Assisted ventilation is an important adjunct to the overall treatment of respiratory failure in the newborn. Ventilation with a bag and mask is a simple and effective means of performing assisted ventilation. The following article will discuss the diagnosis of respiratory failure and this technique as applied to sick neonates.
A brief review of pulmonary physiology in the newborn will serve as a background for the discussion of assisted ventilation. Present research indicates that during intrauterine life the lungs are filled with a volume of fluid approximately ½ of the volume of air to be contained after birth.1 At delivery this fluid must be replaced by air. Factors which aid the entry of air into the chest after birth include the elastic recoil of the rib cage following vaginal compression of the chest during delivery, removal of intrapulmonary fluid by vascular and lymphatic channels, and the presence of surface active phospholipids to minimize the surface forces encountered during expansion of the lungs. Absence or deficiency of one or all of these factors has been implicated in the development of neonatal respiratory distress. Table 1 lists the causes of respiratory failure in the neonate.
The function of the lungs is to oxygenate the blood and remove carbon dioxide. Provision of adequate amounts of oxygen prevents metabolic acidosis while removal of carbon dioxide prevents respiratory acidosis. Thus the lungs also regulate body pH. Pulmonary function may therefore be assessed by determination of blood pH, PCO2 and PO2.
Non- distressed low birth weight infants have an arterial PO2 in the range of 40-60 mm. Hg.* This compares with 85 mm. Hg. or greater in the normal child or adult. Hypoxemia occurs when insufficient oxygen is present in the arterial blood. Cyanosis is apparent when at least five gm. of reduced hemoglobin are present in each 100 ml. of blood. The major causes of arterial hypoxemia include shunting of blood from veins to arteries, as in certain forms of congenital disease or as perfusion of atelectatic areas of the lung; a block to diffusion of oxygen from the alveolar air spaces across the alveolar membrane and into the pulmonary capillaries; or underventilation of areas of the lung, where gas exchange is insufficient to fully oxygenate the blood perfusing these areas.2 While the effects of underventilation or diffusion block may be overcome by administering sufficiently high concentrations of oxygen, hypoxemia due to shunting cannot be relieved. Since the major cause of cyanosis in hyaline membrane disease appears to be shunting of blood through atelectatic areas of the lung, the ability to raise the arterial oxygen tension of an infant with this disease is a reflection of the severity of his illness.
RESPIRATORY FAILURE IN THE NEONATE
In respiratory disease the lung is not homogeneously ventilated and perfused. Normal areas may be found intermixed with atelectatic, underventilated and alveolar-capillary block segments. Segments with alveolar-capillary block, atelectasis and underventilation all contribute to the development of hypoxemia and cyanosis. The use of supplementary oxygen will raise the oxygen tension of blood leaving alveoli which are normal or suffer from either underventilation or alveolar-capillary block. No alveolar gas reaches blood perfusing atelectatic alveoli. These alveoli function as shunts and the use of 100 per cent oxygen tension therefore will not raise the oxygen tension of blood leaving these alveoli. When the proportion of non-ventilated atelectatic alveoli is so large that 100 per cent oxygen cannot raise the arterial oxygen tension over 50 mm. of Hg., then respiratory failure may be said to have occurred.
Adequate removal of carbon dioxide depends on the presence of areas which are both ventilated and perfused. Underventilation in relation to perfusion and ventilation of an area without adequate perfusion can both result in carbon dioxide retention, causing acidemia by production of carbonic acid.* Normally, the PCO2 of neonates is 35-45 mm. Hg. after birth. Elevations of PCO2 indicate progressive degrees of pulmonary insufficiency. A PCO2 of 70 mm. Hg. or greater is indicative of respiratory failure.
Both carbon dioxide retention and hypoxemia contribute to the development of acidosis. After the first hour of life, nondistressed neonates have arterial pHs similar to those of older individuals, 7.35-7.45 units. In respiratory failure, values of 7.2 or less are usually encountered. The sources of acid in respiratory failure are 1) carbonic acid, arising from carbon dioxide retention and 2) lactic acid, arising from anaerobic metabolism. The presence of acidosis suggests inability of the cardiopulmonary system to deliver adequate supplies of oxygen to the tissues and to remove the CO2 produced by the body. Secondly, the presence of acidemia causes pulmonary vasoconstriction, leading to a rise in pulmonary vascular resistance and causing pulmonary underperfusion. This, in turn, leads to further carbon dioxide retention with the development of a still worse acidosis.3
Summarizing the discussion so far, respiratory failure occurs in the neonate when normal pulmonary ventilation cannot be established and maintained following birth. The presence of ventilatory failure is marked by acidemia (pH less than 7,2), hypercapnia (PCO2 70 mm. Hg. or over) and hypoxemia (PO2 under 50 while breathing 100 per cent oxygen). These findings, or the presence of prolonged apnea, are the two major indications for assisted ventilation.
TECHNIQUE OF ASSISTED VENTILATION WITH BAG AND MASK* *
Assisted ventilation is one of the most direct means for relieving the effects of respiratory failure. The bag and mask has been shown to be effective for' such treatment.4 This method has the advantage over most methods of mechanical assistance of not requiring endotracheal intubation, thus avoiding the morbidity and mortality associated with the presence of an indwelling endotracheal tube. Since an endotracheal tube and complicated mechanical equipment are not required, bag and mask ventilation may be easily started, discontinued and resumed if necessary. Although practice in the use of the bag and mask is essential, training in the use of sophisticated mechanical equipment is not.
The following equipment is presently in use at the New York Hospital for bag and mask ventilation:
Conductive breathing bag, 500 cc.**
Female connector, 15 mm.**
Rovenstine angle, 90 degrees, male and female ends.**
Breathing bag plug.**
Infant plastic face masks with rubber cushions, sizes 1-4.***
Additional equipment includes nonadherent dressings for face protection (Telfa, Johnson and Johnson), nasogastric tubes, oxygen measuring equipment, plastic or glass Y or T fittings, and a supply of compressed and humidified gas (both oxygen and air).
Newborn infant being ventilated with bag and mask. For best results the bag should be maintained in an inflated state and the mask firmly applied to the face as shown.
The infant is placed in the supine position with the neck extended by means of a diaper roll. A nasogastric tube is placed in the patient's stomach and allowed to remain in place in order to provide for gastric decompression. If ventilation is to be prolonged, a dressing is applied to the infant's face surrounding the mouth and nose and protecting the eyes. The mask, attached to the bag via the Rovenstine angle, is placed over the infant's face. Gas with the desired oxygen tension is allowed to fill the bag. Flow is regulated between five and 15 liters/minute, depending on the rate and volume of respiration. If 100 per cent oxygen is not required, room air may be mixed with the aid of the Y or T fitting. A leak at the face mask prevents rebreathing of carbon dioxide and overfilling of the bag between respiratory cycles.
With the bag in place, the operator manually compresses the bag with one hand while holding the mask in place with the other. Rate and effort are determined by the ability of the operator to move the infant's chest and by the presence of cyanosis. Generally a rate of 40 breaths per minute is chosen as this is approximately the normal rate at which the infant would breathe. The patient's chest should be seen to move. If the patient is breathing spontaneously, an effort should be made to synchronize the assisted breaths with the patient's own respiratory movements. Cyanosis is an indication to increase the rate and volume of ventilation. If these steps fail, then additional oxygen should be added to the inspired gas mixture. Except in the apneic patient, assisted ventilation with bag and mask is generally begun on an intermittent schedule, e.g., five minutes out of 15 minutes. The effectiveness of a particular regimen of ventilatory assistance is determined by measuring blood gases before the next scheduled period of bagging. If blood gases are normal or improved, the interval between baggings is increased. If the gases have deteriorated, then the time of bagging or the frequency is increased. Weaning thus takes place as the patient is able to maintain normal blood gases while breathing spontaneously in a concentration of oxygen just adequate to relieve cyanosis. Oxygen concentration should be monitored at least hourly and adjusted so that cyanosis is just relieved.*
In addition to measuring routine blood gases and monitoring the oxygen concentration, certain other procedures are necessary. An initial chest x-ray is needed for diagnostic purposes and is not repeated unless clinically indicated. Antibiotics are not used unless indicated clinically for treatment of infection. Since oral feeding is usually impossible, intravenous fluids must be supplied to provide water and calories. Body temperature must be maintained by careful attention to maintenance of a neutral thermal environment (incubator technique.)
Pressures attained during bag and mask ventilation are generally in the range of 50 cm. H2O. The flow about the mask allows the operator to administer continuous positive airway pressure of up to 10 cm. of water by varying the leak around the face of the patient. These pressures are adequate to ventilate the lungs of most sick neonates.
Complications arising from the use of bag and mask have included pneumothorax, pressure necrosis from the mask and dependence on assisted ventilation. The incidence of these complications may be minimized by careful attention to nursing care, the technique of bagging and frequent checking for the opportunity to wean the patient from the bag and mask.
The use of bag and mask is not limited to respiratory failure by specific etiology. All personnel working in delivery areas and nurseries should become proficient in the use of this method, which is the current policy at the New York Hospital. The equipment is inexpensive and may be purchased for under $35 (1970.) Thus several sets may be acquired and placed at strategic locations within the nursery, in the incubators of high-risk infants and in the delivery suites to be ready for use at a moment's notice.
1. Avery, ME. The Lung and Its Disorders in the Newborn Infant. Philadelphia: WB. Saunders. Co.. 1968. 18-30.
2 Com roe. J. H. Physiology of Respiration. Chicago: Year Book Medical Publishers, 1965, 16066. 170-81. 208-211.
3 Chu, J; Clements. J.A.; Cotton. EX. et al Neonatal pulmonary Ischemia. Pediatrics 40 (1967). 709-782.
4. Gruber. H. and Ktaus. M.H. Intermittent bag and mask therapy. Journal of Pediatrics 76 (1970), 194201.
5. Sykes. M.K.; McNicoi, MW. and Campbell, E.J.M. Respiratory Failure. Oxford. England: Blackwell. 1969. 273-285.
RESPIRATORY FAILURE IN THE NEONATE