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Physiologic Monitoring

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INTRODUCTION

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Children with serious illness or injury with the potential for rapid deterioration invariably require close observation to detect changes in function or state. Careful bedside assessment provides the basis for appropriate triage and early awareness of clinical deterioration. Electronic monitoring complements this assessment by providing (1) repetitive or continuous assessment that does not disturb the patient, (2) a means for detecting the effectiveness of interventions, and (3) warning signals for physiological disturbances that permit staff to observe multiple patients simultaneously. The ideal monitoring system has been described by many as one that is accurate, able to predict deterioration, reproducible with a rapid response, operator independent, easy to use, safe, continuous, inexpensive, and integrative with other medical systems. To date, no such monitoring currently exists in clinical use.

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RESPIRATORY MONITORING

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Respiratory Rate

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The respiratory rate (RR) and tidal volume (VT; volume of each breath) together determine minute ventilation (V̇E; volume of air movement into and out of the lungs in 1 minute). Minute ventilation is product of RR × VT. Since arterial blood carbon dioxide levels (PaCO2) are inversely proportional to V̇E, decreases in RR and/or VT will lead to increases in PaCO2. Therefore, determination of RR is an essential component for the evaluation of respiratory function.

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There are several methods for determining RR, ranging from counting visual chest movement and auscultated breaths to using devices that measure changes in electrical bioimpedance, air movement at the nares, or inductance plethysmography. These methods each have advantages and limitations but none measure carbon dioxide (CO2) levels. Auscultation provides the best estimation of RR and depth of breathing. It provides the practitioner the opportunity to listen for air movement within the lungs and to differentiate between problems related to airway obstruction (upper vs lower) and problems with poor lung/chest wall compliance. The drawback is that auscultation is time consuming and a static measure.

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Transthoracic impedance monitoring is the technique most commonly used to continuously measure respiratory rate. A small current is passed between 2 electrodes across the chest (similar to those used for electrocardiography). During inhalation and chest rise, the electrodes move further apart, increasing the impedance between them. During exhalation, the electrodes move closer together and the impedance decreases. The sinusoidal changes in impedance are displayed as the RR. Transthoracic impedance monitoring is safe, relatively cheap, and readily available to all patients. It is important, however, to recognize its limitations. First and foremost, the interpreted signal is dependent on chest movement and not breathing, per se. Therefore, chest movement unrelated to breathing may be mistaken for breathing, overestimating RR. Next, chest movement may negate impedance measurements and also may result in the underestimation of RR. Finally, transthoracic impedance monitoring does not accurately measure the depth of breathing ...

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