21 Bioinstrumentation
Principles and Techniques
Pearls
• Monitoring and support systems provide valuable information and assistance in managing patient care, but the focus should always be on the patient and family as a first priority.
• Assess the patient’s clinical condition before troubleshooting instruments.
• There are many physical characteristics of children that influence the development, use, size, and accuracy of biomedical instruments.
• No single measurement will be as valuable as the evaluation of trends in measurements over time.
• Not all instruments are necessarily valuable or precise.
• ECG waveforms indicate myocardial electrical activity and not the effectiveness of myocardial mechanical function.
• Respiratory monitoring is an invaluable resource to detect cardiopulmonary insufficiency and allow intervention before the development of cardiopulmonary failure.
• The use of mechanical ventilation does not ensure that the child’s ventilation is effective. The child’s clinical condition is the ultimate indicator of the effectiveness of mechanical ventilation.
• Multimodal monitoring, incorporating several neurologic monitoring parameters, may yield valuable information about the patient’s overall clinical status.
Overview of pediatric bioinstrumentation
Bioinstrumentation in the pediatric critical care unit continues to increase in sophistication, paralleling the rapid growth and increasing complexity of pediatric critical care. Currently, a wide variety of instrumentation is available, and can be tailored to the needs of infants and children. The equipment discussed in this chapter is categorized according to the body system for which it is used.
This chapter describes the principles of bioinstrumentation, necessary equipment, specific uses, hazards of devices employed in the care of critically ill children, and troubleshooting techniques. As a first priority, the nurse should always assess the patient’s clinical condition before proceeding to troubleshoot the instruments.
Instrumentation may be used to monitor, measure, or support a patient. Monitoring devices can measure patient physiologic parameters and can give warning or advise the clinician of the status of the parameters. Measuring devices regulate components that are administered to the patient, such as intravenous fluids. Patient support systems, such as ventilators, may both monitor and measure while they provide vital support.
For the sake of brevity, types of equipment described in this chapter are called monitoring devices. These devices may be subdivided further into two types: those considered invasive, which break the normal physiologic barriers (e.g., the skin) and those considered noninvasive, which do not break the physiologic barriers and, in some instances, do not even touch the patient.
Not all instruments are necessarily valuable or precise. The value of each device is affected by the skill of the clinicians and biomedical engineers working with the equipment. Karselis87 maintains that the primary purpose of an instrument is to “extend the range and/or sensitivities of man’s faculties” and, as such, should do so “with speed, reproducibility, reliability, and cost effectiveness.”
Characteristics of Children that Affect Bioinstrumentation
Although the characteristics of the child that can affect selection and reliability of bioinstrumentation vary with the type of device, there are several key characteristics that must be considered during bioinstrumentation. Key characteristics include the child’s body size, cardiovascular structure and function, pulmonary anatomy and physiology, neurologic development and function, metabolic rate, fluid requirements, and immunologic immaturity.
Body Size
Children are obviously smaller than adults. However, children have increased total body surface area in proportion to body mass, which causes higher heat and fluid loss per kilogram body weight than in the adult. Devices requiring exposure of the child to ambient air (e.g., overbed warmers) can increase fluid or heat loss. Compared with adults, infants and children have less absolute surface area available for application of skin electrodes or other contact devices, and this small available surface influences the design and use of some instruments.
Cardiovascular Structure and Function
Several features of the child’s cardiovascular system influence the use of biomedical devices:
• Children have small arteries and veins for cannulation. Multiple lumen catheters in smaller sizes may be difficult to keep patent, influencing the choice of catheter and the nursing care required.
• The resting heart rate of the infant and small child is higher than that of the adult. As a result, when tachycardia develops the heart rate may be extremely high, making it difficult to evaluate the cardiac rhythm. A flexible but sensitive cardiac monitor is required that will accurately document changes in the child’s heart rate or rhythm, and avoid introduction of errors from movement and other artifacts.
• The major compensatory mechanism for the child with cardiovascular dysfunction is tachycardia. The relatively small stroke volume of the young child is generally near the maximum that can be achieved by the child’s small ventricle, so although stroke volume can increase slightly during periods of stress, the major mechanism for increasing cardiac output is an increase in heart rate. If the heart rate falls below normal, cardiac output typically falls. Thus, the child’s cardiac output is much more dependent on heart rate than on stroke volume.
• Pulmonary artery wedge pressure may be elevated artificially by the child’s rapid heart rate.
• The child has a relatively low systolic, diastolic, and mean arterial blood pressure compared with the adult; these low pressures must be measured accurately by monitoring devices. Quantitatively small changes in blood pressure can indicate qualitatively significant changes in the child’s cardiovascular function.
• The technique used to determine cardiac output (CO) by thermodilution may be influenced by the child’s limited tolerance of excessive fluid administration. Thermodilution CO calculations are performed in the adult using several 5-10 cc injections that will likely exceed a child’s fluid requirements. Therefore the CO computer must be capable of calculating CO based on smaller injectate volumes with an acceptable potential error.
Pulmonary Anatomy and Physiology
The entire respiratory tract of an infant or small child is smaller than that of the adult. In addition, immaturity of components of the respiratory system will affect monitors and supportive devices. Specific considerations include:
• The upper airway of the child is smaller than that of the adult. Therefore, endotracheal and tracheostomy tubes used for children must be smaller. In the past, the tubes used in children were uncuffed. In recent years, evidence has accumulated that cuffed tubes may be as safe as uncuffed tubes and use of cuffed tubes in the operating suite allows more accurate selection of tube size.92
• The small tidal volumes, rapid respiratory rates, and short inspiratory times of children require mechanical ventilators that are able to deliver small tidal volumes accurately in short inspiratory times and at low pressures.
• The major compensatory mechanism for the child with respiratory distress is tachypnea. Respiratory monitoring equipment must be capable of accurately measuring rapid respiratory rates, even when tidal volume is low and chest movement is minimal.
• The infant’s small airway closing pressure may be greater than atmospheric pressure, so there may be an increased tendency for alveolar collapse unless continuous positive airway pressure (CPAP) or positive end-expiratory pressure (PEEP) can be provided by pediatric respiratory assist devices.
• The infant’s chest wall is very compliant and provides little resistance to expansion during positive pressure ventilation, so visible chest rise should be observed during positive pressure ventilation. However, inadvertent hyperventilation and barotrauma may occur during hand or mechanical ventilation if excessive volume or pressure is provided.
• Although effective inspiratory inflation pressures are the same in normal patients of all ages, the force required to generate inspiratory pressure decreases with decreasing patient age because the lungs are small. Therefore, unless the inspiratory pressure is monitored closely, pneumothorax may be produced during hand ventilation or ventilation with high inspiratory pressure.
• Respiratory work normally accounts for 2% to 6% of the child’s total oxygen consumption. When the infant develops respiratory distress, this requirement may be as high as 25% to 30% of the total oxygen consumption.
• A rapid respiratory rate produces increased heat and water loss through the respiratory system. Therefore all respiratory assist devices must heat and humidify inspired air.
Neurologic Development and Function
The infant’s skull is thin and fontanelles are present. These characteristics influence the types of intracranial pressure monitoring devices used during infancy.
The critically ill child is typically frightened and uncooperative and is often unable to understand the reasons for monitoring or therapy. Movement artifact can cause interruption or distortion of instrument function or measurements; therefore monitoring devices must be able to differentiate between movement artifact and abnormalities in the child’s clinical status.
Metabolic Rate
The child has a higher metabolic rate than the adult and therefore requires greater daily caloric intake per kilogram of body weight. Numerically small changes in the child’s caloric intake may create significant changes in nutritional status, including the ability to heal.
Rapid heat loss or inadequate cardiac output, especially in the infant or young child may result in temperature instability. Constant temperature monitoring is required. The infant’s oxygen consumption may increase significantly with changes in the ambient temperature.
Fluid Requirements
Infants and small children have a greater proportion of body weight as total body water and extracellular water than older children or adults, yet their absolute fluid requirements are small. Careful attention always must be given to the amounts and types of oral and parenteral fluids that the child receives.
Devices used to regulate the child’s IV fluid administration rate and measure urine output must be calibrated in small units. Infusion pumps must be factory tested and clinically evaluated to ensure that they are able to accurately deliver specific fluid volumes and include appropriate alarms.
Immunologic Immaturity
The risk of healthcare-acquired infection is increased among critically ill children when invasive monitoring is performed. Invasive catheters should be used only when clearly indicated and they should be removed as soon as possible. Each child requires close observation for evidence of infection.
General Problems During Monitoring
Although complications and recommendations for each major equipment category are listed at the end of each major section, the following comments are relevant to all types of monitoring equipment. There are three common challenges in critical care units when electrical or mechanical equipment is used:
• Specialized knowledge is required. The nurse must understand the principles and components of each piece of equipment used to understand its usefulness and hazards. If the nurse is unfamiliar with any equipment, the nurse must immediately obtain instructions in use and troubleshooting of the device.
Equipment is continuously being improved and upgraded and, consequently, this impacts interactive software and electronic medical record systems as well. The continuous improvements provide a challenge for ensuring that all staff are competent in the use of the current models of devices.
• Many alarm systems are in use simultaneously. The number of monitoring devices in use and subsequently the number of audible alarms sounding in the critical care unit are increasing dramatically. The sheer numbers of alarms may result in “alarm fatigue,”65 in which clinicians may disable, silence, or ignore alarms when the number, variety, and sounds of the alarms become overwhelming. In addition, alarms may be turned off or malfunction without the knowledge of the bedside nurse. This may allow a problem to progress to a critical state before detection.
If an alarm fails, the nurse is responsible for the consequences that may follow. All alarm settings and functions must be checked at least at the beginning and end of every shift and whenever vital signs are evaluated.
• Equipment introduces risk of infection. Contamination is more likely when invasive equipment is used, but it can also occur with noninvasive equipment.
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