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How Does a Pulse Oximeter Work

A pulse oximeter consists of a peripheral probe together with a microprocessor unit, displaying a waveform the oxygen

saturation and the pulse rate.

Most oximeters have an audible pulse tone, the pitch of which is proportional to the oxygen saturation which can be useful

when you cannot see the display. The probe is placed on a peripheral part of the body such as a finger, toe, earlobe or the

nose.

Within the probe are two light-emitting diodes (LEDs). One is visible red spectrum and the other is infrared spectrum. The

beams of light pass through the tissues; some light is absorbed by the blood and soft tissue depending on the concentration of

hemoglobin.

Saturation values are averaged out over five to 20 seconds. The pulse rate is also calculated from the number of LED cycles

between successive pulsatile signals and average out over a similar variable period of time, depending on the particularmonitor.

Calibration and Performance

Oximeters are calibrated during manufacture and automatically check their internal circuits when they are turned on. They are

accurate in the range of oxygen saturations of 70 percent-100 percent (+/- 2 percent), but less accurate under 70 percent. The

pitch of the audible pulse signal falls with reducing values of saturation.

The size of the pulse wave is displayed graphically. Some models automatically increase the gain of the display when the flow

decreases; in these, the display may prove misleading. The alarms usually respond to a slow or fast pulse rate or an oxygen

saturation below 90 percent. At this level, there is a marked fall in PaO2, representing serious hypoxia.

Limitations/Pitfalls

Pulse oximetry is a measure solely of oxygenation, not of ventilation, and is not a substitute for blood gases checked in the

laboratory. Pulse oximetry does not give an indication of carbon dioxide levels, blood pH, or sodium bicarbonate levels.False low readings may be caused by hypoperfusion of the extremity being used for monitoring (often because the part is cold

or because of the vasoconstriction secondary to the use of vasopressor agents); incorrect sensor application; highly calloused

skin; and movement (such as shivering), especially during hypoperfusion. Ambient light, abnormal hemoglobin; pulse rate and

rhythm and cardiac function can also cause abnormal pulse oximeter reading. To ensure accuracy, the sensor should return a

steady pulse and/or waveform. Falsely high or falsely low readings will occur when the hemoglobin is bound to something

other than oxygen. In cases of carbon monoxide poisoning, the falsely high reading may delay the recognition of hypoxemia.

Cyanide poisoning can also give a false high reading.

What factors cause errors in the pulse oximeter?

Abnormal hemoglobin -- Blood may contain abnormal hemoglobin such as carboxyhemogolobins and methemoglobin, which

do not contribute to oxygen delivery.

Medical dyes -- If dyes such as cardio green, intravascular dyes and indocyanine green have been injected into the blood, they

may influence the level of transmission of the red and infrared light.

Manicure and pedicure -- If the users wear nail polish, it may absorb the light emitted from the LED and change the light

transmitted through the body, influencing the values calculated.

Major body motion -- Body motion may cause noise that affects the values calculated. When noise -- including that caused by

body motion -- is reduced, the reliability of the values calculated falls, and the pulse oximeter displays a warning.

Blood flow blocked by pressure on the arms or fingers -- The pulse oximeter measures oxygen saturation based on changes in

the blood flow. Therefore, if the blood is blocked, correct measurement becomes impossible. In addition, if the fingers are

flexed at a uniform place, the pulse oximeter may interpret the pressure changes in pulse rate, causing errors.

Peripheral circulatory failures -- The pulse oximeter utilizes blood flow to monitor changes in the amount of light transmitted

to calculated values. If peripheral blood flow is reduced, adequate data may not be obtained, and the result is inaccurate

measurement. In this case, it is necessary to promote blood flow by massaging or warming the fingers.

Excessive ambient light -- The pulse oximeter can usually cancel out the effects of ambient light. However, if the ambient lightis too strong, the device will not be able to cancel out the effects and this may cause errors.

Ambient electromagnetic waves -- If electric appliances such as television, mobile telephones, or medical devices that produce

high levels of electromagnetic waves are used near the pulse oximeter, the electromagnetic waves from these devices may

interfere with accurate measurement.

Probe attached incorrectly -- If the probe is not attached properly, it may detect a variety of noise, resulting in inaccurate

measurement.

What exactly does the pulse oximeter measure?

Blood carries oxygen in two forms. The majority is bound to hemoglobin (oxyhemoglobin), and the rest is dissolved in the

aqueous phase of blood (the plasma). The pulse oximeter measures the saturation of hemoglobin with oxygen. This is

expressed as a percent saturation. Each gram of normal hemoglobin can hold 1.34 milliliters of oxygen. The dissolved fraction

is dependent upon the partial pressure of oxygen. As the partial pressure increases, the dissolved fraction of oxygen increases.For each 1 mm/Hg pressure of oxygen partial pressure, 0.003 milliliters dissolves in the plasma. So under normal conditions,

each 100 mL of blood contains about 20 mL of oxygen bound to hemoglobin and about 0.3 mL dissolved in plasma. The

dissolved fraction is available to tissues first, and then the fraction bound to hemoglobin. So as tissues metabolize oxygen, or if

oxygen becomes difficult to pink up through the lungs, the dissolved oxygen and the hemoglobin-bound oxygen will eventually

become depleted.

The dissolved oxygen can be measured by arterial blood gas analysis, but this is not yet practical for field application. This

fraction is not measured by pulse oximetry. The pulse oximeter waits to sense the pulse of capillary blood from the side of the

capillaries, then, using two different wavelengths of light, calculates the percent of oxyhemoglobin from the total hemoglobin

present. If oxygen transfer across the lungs or lung function is compromised and tissues continue to metabolize oxygen, the

percentage of oxyhemoglobin will decrease. This becomes our quantitative indicator of hypoxia.

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When should supplemental oxygen be administered despite a normal pulse ox? Oxygen should be delivered whenever the

underlying problem is suspected to be organ or tissue ischemia. This would include things like stroke, intracerebral bleed, head

injury, altered mental status from any cause, chest pain of suspected cardiac origin, cardiac dysrhythmias, shortness of breath

from any cause, vascular emergencies like aortic dissection or aneurysm or vascular occlusions, shock from any cause, sickle

cell disease and multisystem trauma.

What respiratory function will pulse ox not measure? The pulse oximeter measures oxygenation. It does not measure

ventilation. Ventilation is the process of removing carbon dioxide from the blood. Hypoventilation from any cause will result in

the accumulation of carbon dioxide in the blood. This leads to respiratory acidosis. If hypoventilation goes on long enough,

blood oxygen will begin to deplete and the pulse oximeter oxygen saturation will begin to decrease. However, short of apnea,

the rate of carbon dioxide accumulation and the development of respiratory acidosis may be greater then the rate of onset of

hypoxia by pulse oximetry. The assessment of adequate ventilation is based on respiratory rate and depth.Should I hold on administering oxygen in order to check a baseline pulse ox? The decision to administer supplemental oxygen

is based on available history and initial examination. If your first impression is that the patient is really sick, then they probably

are. So dont feel compelled to document a baseline pulse ox in a patient you think is in trouble. If the flow of the call results in

an off-oxygen pulse ox reading being available, it certainly can be useful to correlate to symptom severity, but dont hold off

on oxygen waiting to get this value.

Where this might be useful is if you have a patient who is symptomatic and looks OK for the moment, but you would like to get

a better assessment of the severity of the symptoms.

Practical Tips for Successful Use of Pulse Oximetry

Oximetry is not a complete measure of respiratory sufficiency. A patient suffering from hypoventilation (poor gas exchange in

the lungs) given 100 percent oxygen can have excellent blood oxygen levels while still suffering from respiratory acidosis due

to excess carbon dioxide.It is also not a complete measure of circulatory sufficiency. If there is insufficient blood flow or insufficient hemoglobin

(anemia), tissues can suffer hypoxia despite high oxygen saturation in the blood that does arrive.

It is important to remember that pulse oximetry is only one way of monitoring breathing. It is also necessary, as a minimum, to

record respiratory rate, and if pulse oximetry is used, the amount of oxygen the patient is receiving must be recorded. As with

all clinical assessments, you must look at the "whole picture."

The cardiac monitor is a device that shows the electrical and pressure waveforms of the cardiovascular system for

measurement and treatment. Parameters specific to respiratory function can also be measured. Because electrical

connections are made between the cardiac monitor and the patient, it is kept at the patient's bedside.

Purpose

The cardiac monitor continuously displays the cardiac electrocardiogram (EKG) tracing. Additional monitoring components

allow cardiovascular pressures and cardiac output to be monitored and displayed as required for patient diagnosis and

treatment. Oxygen saturation of the arterial blood can also be monitored continuously. Most commonly used in emergency

rooms and critical care areas, bedside monitors can be interconnected to allow for continual observation of several patients

from a central display. Continuous cardiovascular and pulmonary monitoring allows for prompt identification and initiation of

treatment.

Description

The monitor provides a visual display of many patient parameters. It can be set to sound an alarm if any parameter changes

outside of an expected range determined by the physician. Parameters to be monitored may include, but are not limited to,

electrocardiogram, noninvasive blood pressure, intravascular pressures, cardiac output, arterial blood oxygen saturation, and

blood temperature.

Equipment required for continuous cardiac monitoring includes the cardiac monitor, cables, and disposable supplies such aselectrode patches, pressure transducers, a pulmonary artery catheter (Swan-Ganz catheter), and an arterial blood saturation

probe.

Preparation

As the cardiac monitor is most commonly used to monitor electrical activity of the heart, the patient can expect the following

preparations. The sites selected for electrode placement on the skin will be shaved and cleaned causing surface abrasion for

better contact between the skin and electrode. The electrode will have a layer of gel protected by a film, which is removed

prior to placing the electrode to the skin. Electrode patches will be placed near or on the right arm, right leg, left arm, left leg,

and the center left side of the chest. The cable will be connected to the electrode patches for the measurement of a five-lead

electrocardiogram. Additional configurations are referred to as three-lead and 12-lead electrocardiograms. If noninvasive

blood pressure is being measured, a blood pressure cuff will be placed around the patient's arm or leg. The blood pressure cuffwill be set to inflate manually or automatically. If manual inflation is chosen, the cuff will only inflate at the prompting of the

health care provider, after which a blood pressure will be displayed. During automatic operation, the blood pressure cuff will

inflate at timed intervals and the display will update at the end of each measurement.

Disposable pressure transducers require a reference to atmosphere, called zeroing, which is completed before monitoring

patient pressures. This measurement will

occur once the patient is comfortably positioned since the transducer must be level with the measurement point. The pressure

transducer will then be connected to the indwelling catheter. It may be necessary for as many as four or five pressure

transducers to be connected to the patient.

The arterial blood saturation probe will be placed on the finger, toe, ear, or nasal septum of the patient, providing as little

discomfort as possible, while achieving a satisfactory measurement.

Aftercare

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After connecting all equipment, the health care provider will observe the monitor and evaluate the quality of the tracings,

while making size and position adjustments as needed. The provider will confirm that the monitor is detecting each heartbeat

by taking an apical pulse and comparing the pulse to the digital display. The upper and lower alarm limits should be set

according to physician orders, and the alarm activated. A printout may be recorded for the medical record, and labeled with

patient name, room number, date, time, and interpretation of the strip.

Maintenance and replacement of the disposable components may be necessary as frequently as every eight hours, or as

required to maintain proper operation. The arterial saturation probe can be repositioned to suit patient comfort and to obtain

a tracing. All connections will be treated in a gentle manner to avoid disruption of the signal and to avoid injury to the patient.

Normal results

The monitor will provide waveforms and/or numeric values associated with the patient status. These may include, but are notlimited to, heart rate, arterial blood pressure, central venous pressure, pulmonary artery pressure, pulmonary capillary wedge

pressure, left atrial pressure, cardiac output, arterial blood saturation, and blood temperature. Furthermore, these values can

be used to calculate other values, or parameters, or used to diagnose and treat the patient's condition.

Patient movement may cause measurement errors; the patient will be requested to remain motionless. Depending on the

mobility of the patient, assistance should be provided by the health care provider prior to changing from a laying down

position to sitting or standing.

As the patient's condition improves, the amount of monitoring equipment may be decreased. However, the electrocardiogram

and arterial blood saturation probe should be expect to remain attached until discharge is imminent.