Postgrad Med J: first published as 10.1136/pgmj.48.562.507 on 1 August 1972. Downloaded from Postgradulate Medical Journal (August 1972) 48, 507-513. Artificial ventilation, prolonged endotracheal intubation and tracheostomy in paediatric surgery W. J. GLOVER M.B., B.Ch., F.F.A.R.C.S., D.Obst.R.C.O.G. Hospitalfor Sick Children, Great Ormond Street, London, W.C. 1 THE young infant has much less respiratory reserve from the patient to the reading on the spirometer than older children and adults; for example, his (Glover, 1965). The volume of the circuit, the peak resting oxygen consumption, per unit of body weight, inspiratory pressure reached in the circuit, and the is twice that of the adult, 7 ml/kg/min compared with frequency of ventilation determine the size of this 3-5 ml/kg/min. The diameters of distal airways re- compressibility factor. The discrepancy between the main constant from birth until about 5 years of age patient's minute volume and spirometer reading will and only then increase in size (Hogg et al., 1970). therefore be greatest in infants requiring high in- Consequently, infants and young children have a spiratory pressures. high peripheral airways resistance and a greater tendency to airways occlusion than older children. (2) Pressure The average newborn infant has about twice the The inspiratory pressure required to ventilate the surface for heat loss for each kilogram of tissue infant lung varies considerably. It will be less than compared with the average adult (Cross, 1965). If 10 cm H20 if the lungs are normal and it may be as the infant is placed in a cool environment his meta- high as 50 cm H20 in infants with pulmonary oedema copyright. bolism increases to attempt to maintain his body and bronchiolitis. It is therefore essential to be able temperature. This increase in metabolism in turn to apply pressures of this order when the clinical increases his oxygen consumption and minute condition requires it. volume. Nursing an infant in a cool environment places an additional demand on the respiratory (3) Accurate control of inspired oxygen system. In an infant already in respiratory distress In patients of all ages there is a danger of pul- this could result in respiratory insufficiency. The if importance of conserving heat in the care of ill monary damage oxygen concentrations above 500 infants is therefore of the utmost are administered for a number of days. importance. Young animals exposed to 100% oxygen at 1 http://pmj.bmj.com/ In infants, as in adults, the cause of respiratory atmosphere die in 72-96 hours. At autopsy the lungs insufficiency may be decreased respiratory move- are large, dark, heavy and liver-like in appearance. ments or pulmonary complications or a combina- Man appears less susceptible and human volunteers tion of both. For the reasons given above, however, showed a fall in pulmonary diffusion capacity after infants will reach a state of respiratory insufficiency 30 hr exposure to 98% oxygen at 1 atmosphere and more readily than older children. a fall in vital capacity after 60 hr exposure (Caldwell et al., 1966). Essential features of a paediatric ventilator It is well known that high percentages of oxygen on October 1, 2021 by guest. Protected (1) Tidal volume administered to premature infants may result in The tidal volume of an infant varies inversely with blindness due to retrolental fibroplasia. This com- the respiratory rate and ranges from about 10 to 30 plication may arise if the Pao2 is above 150 mmHg ml in patients of about 3.5 kg. The ventilator must in the retinal vessels. If the oxygen tension in the deliver volumes in this range to the patient. The arterial blood is monitored from the umbilical artery volume indicated by a spirometer placed in the con- in the immediate newborn period lower values will ventional position in the expiratory limb of a circuit be obtained than exist in the vessels perfusing the does not represent the patient's minute volume. eye and brain. This is due to shunting from right-to- The figure obtained from the spirometer must be left via the ductus arteriosus. Where a considerable corrected to allow for compression of the gas in the right-to-left shunt exists, as in respiratory distress, patient circuit. Gas compressed in the circuit on a Pao2 of 60-90 mmHg in the umbilical artery blood inspiration will re-expand during expiration when the sample may be acceptable (Baum & Tizard, 1970). pressure falls and contribute with the expired gas In the care of ill patients there should be no Postgrad Med J: first published as 10.1136/pgmj.48.562.507 on 1 August 1972. Downloaded from 508 W. J. Glover hesitation in raising the inspired oxygen concentra- tive provided an artificial airway is in place. If the tion as much as is necessary to maintain acceptable patient is breathing through his nose then neither arterial oxygen tensions. Retrolental fibroplasia will ultrasonic nor compressed air nebulizers are effec- not occur unless the arterial oxygen tension is abnor- tive because the nose filters out the droplets (Wolfs- mally high and the danger of pulmonary damage dorf, Swift & Avery, 1969). When however an endo- from the high inspired oxygen concentration is not tracheal or tracheostomy tube is in place, the output acute enough to outweigh the serious and im- from the nebulizer must be carefully controlled as it mediate consequences of severe oxygen desatura- is easy to overload an infant's lungs with fluid. tion. Saline is more dangerous than water in this respect It is therefore essential to be able to control and consequently distilled water should be used accurately the percentage oxygen delivered by a ven- (Modell et al., 1968). tilator. The inspired oxygen concentration should in An important disadvantage of both compressed addition be checked regularly with an oxygen analy- air and ultrasonic nebulizers is the possible trans- ser. When oxygen-rich mixtures are administered it is mission of bacteria to the patient (Moffet & Allan, also necessary to check the arterial oxygen tension. 1967). Any device producing mists is liable to dis- seminate quantities of bacteria. The smaller particles (4) Humidification produced by ultrasonic nebulizers are particularly Since the artificial airway (endotracheal tube or dangerous as they penetrate deeply into the lungs tracheostomy) by-passes the patient's humidifying where pulmonary clearance mechanisms may not be mechanism, the nose, it is essential to humidify the efficient. inspired gas. This aspect is often overlooked in con- Few ventilators will meet all four criteria described sidering the merits of various ventilators. In infants above. The clinician in choosing a machine must it is of even greater importance because the inspissa- bear in mind the nature of the clinical problems with tion of secretions resulting from the inspiration of which he will be confronted in order that he may dry gas readily blocks the narrow airways of the make the best choice for his particular purpose. patient and also the lumen of the endotracheal or tracheostomy tube. We should try to achieve a rela- Management copyright. tive humidity of 700 or more in order to maintain Continuous nursing supervision is essential in this ciliary activity, as a low relative humidity has a work. The objective in mechanical ventilation is to retarding effect on ciliary motion (Dalhamn, 1956). maintain the patient's arterial Po2 and Pco, near There are three methods of increasing the water physiological levels and this is most easily achieved content in the inspired air and only brief comments by taking complete control of the patient's respira- are made on them here: tion. (a) Standard humidification. This involves the heat- The use of patient-triggered ventilators would ing of water over which gas is passed to the patient. seem attractive but in practice in infants breathing If the water in the humidifier is kept at about 55° C rapidly it can be extremely difficult to achieve satis- http://pmj.bmj.com/ then all vegetative organisms are killed. This elimi- factory patient-triggering. Under-ventilation will nates an important source of proliferation of bac- then occur. teria. The disadvantage of this method is that the If one ventilates children satisfactorily so that the temperature of the inspired gas falls as it passes along Pao2 and Paco2 are approximately normal then the the inspiratory tubing to the patient. It is desirable patient will follow the cycling of the ventilator and to keep the temperature at the patient end of the cease making his own efforts. There are three excep- tubing at approximately 35°C in young infants and tions to this generalization: 33°C in older children. In young infants humidified (1) If the patient is in pain following surgery then on October 1, 2021 by guest. Protected gas is a potent factor in maintaining body tempera- an analgesic such as morphine is required to gain ture as the patient does not have to produce water control. vapour from his respiratory tract and thereby lose (2) If an infant is hungry then a feed is required heat. to establish control. (b) Compressed air nebulizer. This gives a variable (3) If there are large right-to-left shunts causing droplet size and a high proportion of the droplets arterial oxygen desaturation as in respiratory distress may be comparatively large. Droplets greater than syndrome or pneumonia then mechanical ventilation 10 ,u are deposited in the upper trachea. The output will not correct the blood gases and the patient tends from compressed air nebulizers is low even with high to 'fight the ventilator'. When this occurs respiratory gas flow rates.