However, animal studies have shown that a more rapid single bolus gives more uniform pulmonary distribution and dispersion of surfactant.

General

• With any of the methods below it will be necessary to recover or maintain chest movement with each mechanical ventilation breath by increasing the PIP.
• If oxygen saturation SpO2 falls below 90%, increase the FiO2 as well as increasing the PIP.
• If you do need to increase the PIP, make sure you turn it down again soon after: watch the chest wall movement. Volume-targeted ventilation is useful in this situation.
• Check an arterial blood gas within 30 minutes of giving surfactant.

Initial preparation

Position the baby supine and flat (i.e. without any rotation to either side) with the head in the midline. The ETT tip should ideally be in the mid-trachea. The surfactant should be at room temperature.

Method 1

• Use a 5FG infant feeding catheter cut to the length of the ETT and no longer.
• Disconnect the ventilator tubing manifold from the ETT connector.
• Place the cut feeding tube down the ETT. The tip should be at the tip of the ETT and not beyond.
• Instil the surfactant into the distal ETT over 10–20 seconds, then reconnect the manifold to continue IPPV.

Method 2

• Connect a blunt 18G drawing-up needle onto the syringe and instil the surfactant into the ETT connector via the suction port on the ventilator tubing manifold.
• Give over 2–5 minutes as tolerated. Continue mechanical ventilation throughout.

Method 3

• Instil surfactant via the side-port of an ETT connector over 2–5 minutes as tolerated.
• Continue mechanical ventilation throughout.

Assisted ventilation

The decision to ventilate a baby with respiratory distress needs to be taken in discussion with the relevant consultant. Generally accepted criteria are:

1. Apoea unresponsive to stimulation or other treatment, OR:

a frequent: more than 6 episodes in 6 hours requiring stimulation;
b severe: more than 1 episode requiring bag-and-mask ventilation.

2. Acidosis, either:

a respiratory acidosis: arterial pH <7.25 with a PaCO2 (partial pressure of carbon dioxide in arterial blood) above 60 mmHg; or
b metabolic acidosis: pH <7.25 or BE (base excess) worse than –10, not corrected by bicarbonate or volume loading.

3. Babies requiring an FiO2 >0.5 to maintain oxygen saturation >85%.

Babies <1500 g birth weight who need ventilation from delivery should remain ventilated until their respiratory status has fully declared itself.

Remember, respiration aims to:

get O2 in and get CO2 out

• To get O2 in all you need is a supply of inspired O2 and:

— expanded peripheral lung units (usually alveoli) that contain oxygen at a concentration higher than in the pulmonary capillaries, and
— blood flow past those lung units — i.e. ventilation/ perfusion (V/Q) matching.

• To get CO2 out all you need is to wash CO2 out of the peripheral lung units so that the concentration is below that in the pulmonary capillaries — i.e. ventilation or movement of gas in and out of the lung units.

Conventional mechanical ventilation (CMV)

• The basic form of IPPV or CMV used in neonates is pressure-controlled, time-cycled.
• Inspiration continues up to a maximum set pressure (peak inspiratory pressure, PIP) for the duration of the inspiratory time (IT).
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