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It is important to measure the flow rate of gas given to an infant with a flow meter. The flow meter is usually plugged into an oxygen/air blender. However, the flow meter can also be plugged directly into an oxygen wall plug or the reducing value of an oxygen cylinder.
The flow of gas is measured in litres per minute and can be adjusted by turning an adjusting wheel. A flow rate of 5 litres per minute is usually used into a head box. A high flow rate wastes gas and cools the infant while a low flow rate may allow carbon dioxide to accumulate in the head box.
It is also important to use a humidifier together with the flow meter so that water vapour can be added to the dry gas (oxygen, medical air or a mixture). If a humidifier is not used the infant will breathe very dry gas which may damage the airways.
A simple humidifier (‘water bubbler’ at room temperature) is usually used to add water vapour to the dry gas if a head box or nasal cannulas are used. Sterile or boiled water (which has been allowed to cool) is added to the humidifier bottle until the water level reaches the full mark. When the water level approaches the empty mark more water must be added. The water must be changed and the humidifier must be cleaned every day or when the humidifier is to be used for another infant. Dangerous bacteria such as Pseudomonas can grow well in water and, therefore, the humidifier should only be filled with water when it is being used. The humidifier should be cleaned with detergent or soap and water, and be allowed to drip dry. The switch on the humidifier must be kept on ‘bubbles’ and not ‘jet’. The humidifier must be dry during storage.
Some humidifiers both warm and humidify the gas. These are expensive and are usually used with a blender. When infants are given nasal prong CPAP or are ventilated via an endotracheal tube (except during resuscitation), warmed, humidified gas must be used as the high flow rates can cool and dry out the mucosa.
Except during an emergency resuscitation, 100% oxygen from a cylinder or piped source should not be used as pure oxygen is toxic to many tissues, especially the retina of the eye. Whenever possible oxygen should be mixed (blended) with medical air using a blender or with room air using a venturi.
If a blender is not available, a venturi can be used with a head box. A venturi is cheaper than a blender but not as accurate. The venturi is a short plastic tube to which a pipe supplying oxygen is attached. The oxygen passing through the venturi sucks in room air and, thereby, mixes the 2 gases. The venturi is usually attached to a head box (oxygen hood). Some venturis provide a fixed concentration of oxygen while others can be used to give the concentration required. The latter are preferred. When using a venturi attached to a head box, an oxygen flow rate of 5 litres must be used. If possible the percentage of oxygen in the head box should still be accurately measured with an oxygen monitor.
Whenever an infant is given oxygen into a head box the FiO₂ (fraction of inspired oxygen) must be measured with an oxygen monitor as too high or too low a concentration of oxygen may be dangerous for that infant if it results in too much or too little oxygen in the blood. The FiO₂ cannot be controlled accurately with a flow meter alone. If an oxygen monitor is not available then a blender or venturi should be used to determine the approximate FiO₂, provided a flow of 5 litres or more is used.
Place the sensor in room air and switch on the monitor. The display should read 21%. If not, adjust the calibration knob until the display reads 21%. The monitor should always be calibrated before it is used. It should also be calibrated at least daily while in use.
First calibrate the monitor with room air. Then place the sensor into the head box. The display should now give the FiO₂ in the head box. Set the high and the low alarm limits to 5% above and 5% below the required FiO₂. If the display falls outside these limits, the red alarm light will come on and the alarm buzzer will sound. Silence the alarm by correcting the air/oxygen mixture to the required FiO₂. The display should be read and recorded on the observation chart at regular intervals while the infant is receiving extra oxygen. Remember that the monitor measures the FiO₂ but does not control the FiO₂. The FiO₂ cannot be changed by simply adjusting the oxygen monitor!
A pulse oximeter (also called an oxygen saturation monitor) measures the saturation (amount) of oxygen in the red cells of small arteries under the skin. The result is expressed as a percentage and the normal saturation of oxygen (SaO₂) in a newborn infant is 86–92%.
A SaO₂ above 92% is safe only if the infant is breathing room air.
A saturation below this range may be dangerous to the infant. The measurement is made by shining a bright light through the skin and then determining the colour of the transmitted light on the other side with a sensor. If the blood is red (well saturated) the SaO₂ reading will be normal or high. A low reading will be obtained if the blood is cyanosed. The monitor also measures the pulse rate by detecting the arterial pulsations in the small vessels in the skin.
The monitor is attached to a skin sensor by a thin cable. The monitor is powered by electricity (via a power cable which plugs into a wall fitting) or battery and displays a pattern of the pulse wave together with the percentage saturation and pulse rate. A number of different designs of sensor are available. One type looks like a clothes peg and can be clipped onto the infant’s hand, foot or ear lobe. Another type can be strapped onto a hand or foot with tape, while an adult finger sensor can, with difficulty, be slipped over the infant’s foot. A regular pulse wave indicates that the skin sensor is correctly positioned. The pulse wave may be displayed as a moving line on a screen or a digital display of vertically arranged lights.
The pulse oximeter should be used when the measurement of SaO₂ is needed on a sick infant. The sensor can be left attached for continuous monitoring or the sensor can be attached at regular intervals for a single reading. The monitor should not be used simply to obtain the pulse rate. If the pulse rate recorded by the monitor differs from the correct heart rate, then the monitor is not functioning properly and, as a result, the SaO₂ displayed may be incorrect. When moving the sensor from one infant to another, the sensor should first be wiped with an alcohol swab to prevent the spread of infection.
This is the best way of providing an infant with extra oxygen if CPAP or ventilation is not required.
The nasal cannula set is slipped over the infant’s head so that both short cannulas sit comfortably in the nostrils. The two tubes are then gently pulled together at the back of the head. Usually the tubing is taped to the infant’s face on either side of the nose. This will keep the nasal cannulas in place and prevent them pulling out.
It is important not to attempt to provide nasal CPAP unless the medical and nursing staff have been trained in the correct method to apply this management. A number of commercial and localy made CPAP devices are available. The Pumani bubble CPAP device has been designed for under resourced countries and will be used as an example of how to provide nasal CPAP. The device is cheap, easy to use, very effective, portable and easy to repair. Pumani means “breathe easy” in Chichewa, the language of Malawi.
Figure 11a-1: Pumani CPAP device
The Pumani CPAP device consists of a number of parts:
Figure 11a-2: Nasal prongs in place
Correct nasal prong size:
|Infant weight||000-1249 g||1250-1999 g||2000-3000 g||Over 3000 g|
At a total (blended) flow rate of 6 litres per minute the approximate FiO2 can be determined by altering the oxygen flow rate as follows:
|Oxygen % needed||21%||30%||40%||50%||60%||70%||80%||90%|
|Total flow rate||0||1.5||2.5||3.5||4||4.5||5||5.5|
If the total flow rate has to be increased the oxygen flow rate will also need to be increased to keep the FiO₂ the same. It is best to use a saturation monitor to decide what oxygen flow rate is needed. A higher oxygen flow rate will give a higher FiO₂.