For many decades, acute lung injury (ALI) has been considered a clinical diagnosis that is characterized by severe inflammation resulting in hypoxemia and respiratory failure.1 Primary criteria for ALI in adult and pediatric patients were established in the 1990s by the North American European Consensus Conference (NAECC) as a clinical presentation of acute inflammation, severe hypoxemia defined by a Pao2/Fio2 ratio of <300 (ALI), the presence of bilateral infiltrates on chest radiograph, and absence of physiological shunts or left atrial hypertension.2 Pediatric ARDS, the most complex form of ALI, is prevalent in <4% of pediatric intensive care unit admissions and carries a risk of 10% to 40% mortality based on these criteria.2 In infants, the primary risk for morbidity and mortality is associated with an escalation of injury attributed in part to biotrauma and maternal infection that results in inflammation that precedes the need for oxygen and mechanical ventilation and may later result in neonatal infection.3 Premature infants are at greater risk for ALI in the presence of lung immaturity and lack of alveolarization and are subject to rheotrauma during mechanical ventilation in the presence of high flow rates and gas turbulence that result in inadvertent PEEP, subsequent overdistention, and further lung injury.4 While the causes of ALI may be multifactorial, significant alveolar damage may result in air leak, and prolonged mechanical ventilation poses the greatest risk for impairment. With the help of noninvasive technology, the oxygen saturation/Fio2 (S/F) ratio and oxygen saturation index (OSI) are being evaluated as potential tools for pediatric ALI criteria. An oxygen S/F ratio of 253 and OSI of >6.5 in the presence of oxygen saturation <98% is indicative of ALI.5 Diagnosis, monitoring, and management of ALI are challenging and require an integrated approach from all members of the health care team.
Assessment of Gas Exchange
Patients identified as being at risk for ALI require intermittent or continuous, invasive or noninvasive monitoring of gas exchange. The type of monitoring used is dependent on the patient’s access to the specific method and its equipment. Short- and long-term monitoring is essential to management of the underlying inflammatory process and may impact its treatment.
Invasive Monitoring of Oxygenation
Assessment of acid base status, oxygenation, and ventilation can be accomplished by monitoring blood gases. Invasive monitoring via arterial line in acute and serious clinical situations is suggested. Although Pao2 obtained from an arterial blood gas captures only a particular moment in a clinical presentation, it plays an important role in the Pao2/Fio2 ratio from a diagnostic perspective. This ratio is used to track progression of the disease and is often referred to as an indicator of oxygenation.6 While a value of <300 has typically been used to define ALI, the most critical form of ALI, also known as acute respiratory distress syndrome (ARDS), is defined by a Pao2/Fio2 ratio <200. A rise in the Pao2/Fio2 ratio may imply that the patient is improving. Utilizing this ratio to gauge progress is in question and creates a dilemma, as the Pao2 may initially rise in response to an increase in supplemental oxygen alone while a change in lung pathology may not respond at the same interval.
Central Line Monitoring
Central lines are often inserted in ALI pediatric patients to monitor central venous pressure (CVP), assess fluid status and need for resuscitation, and assess central venous saturation Svo2.7 Svo2 is compared with arterial saturation (Sao2) to assess ventilation and perfusion mismatch. The central venous oxygen saturation cannot accurately reflect mixed venous saturation when <70% but may be useful if saturation is >70%.8 Sao2 and Svo2 are also used in pediatric patients to estimate the oxygen extraction ratio and oxygen delivery (Sao2) and the degree of tissue oxygenation in the face of hemodynamic changes. In ALI, during which oxygen delivery may be decreased, oxygen extraction increases. The rapid changes in these values will provide valuable information as the condition changes.
Pulmonary Artery Catheters
Pulmonary artery catheters (PAC) provide the most accurate measurement and establish baseline hemodynamic parameters. Fluid status, cardiac output, Svo2, values for extraction ratio, pulmonary and systemic vascular resistance, and cardiac versus noncardiogenic pulmonary edema are monitored by pulmonary artery catheters.9 The use of PAC has not been shown to improve outcomes in pediatric or adult patients, however, and is therefore not recommended as a primary method of routine monitoring in these patients.9,10
Noninvasive Monitoring of Oxygenation
As technology has advanced, the availability of continuous noninvasive monitoring has changed the management of many conditions, including ALI. Monitoring oxygenation noninvasively has become a routine component of physiological monitoring, and pulse oximetry has been shown to be beneficial in monitoring most types of cardiac and pulmonary illnesses, including ALI.
Pulse oximetry measures arterial oxygen saturation noninvasively and, with its advanced concepts in perfusion index and ability to trend recorded values, has been shown to be accurate at saturations above 80%. Historically, clinicians have relied on pulse oximetry as part of routine management of oxygenation in infants with acute and chronic lung injury. Oxygen saturation (sPo2) value in the OSI equation in place of Pao2 (Pao2/Fio2 ratio) is on the horizon of being validated. Despite the subtle differences in each measurement methodology, the error associated with Sao2 measurement that occurs at low perfusion states, the principles of the oxygen dissociation curve, and an sPo2 >80% and <98%, pulse oximetry may be used effectively for monitoring response to treatment. OSI is currently being used to determine success of ventilation strategies with high frequency ventilation versus conventional strategies in pediatric centers. Children or infants without access to invasive monitoring can be monitored closely using sPo2 with minimal effort or adverse events.
Noninvasive Monitoring of Ventilation: Capnography and End Tidal CO2 Monitoring
Capnography, the measurement of end tidal CO2 over time (PetCO2) at the airway, is a noninvasive method that does not reflect levels that directly correlate with PaCO2. PetCO2 is used as a trending value in ALI for continuous monitoring of CO2 (via capnogram) in the absence of or in conjunction with invasive monitoring. PetCO2 is a valuable parameter used to estimate ventilation with or without an artificial airway. In order to accurately reflect CO2, a sensor must capture a minimum tidal volume, and in cases where small tidal volumes exist, PetCO2 may read falsely low. In addition, it is subject to some error in the face of uncuffed endotracheal tubes or leaks. PetCO2 is routinely used to monitor and trend CO2 and changes in ventilation during noninvasive or invasive ventilation in ALI.11
The PaCO2/ETCO2 Gradient (a-ET) PCO2 Difference
The combined use of invasive PaCO2 measured from an arterial blood gas and the PetCO2 measured by capnography has guided clinicians in monitoring progression of respiratory failure and recovery related to ALI.11 This gradient implies that there is some degree of ventilation and perfusion mismatch. A change in the (a-ET) PCO2 gradient may be associated with the presence of increased dead space. An increased dead space to tidal volume ratio or Vd/Vt (that reflects the degree of dead space present in the airway) may account for an increase in the (a-ET) PCO2 gradient. The (a-ET) PCO2 difference may be used to optimize PEEP in ALI. An increase in oxygenation related to PEEP is associated with a reduction in the (a-ET) PCO2 gradient.12 Changes can be easily monitored in ALI during lung protective strategies for monitoring the prevention of overdistention and atelectrauma.
Volumetric CO2 (VCO2) integrates CO2 and flow in a single or breath by breath measurement. VCO2 is utilized to monitor CO2 elimination, alveolar ventilation, dead space measurements, and pulmonary capillary blood flow.11 In all patients, VCO2 has been utilized to determine cardiovascular and metabolic status during ALI via measurement at the airway. When managing ALI, VCO2 is useful in determining the effects of mechanical ventilation on CO2, weaning from support, and optimizing treatment strategies by assessing gas exchange.13 Early recognition of changes in VCO2 may assist in decisions related to changes in vasopressor or ventilation support.
Transcutaneous CO2 Monitoring
Transcutaneous CO2 monitoring (PtcCO2) provides a good alternative to PetCO2 as an approximation of carbon dioxide measured indirectly on the skin surface.14 Its best attribute involves trending ventilation by measuring tcCO2 values over time when adequate skin perfusion exists. PetCO2 has been successful in monitoring infants or pediatric patients receiving high frequency mechanical ventilation.15 Advances in the technology have improved its reliability, reduced the incidence of thermal injury, and resulted in more frequent utilization. In the absence of PetCO2 monitoring in infants or in conditions where tidal volumes are very low, PtcCO2 monitoring can function as a reasonable option for assessing ventilation.
Pulmonary mechanics measurements are important for optimizing mechanical ventilation parameters to prevent ventilator-induced lung injury resulting from ALI. Pulmonary graphics depict real-time representation of hyperinflation, gas trapping, and optimal PEEP when using lung protective strategies in infants or pediatric patients.13 The pressure volume loop helps determine the degree of airway resistance, adequate level of PEEP, and overdistention. Flow waveforms are used to depict the presence of trapped gas on expiration.13 In larger patients, the intrapulmonary gas volume or FRC measurements have been introduced at the bedside for stabilizing and optimizing PEEP. However, using this technique is limited by the patient’s weight and the type of device used.16 Assessment of mean airway pressure, plateau pressure (to be kept <30 cm H2O), and pulmonary compliance is important in monitoring lung mechanics, as abnormal values are associated with the degree of risk of ALI morbidity and mortality.17
Acute lung injury affecting infants and children presents challenges for the multidisciplinary team. The goal is to prevent long-term complications associated with its treatment. Effective monitoring during ALI relies heavily on attention to detail during both the acute and subacute phases of the disease process. Expert management, monitoring, and response to lung protective measures inherently affect the outcomes.
Kathleen Deakins, MSHA, RRT-NPS, FAARC, is supervisor, pediatric respiratory care, Rainbow Babies & Children’s Hospital, Cleveland. For further information, contact [email protected]
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