The Mechanical Respirator and the Pediatric Patient KARIM F. RASHAD, M.D.*
Mechanical ventilators have been widely used on adult patients in the last decade. Only recently have respirators been adopted for use in the pediatric age group. Apparatus for infants have been more difficult to perfect because of the different nature of respiration in the infant as compared to the adult. Infants' rapid respiration, greater resistance to control, small tidal volume, and sudden development of pressure changes within the thoracic cavity make the adaptation of respirators for use in newborns and small infants extremely difficult. 1 • 2 A child of three years or more may be treated much like an adult with respect to choice and use of ventilation.
HOW TO CHOOSE A RESPIRATOR FOR A PEDIATRIC UNIT Respirators are generally classified as pressure-limited (Bird, Bennett) or volume-limited (Engstrom, Emerson). 7 In the pressure-limited type, the respirator delivers a volume of air or gas to the patient until a pre-set pressure is reached. The volume delivered to the patient therefore will depend on the patient's compliance and the presence or absence, of airway obstruction or kinking of tubes that connect the patient to the pumping port of the respirator. This type of respirator is used mainly to assist respiration and very rarely to control respiration. It is very difficult to adapt this type for use in infants. Its use requires someone who is very familiar with the mechanics of the respirator and also knows a good bit of pulmonary physiology. Volume-controlled respirators deliver a pre-set volume with no regard to any pressure build-up. This type is more effiqient in providing good ventilation in the pediatric age group, especially the types that are provided with a means to vary the inspiratory and expiratory phases of respiration (Emerson). ''Assistant Professor of Anesthesiology, The Johns Hopkins University School of Medicine, Baltimore, Maryland
Surgical Clinics of North America- Vol. 50, No, 4, August, 1970
Another point that should be taken into consideration in selecting a respirator is the simplicity and the durability of the machine. This factor is of great importance. Most personnel handling these machines in recovery rooms and intensive care units have little or no mechanical ability. To provide them with a complex respirator and expect them to comprehend in detail how the machine functions, or to recognize any breakdown in function at a moment's notice, is asking too much. The respirator should also provide a good means for humidification and nebulization, and it should be easily disassembled for sterilization. Last, but not least, the cost of purchasing the machine and maintaining it should be reasonable. From our own experience we have found that the Emerson respirator best fits these criteria for use in the pediatric age group.
INDICATIONS FOR USE OF RESPIRATORS Before we consider any indication, it must be clearly understood that there must be someone in the hospital who is vitally interested in respiratory care and who understands the mechanics of the particular respirators to be used. The margin of safety during constant artificial ventilation is very small, and failure of a respirator will lead to death or irreversible brain damage in a matter of minutes. Safeguards must be taken against accidents, whether caused by failure of the respirator, accidental disconnection of the patient from the respirator, or airway obstruction. These patients should be cared for in a special unit under constant observation by experienced nursing staff and physicians. These patients should never be left unattended. 6 Respirators are used in numerous situations involving actual or threatened respiratory failure. At present the most common indication for their use in this age group is following open heart surgery. 3 • 4 Examples of nonsurgical conditions requiring the use of respirators include respiratory distress syndrome, Guillain-Barre disease, status asthmaticus, pneumonia, tetanus, other pulmonary and neuromuscular disorders, and trauma. From a practical point of view, the only absolute indication for using a respirator is a patient who is breathing inadequately, whatever may be the cause. Relative indications include patients who are hypoventilating, whether the hypoventilation is actual, relative, or potential. These relative indications are not easily defined but in many respects are similar to those that have been used for tracheostomy, and mainly call for good judgment. Specifically, the general appearance of the patient is of great importance- not only the color of his mucous membranes and his nail beds, but also any change in his level of responsiveness. If a child who is alert and crying vigorously becomes more and more lethargic, this perhaps indicates that the child needs respiratory help. Anxiety is another important factor. The more a child needs respiratory assistance, the more anxious he gets. The amount of work a child
THE MECHANICAL RESPIRATOR AND THE PEDIATRIC PATIENT
is doing to breathe and any signs of exhaustion should also be taken in consideration. Also, is the child using his accessory muscles to maintain his respiration? Is there subcostal retraction? Is the respiratory frequency excessive? The presence of any two of these signs is an indication for need of respiratory assistance. 5 Vital signs are indications if they are abnormal. It is important to realize that a person may have an extremely high Pco 2 or may have a relatively low Po 2 , and yet may have a perfectly normal blood pressure. Thus, if the vital signs are normal it does not necessarily mean that the child is not in severe distress. Laboratory data are of extreme help. Today there is equipment available for measurement of pulmonary function and blood gases that is very reasonably priced and easy to use. It should be available in every hospital. A patient whose vital capacity is less than 15 ml. per kg. of body weight is not going to be able to maintain adequate spontaneous ventilation. A vital capacity under 10 ml. per kg. is totally inadequate, and this patient very likely will go into respiratory failure. Blood gas determinations offer the most definite guidance. Most agree that a Po 2 <40, Pco 2 >65 and a pH under 7.25 indicate the need for active respiratory assistance. It is more important, however, to look for a change in blood gas values. A patient may carry a relatively high Pco 2 of 55 to 65 mm. Hg and do perfectly well. However, if a patient's Pco 2 is initially 45 mm. Hg, and one hour later it is 55 mm. Hg, still later climbing to 60 mm. Hg, this patient has a definite indication for therapy.
COMPLICATIONS AND HAZARDS Leading the list of complications and hazards are the so-called plumbing errors. Here the respirator becomes disconnected, is connected improperly, or develops a leak, or a valve malfunctions. While most of the manufacturers sell a device that is supposed to give an alarm if a part gets disconnected and pressure drops, anyone who has any experience with respirators knows that these are not infallible and that they must not be relied upon. It takes a good baby sitter, someone who is really interested in following that patient, to keep close watch to prevent these types of errors from occurring. Inadequate humidification is a most serious error. In bypassing the nose and mouth with a tracheostomy or endotracheal tube, adequate humidification must be provided or irreparable damage will result. Hypertension is not at all uncommon. If a patient is hypovolemic to begin with and he is started on intermittent positive pressure respiration, the normal filling gradient of the right side of the heart may very well be decreased and the patient will go into shock. If this happens, this should be a clue that perhaps the patient needs more blood or fluids, or both. Pneumothorax is a very serious complication unless detected and treated promptly. When any patient who is doing well on a respirator
suddenly deteriorates, think of pneumothorax and exclude it as a complication by physical examination and chest x-ray. The treatment is underwater drainage of the thoracic space with chest tubes. This is sometimes best done before placing a patient on a respirator, especially in instances in which lung damage has occurred. In spite of the best possible care, pulmonary infections often develop with prolonged use of respirators, particularly in patients with flail chests and in debilitated patients. It is not uncommon to culture gramnegative bacilli out of the sputum of patients after a period of 2 to 3 days. Our experience suggests that the patient should not be started on antibiotics, not even prophylactically, until a definite organism has been detected, at which time the choice of an antibiotic can be made on the basis of culture and sensitivity. Prevention of pulmonary infection is difficult at best. It depends on good nursing, and requires a physician who is interested enough in the subject to convey its importance to the nursing staff. The tracheostomy or endotracheal tube should be treated like a surgical incision. When it is changed or suctioned, sterile gloves should be worn and the area should be draped with sterile towels. Only sterile suction catheters should be used, and these should be discarded after every use. The parts of the respirator that come in contact with the patient should be changed daily and replaced by sterile parts. For this we advocate gas sterilization. The maintenance of the maximum host resistance to infection is insured by an adequate state of hydration and nutrition. An adequate blood volume with tissue perfusion and tissue oxygenation helps greatly in reducing the incidence of infection. Gastrointestinal complications include upper gastrointestinal bleeding, gastric dilatation, and ileus. They should be looked for and treated promptly if they do occur.
EVALUATION AND MONITORING OF PATIENTS ON RESPIRATORS Evaluation can be divided between clinical observation and physiologic measurements if they are available. The strict observation of the patient should include observation of how the chest is moving. Anytime a patient is placed on a respirator, the first thing that must be done is to observe the chest movement. Does it move adequately and in phase with the respirator? If not, the respirator probably has to be _adjusted to a greater volume or a faster rate to better ventilate the patient. It may also be necessary to obtund the patient with small increments of morphine until he is under control. Periodic auscultation of the chest is of great value in estimating adequacy of ventilation and detecting changes. The color of mucous membrane and nail beds is very helpful in this age group. The absence of anxiety or fighting the respirator, all other things being equal, is a sign of a well ventilated patient. If the patient becomes anxious or breathes out of phase with the respirator, then something
THE MECHANICAL RESPIRATOR AND THE PEDIATRIC PATIENT
is amiss, usually within the patient (low cardiac output, airway obstruction, pneumothorax, etc.). In physiologic measurements, the effective compliance is the easiest thing to measure because almost all respirators have an aneroid manometer that indicates the airway pressure. If, for example, when a child is placed on the respirator, there is adequate movement of the chest at say 25 em. of water pressure, but the next day in order to get adequate movement of the chest it takes 45 em. of water, this indicates that there is something going wrong in this child's lungs. The compliance is decreasing, and it may very well be that the child is developing atelectasis and perhaps is being underventilated. The arterial blood gases and the pH should be checked frequently if at all possible. Carbon dioxide elimination as attested to by Pco 2 usually has to be kept at least in the low 30's or high 20's to keep patients comfortable. The accompanying pH will usually be on the mildly alkalotic side. An abnormally low Po 2 usually indicates some type of intrapulmonary shunting, most likely atelectasis. This calls for increasing the tidal volume, retarding the expiration or frequent manual hyperexpansion with a self-filling bag, or all three. The dead space/volume ratio is a handy measurement to follow if it can be done. It is helpful indication of when a patient needs a respirator (less than 0.6) or can get by without a respirator (above 0.6). To wean a patient from the respirator, the same clinical and laboratory criteria can be followed in reverse. If one can take a patient off the respirator and he remains comfortable, he doesn't develop anxiety, he doesn't start thrashing around, his color remains good, and his vital signs remain stable, then the chances are that the patient can remain off the respirator. Someone must be there, however, who can observe the patient very closely. If not, one can expect an emergency call that the patient is in arrest. Laboratory criteria such as blood gases are of value if one knows what they were before weaning, and if they remain constant. Weaning may take considerable time and effort, and must not be rushed.
REFERENCES 1. Ahlgren, E. W., and Stephen, C. R.: Mechanical ventilation of the infant. Anesthesiology, 27:692, 1966. 2. Ahlgren, W. W., and Stephen, C. R.: Experience in the management of hyaline membrane with a new mechanical ventilator. Anesthesiology, 28:237, 1967. 3. Brown, K., Johnston, A. E., and Conn, A. W.: Respiratory insufficiency aad its treatment following pediatric cardiovascular surgery. Canad. Anesth. Soc. J., 13:342, 1966. 4. Dammann, J. F., Jr., eta!.: The management of severely ill patients after open heart surgery. J. Thorac. Cardiovasc. Surg., 45:80, 1963. 5. Downes, J. J., and Wood, D. W.: Mechanical ventilation in the management of status asthmaticus in child. In Eckenoff, J. E., ed.: Science and Practice in Anesthesia. Philadelphia, J. P. Lippincott Co., 1965. 6. Holmdahl, M. H.: The respiratory care unit. Anesthesiology, 23:559, 1962. 7. Safar, P., and Kunkel, H. G.: Prolonged artificial ventilation. In Artusio, J. F., Jr., ed.: Clinical Anesthesia Series, Vol. 1, 1965, Chapter 5.
The Johns Hopkins Hospital Baltimore, Maryland 21205