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Gauge trans-thoracic impedance for patient monitor

25 Feb 2013  | Catherine Redmond

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Patient monitors are used to measure and display the various vital signs of the connected patient. The main signature of interest is the patient's electrocardiogram (ECG) signal, but other parameters of interest and importance include temperature, blood pressure, and respiration rate. This article describes the nature of the respiration measurement based on thoracic impedance.

The respiratory system supplies our blood with adequate oxygen through breathing. All cells in the body require oxygen to survive, grow, and turn food into energy. When we breathe, we inhale oxygen and exhale carbon dioxide and water vapour as by-products of cellular respiration. Breathing is mostly an involuntary—and usually effortless—process under control of the autonomic nervous system, which instigates contraction and relaxation of the diaphragm and the muscles around the lungs.

This contraction and relaxation produces a rhythmic respiratory rate and pattern. Relaxed respiration is constant, interspersed with the occasional yawn or sigh. At rest, only the inspiration muscles are in use, with exhalation usually being a passive process as the lungs rebound after the inhalation stretch.[1]

Normal respiration depends on a variety of factors, such as age, fitness level, and stress level; and is generally at a constant rate and volume. The breath intake for a newborn could be in the range of 30 to 60 breaths per minute, whereas a normal respiration rate for an adult might be around 12 to 20 breaths in that same 1 minute timeframe, increasing with stress, illness, and activity level. More relaxed individuals using breathing techniques or in a meditative state might achieve rates as low as a mere 3 to 5 breaths per minute.

Table: Range of respiration rates 2.

In hospital environments, physiological observations of pulse, blood pressure, temperature, respiration, and consciousness levels give physicians and nurses timely information related to patient health. Of these parameters, respiration rate, a critical vital sign that provides important information about patient distress or respiratory issues, is sometimes underutilised[3],[4]. An abnormal respiratory rate (in excess of that shown in the table), changes in respiratory rhythm, or more effortful breathing may indicate some physiological instability, and could help to identify patients at risk of cardiac issues such as CHF (Chronic Heart Failure) [5].

The key to determining the patient's respiration rate is to measure the changing impedance of the thoracic cavity, which varies with each inhalation and exhalation. The impedance increases as the patient inhales and decreases as he exhales. A circuit designed to detect this impedance variation—based on impedance pneumography—delivers a high-frequency differential current driven onto the patient through a pair of electrodes. The impedance variation caused by breathing results in a corresponding voltage change that can be measured on the same electrodes (2-wire respiration measurement), or on a different pair of electrodes (4-wire respiration measurement).

Achieving the optimum respiration measurement can greatly depend on patient position. For example, if the patient is sleeping or in a lying position, breathing tends to be in the abdomen area, so, Leads II or III might provide the best 2-wire measurement. Alternatively, if the patient is in an upright position, a better signal might be available on the Lead I electrode pair. In addition, stress tends to make us breathe only in our upper chest, so Lead II or III might still be the appropriate choice for the calmer individuals among us. A respiration circuit designed to multiplex different pairs of electrodes will ensure full coverage so the best respiration measurement can be captured.

Figure 1: Physical electrode placement for limb leads and lead configuration.

Drive circuit
A typical arrangement consists of a drive-and-measure circuit. The drive portion may be a DDS or a DAC that delivers the two out-of-phase ac-coupled currents at the programmed frequency onto the pair of electrodes. The current is driven onto the patient using series resistors and capacitors. The ac coupling serves to isolate the patient from dc, alleviating any concerns around supplying common-mode voltage to the patient.

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