Recent diagnostic and therapeutic advances in perinatology (fetal monitoring, scalp sampling and amniocentesis) have significantly altered perinatal mortality; better understanding of pulmonary physiology and the use of improved technology with mechanical ventilators have had a dramatic impact on the treatment of the idiopathic respiratory distress syndrome (IRDS). In several leading institutions, inclusion of anesthesia colleagues in the team approach to treating infants with severe respiratory distress has added another dimension to the understanding and treatment of the disease. However, the improved mortality in these sick infants is not due solely to improve ventilatory support; attention to the meticulous details of total care from birth to resolution of respiratory distress has played an equally important role.
Amniocentesis provides a prognostic tool that frequently alters obstetrical management of the immature infant most likely to develop IRDS. Postponement of delivery of infants with immature L/S (lecithin/sphingomyelin)* ratios allowing further maturation of surfactant is constantly weighed against the intrauterine risks for the fetus. In the event of imminent fetal demise, a foreknowledge of the infant's L/S ratio predicts the likelihood of subsequent development of IRDS. Fetal monitoring and scalp sampling, prompt and vigorous resuscitation, maintenance of body temperature in the delivery room and prompt correction of acid-base disorders are particularly important factors in the treatment of high-risk infants. Upon subsequent development of IRDS, intensive care of the sick infant with particular attention to maintenance of blood pressure, arterial oxygenation, body temperature and chemical homeostasis is critically important. All too often, credit for the recent improvement in perinatal care is attributed solely to better ventilation when in fact such improvement results from advances in many areas.
Ventilatory support of the infant with IRDS demands a single nurse per patient, per shift, and the availability of a trained physician 24 hours per day. Equating newborn intensive care units with machines rather than with adequate numbers of trained personnel is a common mistake. A six to eight week orientation period for new nurses is a routine practice in our nursery before a nurse assumes primary responsibility for patient care. Realistically, another several months are required before a good nurse develops the needed judgment and expertise. An on-going program of education with review of principles and old methods in addition to introduction of new techniques is mandatory to maintain the skill of the nursing staff. Supporting a nursing staff of this quality is obviously an expensive proposition, but in my opinion, any institution unable or unwilling to support such a nursing staff in addition to the full-time physicians should not ventilate sick infants with IRDS. The need for regionalization of care in this area is particularly obvious.
MONITORING OF BLOOD GASES
The present methods of determining PO2 are not entirely satisfactory. Catheterization of the umbilical artery is not without risk and its indiscriminate use is not justified.
Catheterization of the umbilical artery facilitates the determination of blood gases and acid-base status in critically ill infants; it also permits constant recording of arterial blood pressure which can be an important aid in the management of these sick infants. This knowledge is of particular value in caring for infants requiring assisted ventilation; it is indispensable for newborn infants requiring continuous positive pressure breathing. Radial, brachial or temporal artery puncture can be used, but repeated blood sampling from these sources is difficult and demands skilled experience. Blood sampling from these sites is very useful in the recovery phase of hyaline membrane disease after the catheter has been removed; in infants requiring occasional blood gas determinations in whom umbilical artery catheterization is not indicated; and in infants suspected of having a significant right-to-left shunt through the ductus arterious.
Employing the standards of Dunn,3 we thread the catheter to a position one cm. above the diaphragm and carefully secure it with suture or tape.
The position is then verified radiologically. Using this route demands constant observation of the umbilicus for bleeding, meticulous care of the catheter to prevent propagation of thrombus or air embolism, continuous scrutiny of the lower extremities for evidence of arterial spasm (cyanosis or blanching) and limitation of the duration of catheterization to a period as short as the clinical course allows. The catheter should be removed cautiously so as to allow proximal arterial spasm and to minimize further bleeding. Both the acute and chronic risks from the umbilical artery catheter demand careful study of each individual patient and should contraindícate its routine employment solely for the purpose of parenteral therapy. Kitterman,9 Gupta7 and Wigger11 have previously recorded the experience of their institutions with the risks of catheterization; their findings emphasize the dangers involved if there is laxity in its use.
Oxygen and compressed air are generally supplied to an intensive care unit from a central source. The capability to provide mixtures of air and oxygen producing concentrations from 25 per cent to 100 per cent must be available. In addition, these mixtures must be available at both atmospheric pressure and at pressures up to 50 lb. per square inch in units where positive pressure ventilators are used. Warning devices to alert personnel to a fall in pressure should be part of each system.
Having determined that an infant requires supplemental oxygen and being aware of its potential hazards, its delivery should be carefully monitored. When small increases in ambient oxygen are required, delivery directly into the incubator often suffices. With concentrations exceeding 50 per cent using a head box within the incubator is a simple method for more accurate provision of the correct concentration with simultaneous limitation of significant fluctuations in oxygen, especially when the incubator ports must be opened. Some incubators have cut off mechanisms designed to prevent administration of oxygen exceeding a set point. Reliance solely on these controls is unreasonable and the ambient oxygen concentration should be checked at regular intervals by a paramagnetic oxygen analyzer* that is commercially available or continuously monitored by means of an oxygen electrode with a high and low alarm.** Orders for oxygen therapy should never be written in terms of flow (liters/minute), but rather in terms of the ambient concentration.
Oxygen should be humidified and warmed prior to delivery to a sick infant. Presentation of dry gas to the patient requires humidification by means of evaporation from the respiratory mucosa. Under normal circumstances this process presents no problem, but dry gas in a sick infant, particularly with respiratory distress, compounds existent problems. By drying the respiratory mucosa, the viscosity of pulmonary secretions is increased and effective ciliary action is impaired. Such therapy may also aggravate problems of water balance in infants with precarious states of hydration by significantly increasing insensible water losses from the respiratory tract. Oxygen that is not warmed may also be a significant cold stress to the premature infant with respiratory distress syndrome.
INDICATIONS FOR VENTILATION
With the exception of apnea, indications for assisted ventilation vary between institutions. In reviewing their experience with ventilation, Murdoch et al. 10 found that infants treated with apnea as the primary indicator for ventilation had the worst prognosis of all their groups. This finding is based on several factors including the existence of more immature, sicker infants and probable central nervous system hemorrhage. However, in less experienced institutions apneic infants are often ventilated as a result of errors in judgment as to the severity of illness with failure to recognize earlier signs of impending respiratory failure. Prevention of severe apnea with its known neurologic sequelae is possible in many cases through employing accurate mechanical monitors. Such prevention is dependent on the proper use of available monitors. Frequently because of inadequate instruction from the medical staff, nursing personnel do not understand the principles of monitoring including how to properly place leads and blame false alarms on the monitor rather than on technical errors. All too often with false alarms, the natural tendency is for the staff to turn off the alarm assuming the error is inherent in the monitor, and the apneic infant may go unnoticed for valuable seconds of time.
Other indications for ventilation include hypoxia, hypercarbia and acidemia. Generally speaking, the aim of therapy should be to maintain the arterial oxygen tension between 40-60 mm.Hg. in the aorta below the level oí the ductus. By varying the ambient oxygen concentration this goal may be easily achieved in mild IRDS without exposing the infant's lungs to high concentrations of oxygen for prolonged periods. In severe IRDS, arterial oxygen tensions may rapidly fall despite increasing concentrations of oxygen, and this group of infants may require assisted ventilation. In our hands, this point is generally reached when the arterial oxygen tensions fall below 50 mm.Hg. with the infant breathing 100 per cent oxygen. Generally speaking, if therapy can maintain the pH above 7.20 and the pC02 between 60-70 mm.Hg., ventilation is not indicated. But if despite correction of the metabolic acidosis that commonly accompanies IRDS and intermittent bagging to assist ventilation, the pH falls below 7.2 and the pCOi exceeds 70 mm.Hg., mechanical ventilation is initiated.
Methods of intubation are detailed elsewhere and will not be covered in this article.8 The route of intubation is dependent on institutional experience and opinion. Most institutions use nasotracheal tubes for infants requiring mechanical ventilation, but a recent paper details experience with oral-tracheal intubation and its advantages.
We employ nasotracheal intubation for assisted ventilation because we have extensive experience with this method and feel that we can provide better airway care with this route. When an infant with severe IRDS is admitted to the nursery, his length is measured so that the approximate position of his nasotracheal tube is known (Figure I)2 should mechanical ventilation subsequently be indicated. Once the decision to ventilate an infant has been made, the infant is initially intubated with an oral-tracheal tube and ventilated until he is well-oxygenated. At this point, after adequate suctioning and direct visualization of the cords, the oraltracheal tube is replaced with a nasotracheal tube. Once the tube is in place, its position should be checked clinically by assuring good bilateral excursion of the chest and clear, equal breath sounds bilaterally. On small infants, listening to the chest may occasionally be misleading to the inexperienced observer since even with the tube in the esophagus breath sounds may appear to be equally distributed bilaterally. In such infants, by listening directly over the tube with either the ear or the stethoscope for the presence or absence of breath sounds, one can determine the position of the tube immediately. This simple step avoids life threatening hypoxic episodes secondary to improper placement of the tube.
Once the position of the tube is verified clinically, the tube is secured in place marking the point of exit from the nares. This point serves as a useful marker during the course of ventilation to check for inadvertent withdrawal or advancement of the tube. The method of securing the tube in place is again one of personal experience and opinion. Whatever the method employed, once the tube is secured, a chest x-ray should be done to confirm the position of the nasotracheal tube, ideally one-two cm. above the carina.
The nasotracheal tubes that we use are clear, sterile and nontoxic, and have an internal diameter of 3.0 to 3.5 mm. We attempt to avoid use of nasotracheal tubes with an internal diameter of less than 2.5 mm. because of the associated problem of increased airway resistance with the small tube.1
Regression lines for naris-cord and naris-carina distancée versus crown-heel length from Colderon.2
CONTINUOUS POSITIVE PRESSURE BREATHING(CPPB)
One of the most significant advances in the treatment of IRDS was the introduction of CPPB by Gregory et al.6 Prior to the institution of this therapy, infants weighing less than 1,500 grams and requiring ventilation in the first day of life had mortality rates approaching 90 per cent. After the initial experience with CPPB, Gregory et al. report a 63 per cent survival rate.4
Figure 2 shows one of the systems used by Gregory to apply continuous positive pressure. An alternate system using a head hood is also available when an infant meets the criteria for assisted ventilation. Initially five mm. of water positive pressure is applied either via a tube or the head box with the child breathing 100 per cent oxygen. If there is no immediate rise in the PO2 to the desired level of 50-70 mm.Hg., the pressure is increased by increment of two mm. until improvement is noted. Once the PO2 rises above 60-70 mm.Hg., the oxygen content is gradually lowered by 10 per cent with blood gases rechecked. Over 12 to 18 hours, the ambient oxygen is often lowered to 40-50 per cent. When the infant has been without continuous positive pressure for 4-6 hours and is on 40 per cent oxygen, extubation and removal from the hood is considered. Prior to either maneuver, the ambient oxygen concentration is increased 1020 per cent because of laryngeal incompetence. Usually, no or ineffective grunting for variable periods follows extubation.11
System for continuous positive pressure breathing from Gregory. G. A5
If following the initiation of CPPB the PO2 continues to fall and the PCO2 rises despite intermittent bagging, mechanical ventilation is started and a positive end expiratory pressure (PEEP) may be applied. The principles of management of PEEP are the same as with CPPB with gradual increases in pressure until an improvement in PO2 is noted. Gradual reduction of oxygen, followed by reductions in PEEP are determined by serial blood gases. The likelihood of pneumothorax is greater with PEEP than with CPPB so the infant with PEEP must be carefully observed for sudden changes in his clinical course.
The most commonly used respirators are positive pressure ventilators, either volume limited or pressure limited. Again institutional opinion and preference determines which particular respirator is used. Volume limited respirators are generally preferred to pressure set respirators assuming that the proper sized airway is used. Most infants require the control mode of the respirator for adequate ventilation, and use of the assist mode is generally limited to short periods during the recovery phase of the disease. In some infante coordination of the infant with the ventilator may be a problem. Most infants require the control mode of the respirator for adequate ventilation, and use of the assist mode is generally limited to short periods during the recovery phase of the disease. In some infants coordination of the infant with the ventilator may be a problem. Most infants who are well oxygenated will synchronize with the ventilators, but a few will require sedation with either morphine or phénobarbital in order to be adequately ventilated.
Our experience is primarily limited to volume controlled ventilators (Bourns and Bennet MA-I). In selecting initial rate and tidal volume, one depends on clinical findings (color and auscultation of the chest) with subsequent adjustments based on serial determinations of PCO2 and PO2. Because these results determine subsequent therapy, a blood gas machine in dose proximity to the unit is essential. If rapid results (within minutes) can not be provided by the laboratory, a blood gas machine should be installed within the intensive care unit. If neither option is possible, infants requiring ventilation should be transferred to a hospital with adequate facilities. The frequency of arterial sampling is determined by the clinical course of the patient. In acute stages of IRDS, more frequent determinations are required to avoid both hypoxia and hyperoxia and to maintain the PCO2 within an acceptable range. In using volume limited ventilators one must also consider the compression volume of the ventilator, i.e., the gas compressed within the respirator system in selecting the volume settings. Failure to note this factor will result in underventilation of the sick infant. Several reports on extensive experience with negative pressure respirators, which have the obvious advantage of avoiding a tube and its hazards, are available.
After a period of stabilization, changes in ambient oxygen concentration, tidal volume and rate will be based on serial blood gases. If the infant requires 100 per cent ambient oxygen to maintain PCO2 within the desired range (50-60 mm.Hg.), a positive and expiratory pressure (PEEP) is applied to the system. This method improves oxygenation in infants with severe pulmonary disease without significantly impairing cardiac output or venous return. The end expiratory pressure required for a particular infant must be determined arbitrarily and its effects determined by blood gases. We generally start with five cm. water pressure and increase or decrease from that point as necessary. By improving oxygenation, ambient oxygen concentration can be gradually lowered over a 12-18 hour period while maintaining the PCO2 between 50-70 mm.Hg. Pneumothorax is the most frequent complication of this therapy, and sudden changes in clinical course require a chest x-ray to exclude its presence.
The acute stage of IRDS varies from infant to infant, ainical improvement is accompanied by a return of the PCO2 to normal or near normal values and then lower levels of ambient oxygen are needed to maintain an accepted PCO2. When the ambient oxygen is less than 50 per cent, PEEP is gradually decreased with serial blood gases to document improvement. Over several hours or days the PEEP maybe gradually discontinued. At this point, the pressure required to ventilate the infant may be then lowered with blood gases again verifying improvement. As the patient becomes more vigorous, trial periods on a head box with the nasotracheal tube in place are attempted. The duration of these periods is gradually increased to the point at which the infant's clinical condition and blood gases suggest that extubation is indicated. In most infants the interval from gradual decrease in PEEP to extubation is several days, but in other infants the weaning process requires weeks.
Prior to extubation the infant is vigorously suctioned and then ventilated with higher ambient oxygen concentrations for several minutes. The infant should not be suctioned during extubation because this practice may result in significant atelectasis. After extubation, the infant is placed in a head box in an ambient oxygen concentration of 10-20 per cent higher than the prior level while in the respirator. During the first eight-12 hours the baby should be carefully monitored with blood gases; vigorous chest percussion and direct tracheal suction as indicated should be performed at regular intervals (at least every two hours). During this critical period the infant should receive intravenous fluids rather than risking aspiration of formula. Some infants may require reintubation and ventilation for variable periods and repeating of the weaning process a second time.
Some institutions have used steroids or racemic epinephrine in older infants with respiratory failure during the weaning stage, but we have no experience with steroids; racemic epinephrine given by nebulizer and mask has proved beneficial in extubating several infants who have been entubated for prolonged periods.
Not only has mortality improved in infants with IRDS, but perhaps even more importantly the long term neurologic, mental and behavioral status of survivors is particularly promising. The previously quoted change in mortality following the introduction of CPPB by Gregory et al. has not yet been equalled by other institutions, but most centers have reported a significant improvement in mortality figures.
The report of Rawlings et al. on infants with birth weights less than 1,500 grams supported the earlier findings of Stahlman in infants with IRDS that most (80-85 per cent) of the survivors of IRDS were neurologically intact and had development or intelligent quotients within the normal range. These recent reports supported by our own experience are in direct contrast to reports made before the present comprehensive approach in neonatal intensive care was adopted.
An approach to the perinatal treatment of an infant with IRDS has been presented. The requirements for maintaining an effective program in terms of physicians, nurses and paramedical personnel are immense, and the cost of supporting personnel, equipment and space are continually increasing. The necessity for well organized units, including an effective means for transporting sick infants to the center, demands regionalization of care if we are to be effective in lowering existent perinatal mortality and morbidity.
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