Selecting Appropriate Ventilator Parameters

RCPs at Rainbow Babies and Children’s Hospital are required to have a thorough understanding of theory, operation, and application of ventilator mechanics when selecting appropriate parameters

Patients often require mechanical ventilation in disease states characterized by impaired gas exchange, ventilation-perfusion mismatching, reduced alveolar ventilation, increased work of breathing, and poor oxygenation.¹ The ability to achieve the goals of mechanical ventilation without causing adverse cardiovascular effects and lung tissue damage is essential. Past work² has documented the efficiency and success of the RCP in implementing and using disease-specific protocols. The ability of RCPs working with pediatric and neonatal populations to select mechanical ventilator parameters such as mode, frequency, inspiratory time, positive end-expiratory pressure (PEEP), pressure support, fraction of inspired oxygen (Fio₂), and tidal volume, as well as to achieve target blood gas values, has not been described.

Rainbow Babies and Children’s Hospital, Cleveland, is a division of University Hospitals of Cleveland and is a 244-bed facility. The pediatric respiratory care staff at the hospital has played an important role in the evolution, implementation, and management of mechanical ventilation of infants and pediatric patients for many years. RCPs strive to promote strong relationships with physicians, nurses, and other health care personnel by exhibiting excellence in pediatric and neonatal mechanical ventilation. RCPs are involved in the selection of appropriate ventilation parameters, which requires them to hold a fundamental understanding of the theory, operation, and application of ventilator mechanics while considering the physiologic differences between the mechanics of ventilation of infants and children and of adults.

Initiating Mechanical Ventilation
When patients are admitted to the neonatal intensive care unit (NICU) or pediatric intensive care unit (PICU), the role of the RCP is to choose the appropriate ventilator based on background clinical information and potential ventilatory needs. Generally, pressure, volume, or flow controllers are used for pediatric patients. With advanced technology and monitoring capabilities, changes in compliance and resistance can be assessed using these ventilators or using separate monitors. In neonatal mechanical ventilation, pressure or flow controllers continue to be the ventilators of choice at Rainbow Babies and Children’s Hospital. During time-cycled, pressure-limited ventilation, the clinician must be cognizant of changes that affect gas exchange, such as alterations in respiratory system compliance or resistance.

Initial mechanical-ventilator settings are established based on the patient’s age, weight, severity of disease, and level of consciousness. Modes of ventilation are chosen based on the patient’s diagnosis and clinical condition. For neonatal mechanical ventilation, intermittent mandatory ventilation and synchronous intermittent mandatory ventilation (SIMV) are the standard modes of ventilation used at this institution. For patients with normal lungs (for example, postsurgical patients or those presenting with apnea of prematurity refractory to continuous positive airway pressure), peak inspiratory pressure (PIP) is normally set at 10 to 14 cm H₂O with a PEEP of 3 to 4 cm H₂O. Frequency will be set at 15 to 20 breaths per minute, with an inspiratory time of 0.5 seconds. An Fio₂ of 21% to 30% is frequently used.

For patients with respiratory distress syndrome (RDS), ventilator parameters will vary based on the severity of RDS. Generally, a PIP of 18 to 25 cm H₂O and a PEEP of 4 to 6 cm H₂O will be used. Frequencies of 25 to 40 breaths per minute with inspiratory times of 0.4 to 0.5 seconds are used in RDS where areas of decreased compliance and short time constants (resistance multiplied by compliance) exist. Maintaining oxygen saturation as determined using pulse oximetry (Spo₂) or alveolar oxygen tension (Pao₂) in a specified range often requires an Fio₂ of 40% to 100%.

When initiating high-frequency oscillatory ventilation (HFOV), an Fio₂ of 100% and inspiratory time of 33%, and a mean airway pressure (MAP) 2 to 6 cm H2O above the conventional ventilator MAP, are preferred. Pressure amplitude is used to achieve an adequate chest wiggle. Chest radiographs are used to assess lung volumes. Bias flows of 15 to 20 L/min are selected. The chosen frequency depends on the disease process, but is generally 10 to 15 Hz for infants.

In pediatric patients using volume or flow controllers, SIMV volume control with pressure support is commonly selected, with target tidal volumes of 6 to 10 mL/kg, frequencies of 12 to 25 breaths per minute, pressure support of a minimum of 5 cm H2O, PEEP of 4 cm H2O (increased as necessitated by the condition), and an Fio₂ of 25% to 100%. Pressure controllers are also used for pediatric patients in SIMV pressure control modes at the same baseline frequencies and targeted tidal volumes, along with a minimum PEEP of 5 cm H2O (increased as needed) and inspiratory times determined according to the lung disease present.

After selection of ventilator parameters, the physician or NICU nurse practitioner is informed of the selected parameters and makes recommendations as necessary. Arterial, venous, or capillary blood-gas analysis is used, as a standard, to assess gas exchange and oxygenation and to evaluate therapeutic responses to changes in ventilator parameters.³ Noninvasive respiratory monitoring such as pulse oximetry, end-tidal carbon dioxide monitoring (for PICU patients), and pulmonary mechanics monitoring (including pressure, flow, and volume waveforms) is used on a routine basis to assess oxygenation, ventilation, and patient-ventilator interaction.

Study Methods
The purpose of a study undertaken at Rainbow Babies and Children’s Hospital was to assess the ability of RCPs to initiate mechanical ventilation by selecting appropriate ventilator parameters and achieving adequate gas exchange, as confirmed by blood-gas values, in both neonatal and pediatric populations.

Between January 8 and April 8, 2000, 202 patients used mechanical ventilators in the PICU or NICU at the institution. Demographic data were collected on each patient receiving mechanical ventilation; these data included age, weight, diagnosis, mode of ventilation, initial ventilator settings, and blood-gas–analysis results following the initiation of mechanical ventilation. Data were recorded on a document created for this study, the Mechanical Ventilation Initiation Data Sheet. Blood-gas values were used to assess the adequacy of ventilation (pH/Pco₂) and oxygenation (Spo₂/Po₂) following the initiation of mechanical ventilation. The goal of ventilation was to achieve blood gas values in the normal range for each patient. In determining that normal range, respiratory acidosis was defined as pH <7.35 for PICU patients and as pH <7.25 for NICU patients. Respiratory alkalosis was defined as pH >7.45 and Pco₂ <35 mm Hg. Hypoxic status was defined as Pao₂ <55 for NICU patients and as Spo₂ <93% or mixed venous oxygen saturation <70% for PICU patients.

Results
Study data were first analyzed for outcomes based on mode of ventilation: volume control, pressure control, or HFOV. Successful initiation of ventilation was achieved best in the conventional modes of volume control and pressure control. Patients using the volume-control mode exhibited the best oxygenation status, compared with patients using other modes. In the second analysis, data from neonatal (25-40 weeks) and pediatric (newborn to 17 years) populations were compared. The age of patients receiving mechanical ventilation exerted little influence on the ability to achieve gas exchange by meeting target blood-gas values after the selection of appropriate ventilator parameters and the initiation of ventilation.

Conclusion
Evaluation of RCP selection of appropriate ventilator parameters was necessary to assess the value of RCPs and their role in the management of mechanical ventilation. The ability to achieve target results based on selection of appropriate ventilator parameters in 80% of pediatric and neonatal patients is valuable in clinical practice. Differences seen between conventional and high-frequency modes of ventilation in attaining target results (80% were in normal range in conventional ventilation and 60% were in normal range for HFOV for acid-base status) are most likely to be due either to an unbalanced number of patients (the majority of patients received conventional modes of ventilation) or to the use of HFOV as a late rescue method in some cases.

The results of this study suggest that utilizing the RCP’s abilities and expertise in mechanical ventilation has been beneficial in physician-directed management of the mechanical ventilation of infants and children. This traditional approach to choosing appropriate mechanical-ventilation parameters continues to be successful in achieving the basic goals of gas exchange. With economic pressures influencing mechanical ventilation practices, urging physicians and other clinicians to increase the effectiveness of their management, protocols, guidelines, and fast tracks have made their way into clinical practice, streamlining mechanical ventilation management.⁴ Comparisons of physician-directed and protocol-driven mechanical ventilation management are appearing after more than a decade of protocol implementation.⁵

The RCP’s role in documented, traditional management reflects superior clinical abilities that are a prerequisite for the proper utilization of protocols. Mechanical-ventilation management has evolved toward the protocol-driven arena due to the benefits of cost-effectiveness, earlier extubations, reduced ICU days, and shorter overall lengths of hospital stay.⁶ Selection of appropriate ventilation parameters requires the same expertise, knowledge, and foresight for successful management of mechanically ventilated patients in physician-driven or protocol-driven mechanical ventilation. The future of RCP involvement in the selection of appropriate mechanical ventilator settings and in the management of mechanical ventilation appears to be strong.

Kathleen M. Deakins, RRT, is a lead pediatric RT for pediatric respiratory care; and Timothy Myers, RRT, is administrative manager for pediatric respiratory care both at Rainbow Babies and Children’s Hospital, Cleveland.

References
1. Chatburn RL. Principles and practice of neonatal and pediatric mechanical ventilation. Respiratory Care. 1991;36:569-595.
2. Wood G, MacLeod B, Moffat S. Weaning from mechanical ventilation: physician directed vs respiratory protocol. Intensive Care Med. 1996;22:567-575.
3. AARC Clinical Practice Guidelines: sampling for arterial blood gas analysis. Respiratory Care. 1994;39:1180-1183.
4. Kollef MH, Shapiro SD, Clinkscale D, et al. The effect of respiratory therapist initiated treatment protocols on patient outcomes and resource utilization. Chest. 2000;117:467-475.
5. Kollef MH, Shapiro SD, Silver P. A randomized, controlled trial of protocol directed versus physician directed weaning from mechanical ventilation. Crit Care Med. 1997;25:1052-1056.
6. Raake J. Ventilator weaning protocols in the CICU: a pediatric perspective. RT. 2001;14(2):39-40.