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
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
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 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.
General Problems During Monitoring
• 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
