Successful treatment of these patients requires RCPs to adopt a multisystem approach to therapy.
Treating hypoxia in the neonatal patient presents the RCP with a serious challenge. Successful treatment of a newborn with hypoxic failure requires the RCP to adopt a multisystem approach to therapy, as there are numerous reasons for hypoxia in these patients. Treatment plans for the neonatal patient should begin as soon as possible following birth to prevent hypoxia from damaging the major organs. Early intervention may require that the care plan begin in the delivery room, immediately following the patient’s admission to the intensive care unit (ICU). Advanced medical technology will assist the RCP in treating newborns and will present the practitioner with immediate feedback concerning the patient’s response to treatment.
Fetal And Transitional Circulation
During fetal circulation, blood shunts from the right side (venous blood) to the left side (arterial blood) of the heart through the foramen ovale and the ductus arteriosus, creating a right-to-left shunt. The major element in this shunt is elevated pulmonary vascular resistance (PVR).
The RCP must be aware of the physiology of fetal circulation so the appropriate therapy can be selected to stabilize the infant experiencing hypoxia immediately following birth. Once the infant moves from fetal to transitional circulation, numerous physiologic phenomena occur simultaneously. The most notable response during this time is the first breath (and the associated alterations in pulmonary and systemic blood flows, which correspond to the level of pulmonary vascular resistance). As pulmonary blood flow increases and pulmonary vascular resistance decreases, an associated reversal of shunted blood through the foramen occurs. This reversed flow from the left to the right atrium (left-to-right shunting) assists in maintaining pulmonary perfusion as vascular resistance decreases. In conjunction with the reversed shunting through the foramen, blood flow also reverses through the ductus arteriosus to assist in maintaining adequate pulmonary perfusion. As pulmonary perfusion and oxygenation increase, the ductus will constrict and will no longer allow the left-to-right shunting of blood.
The RCP must keep in mind the clinical physiology of the ductus arteriosus for the development of an appropriate treatment strategy for the infant suffering from hypoxia. For some children born with specific congenital cardiac defects, closure of the ductus would lead to death from insufficient systemic or pulmonary perfusion. For these patients, the administration of prostaglandin will promote dilation of the ductus and maintain adequate blood flow.
Neonatal Blood Gases
Assessing the basic variables of arterial blood gases is an essential component of treating infants with hypoxia. Figure 1, page 90, is a simple presentation of the oxygen-hemoglobin dissociation curve as it relates to fetal and adult arterial blood. As the figure demonstrates, fetal hemoglobin has a greater affinity for oxygen than does the hemoglobin in adult blood. Therefore, fetal blood has a left shift in the dissociation curve; this means that less oxygen is required to saturate the blood at a level of 50 percent. This comparison to adult blood is important to the determination of the level of inspired oxygen needed in treating a newborn with difficulties
in maintaining appropriate gas exchange. The normal values
of blood gases in the fetal blood are 7.33 for pH,
45 mm Hg for Pco2, and 24 mm Hg for Po2. For maternal blood, normal values are 7.44 for pH, 36 mm Hg for Pco2, and 82 mm Hg for Po2. As these values indicate, fetal blood is more acidic, with a corresponding decrease in oxygen levels. At the conclusion of labor, substantial decreases in oxygen saturation and Po2 are present in fetal blood. These alterations in fetal oxygen values are an important aspect of treating infants with specific congenital cardiac defects (such as hypoplastic left-heart syndrome). In these children, hypoxic inspired gases or increased levels of carbon dioxide are used to offset pulmonary hyperperfusion.The use of hypoxic gases (Fio2<0.21) is preferred, as the newborn existed in a hypoxic environment during fetal life.
Noninvasive Patient Monitoring
Treating the neonatal patient with various respiratory care modalities requires constant surveillance of the patient’s response to therapy. Noninvasive monitoring technology provides a mechanism for the RCP to assess the patient’s immediate treatment response and make alterations in treatment when required. Pulse oximetry’s oxygen saturation reading has become an important parameter to monitor in treating neonatal patients with increased levels of inspired oxygen. Pulse oximetry provides a means of monitoring the effects of inspired oxygen in maintaining appropriate therapy. Pulse oximetry monitoring requires little clinician involvement once stable readings have been noted. The clinician, however, must bear in mind that pulse oximetry is a gross assessment of oxygenation, not a substitute for the serial monitoring or analysis of arterial blood gases. Recent research has demonstrated that values determined by different brands of pulse oximeters can vary greatly.
In my experience, transcutaneous monitoring of oxygenation (in conjunction with pulse oximetry) assists the clinician in assessing oxygenation better than does pulse oximetry alone. Transcutaneous monitoring reflects the patient’s oxygen delivery status from a cardiopulmonary standpoint. Assessing oxygen delivery provides information for maintaining appropriate levels of inspired oxygen and for monitoring cardiac function in relation to tissue oxygenation. Because the skin is heated to improve accuracy in transcutaneous monitoring, the clinician must alter the placement of the electrodes more frequently in patients who have compromised cardiac function in order to prevent minor skin burns.
Capnography provides information concerning the entire cardiopulmonary system. Capnography not only assesses end-tidal carbon dioxide, but provides a graphic display (capnogram) that is useful in detailed analyses of ventilation and pulmonary perfusion. Figure 2, page 92, presents the normal capnogram as it relates to normal cardiopulmonary function.
As this figure demonstrates, the alveolar plateau (phase III) represents a normal cardiopulmonary system. With compromises in pulmonary perfusion, the slope of the alveolar plateau alters during conditions of pulmonary hypoperfusion. These conditions are related to recent ventilator settings (as reflected by capnography), which is therefore a useful tool for setting and resetting the ventilator to prevent overdistension of the lungs and barotrauma.1
Selecting a ventilator strategy for the newborn with hypoxic failure requires a multisystem approach from the moment the patient is admitted to the neonatal ICU. The goal of the ventilator strategy is to minimize barotrauma to the infant’s immature lungs. As the RCP increases the mean airway pressure to improve gas exchange, monitoring is essential to prevent overdistension of the lungs and barotrauma. Monitoring for lung damage that would elevate pulmonary vascular resistance and divert blood through the foramen ovale and/or ductus arteriosus (and create right-to-left intracardiac shunting and the associated hypoxia) should be done in tandem.
Transcutaneous monitoring, in conjunction with capnography, will display the patient’s response to the ventilator’s settings on a breath-by-breath basis. A gradient of approximately 10 mm Hg between transcutaneous Pco2 and end-tidal carbon dioxide will represent adequate tissue perfusion and oxygen delivery. The slope of phase III of the capnogram will indicate immediate alterations in pulmonary blood flow. Attention to lung mechanics and graphics will help the RCP determine the correct ventilator settings (such as inspiratory time, peak airway pressure, positive end-expiratory pressure, and respiratory rate) on a real-time basis.
Patients with surfactant deficiencies may require surfactant replacement to improve gas exchange and static lung compliance. When administering surfactant, careful surveillance of static lung compliance is essential in preventing a sudden increase in tidal volume and possible pneumothorax and barotrauma.
Many newer-generation mechanical ventilators are equipped with graphic displays that measure and trend lung mechanics data over several hours so that strategies such as surfactant replacement can be monitored. By decreasing ventilator pressures and monitoring the patient’s responses to ventilator-setting alterations, the RCP can adjust therapy to meet the needs of the patient while minimizing the lung damage created by the ventilator. Whenever monitoring results seem questionable, arterial blood gas analysis is necessary to correlate data from the monitors and to check for changes in the acid-base status of the patient.
Those patients who do not respond to conventional ventilation and surfactant replacement may require high-frequency ventilation or the use of extracorporeal membrane oxygenation (ECMO). All three of these respiratory modalities require neonatal patients to be admitted to a tertiary neonatal center for proper administration of therapy. RCPs are a valuable asset on tertiary treatment teams, both in selecting treatment plans and in knowing the technical aspects involved in providing care.
Newer-generation mechanical ventilators offer the RCP novel approaches to weaning the patient. Modes of ventilation such as pressure-regulated volume control and volume-supported ventilation provide a predetermined tidal volume and automatically adjust the peak inspiratory pressure (PIP) to deliver the desired volume.2 Excessive pressures are prevented by limiting the upper-pressure threshold of PIP. Because the ventilator adjusts PIP, the work of breathing is also increased or decreased as selected by the patient. As the patient assumes more of the work of breathing, PIP will decrease. If the patient tires and requires more support, PIP will increase to meet patient demands by decreasing the imposed work of breathing. The clinical application of noninvasive monitors will aid the clinician in determining the patient’s response to weaning. The weaning plan must involve a multisystem approach and must incorporate input from the entire bedside team, with the RCP at the core of the decision-making process.
David H. Walker, MA, RRT, RCP, is chief technology officer at Valley Children’s Hospital, Fresno, Calif.
1. Fletcher R, Niklason L, Derefeldt B. Gas exchange during controlled ventilation in children with normal and abnormal pulmonary circulation. Anesth Analg. 1986;3:155-163.
2. Pitotrowski A, Sobala W, Kawczynski NO. Patient-initiated, pressure-regulated, volume-controlled ventilation compared with intermittent mandatory ventilation in neonates: a prospective, randomized study. Intensive Care Med. 1997;23:975-981.