[转帖]脉搏氧饱和相关背景知识--英文

Pulse oximeter

A portable saturometer (for emergencies)

Typical measurement through the fingernail

A portable pulse oximeter registering a satisfactory saturation reading

A pulse oximeter is a medical device that indirectly measures the oxygen saturation of a patient's blood (as opposed to measuring oxygen saturation directly through a blood sample) and changes in blood volume in the skin, producing a photoplethysmograph. It is often attached to a medical monitor so staff can see a patient's oxygenation at all times. Most monitors also display the heart rate. Portable, battery-operated pulse oximeters are also available for home blood-oxygen monitoring. The original oximeter was made by Milliken in the 1940s. The precursor to today's modern pulse oximeter was developed in 1972, by Aoyagi at Nihon Kohden using the ratio of red to infrared light absorption of pulsating components at the measuring site. It was commercialized by Biox in 1981. The device did not see wide adoption in the United States until the late 1980s.

Contents   1 Function 2 Advantages 3 Limitations and Advancements 4 Increasing usage 5 See also 6 References 7 External links

 Function

A blood-oxygen monitor displays the percentage of arterial hemoglobin in the oxyhemoglobin configuration. Acceptable normal ranges are from 95 to 100 percent, although values down to 90% are common. For a patient breathing room air, at not far above sea level, an estimate of arterial pO₂ can be made from the blood-oxygen monitor SpO₂ reading.

A pulse oximeter is a particularly convenient noninvasive measurement instrument. Typically it has a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body, usually a fingertip or an earlobe. One LED is red, with wavelength of 660 nm, and the other is infrared, 905, 910, or 940 nm. Absorption at these wavelengths differs significantly between oxyhemoglobin and its deoxygenated form, therefore from the ratio of the absorption of the red and infrared light the oxy/deoxyhemoglobin ratio can be calculated. The absorbance of oxyhemoglobin and deoxyhemoglobin is the same (isosbestic point) for the wavelengths of 590 and 805 nm; earlier oximeters used these wavelengths for correction for hemoglobin concentration.^[1]

The monitored signal bounces in time with the heart beat because the arterial blood vessels expand and contract with each heartbeat. By examining only the varying part of the absorption spectrum (essentially, subtracting minimum absorption from peak absorption), a monitor can ignore other tissues or nail polish^[2] and discern only the absorption caused by arterial blood. Thus, detecting a pulse is essential to the operation of a pulse oximeter and it will not function if there is none.

A recent use has been found for the pulse oximeter, in the area of detecting blood loss. Kirk ­Shelley, a Yale University anesthesiologist, through gathering pulse oximeter data for over seven years, developed an algorithm that turns absorption changes into accurate estimates of blood volume.^[3]

 Advantages

A pulse oximeter is useful in any setting where a patient's oxygenation is unstable, including intensive care, operating, recovery, emergency and hospital ward settings, pilots in unpressurized aircraft, for assessment of any patient's oxygenation, and determining the effectiveness of or need for supplemental oxygen. Assessing a patient's need for oxygen is the most essential element to life; no human life thrives in the absence of oxygen (cellular or gross). Although a pulse oximeter is used to monitor oxygenation, it cannot determine the metabolism of oxygen, or the amount of oxygen being used by a patient. For this purpose, it is necessary to also measure carbon dioxide (CO₂) levels. It is possible that it can also be used to detect abnormalities in ventilation. However, the use of a pulse oximeter to detect hypoventilation is impaired with the use of supplemental oxygen, as it is only when patients breathe room air that abnormalities in respiratory function can be detected reliably with its use. Therefore, the routine administration of supplemental oxygen may be unwarranted if the patient is able to maintain adequate oxygenation in room air, since it can result in hypoventilation going undetected.

Because of their simplicity and speed, pulse oximeters are of critical importance in emergency medicine and are also very useful for patients with respiratory or cardiac problems, especially COPD, or for diagnosis of some sleep disorders such as apnea and hypopnea. Portable, battery operated pulse oximeters are useful for pilots operating in a non-pressurized aircraft above 10,000 feet (12,500 feet in the US)^[4] where supplemental oxygen is required. Prior to the oximeter's invention, many complicated blood tests needed to be performed. Portable pulse oximeters are also useful for mountain climbers and athletes whose oxygen levels may decrease at high altitudes or with exercise. Those using portable pulse oximeters are also making use of blood oxygen charting software. These charting methods provide print outs for the patients physician of blood oxygen and pulse, and reminders to check blood oxygen levels.

 Limitations and Advancements

Oximetry is not a complete measure of respiratory sufficiency. A patient suffering from hypoventilation (poor gas exchange in the lungs) given 100% oxygen can have excellent blood oxygen levels while still suffering from respiratory acidosis due to excessive carbon dioxide.

It is also not a complete measure of circulatory sufficiency. If there is insufficient bloodflow or insufficient hemoglobin in the blood (anemia), tissues can suffer hypoxia despite high oxygen saturation in the blood that does arrive.

A higher level of methemoglobin will tend to cause a pulse oximeter to read closer to 85% regardless of the true level of oxygen saturation. It also should be noted that the inability of two-wavelength saturation level measurement devices to distinguish carboxyhemoglobin due to carbon monoxide poisoning from oxyhemoglobin must be taken into account when diagnosing a patient in emergency rescue, e.g., from a fire in an apartment. A Pulse CO-oximeter measures absorption at additional wavelengths to distinguish CO from O₂ and determines the blood oxygen saturation more reliably. In 2005 Masimo Corporation introduced the first FDA-approved pulse oximeter to monitor carbon monoxide levels noninvasively. Masimo Pulse CO-oximeters can now measure total hemoglobin, oxygen content, methemoglobin and PVI, in addition to carboxyhemoglobin.^[5]

PVI has been shown in initial clinical studies to provide clinicians with a new method for noninvasive and automatic assessment of patient fluid volume status.^[6] Appropriate fluid levels are vital to reducing postoperative risks and improving patient outcomes as fluid volumes that are too low (under hydration) or too high (over hydration) have been shown to decrease wound healing, increase risk of infection and cardiac complications.^[7]

 Increasing usage

According to a report by Frost & Sullivan entitled U.S. Pulse Oximetry Monitoring Equipment Market, US sales of oximeters were worth $201 million in 2006. The report estimated that oximeter sales in the US would increase to $310 million annually by 2013.^[8]

In 2008, more than half of the major internationally-exporting medical equipment manufacturers in China were producers of pulse oximeters.^[9]

In June, 2009, video game company Nintendo announced an upcoming peripheral for the Wii console, dubbed the "Vitality Sensor," which consists of a pulse oximeter. This marks the onset of the use of this device for non-medical, entertainment purposes.^[10][11]

 See also

• Arterial blood gas
• Medical equipment
• Medical monitor
• Pulse oximetry
• Capnography, measuring of carbon dioxide (CO2) in the respiratory gases

 References

1. ^ Pulse Oximetry (a review) Anaesthesia UK 11 Sept 2004.
2. ^ Brand TM, Brand ME, Jay GD. Enamel nail polish does not interfere with pulse oximetry among normoxic volunteers J Clin Monit Comput. 2002 Feb;17(2):93-6.
3. ^ Bullis, Kevin. Detecting blood loss.- A simple finger-clip device is able to monitor blood loss accurately -- without the need for more invasive or expensive procedures Technology Review December 2005.
4. ^ Code of Federal Regulations Federal Aviation Administration
5. ^ Masimo Corporation website
6. ^ Breaking Study: Masimo Pleth Variability Index (PVI) Shown Effective in Noninvasive Detection of Changes in Ventricular Preload and Fluid Volume Bio-Medicine 15 October 2007.
7. ^ Ishii M, Ohno K. Comparisons of body fluid volumes, plasma renin activity, hemodynamics and pressor responsiveness between juvenile and aged patients with essential hypertension. Jpn Circ J 1977 Mar; 41(3):237-46.
8. ^ "Pulse Oximetry Market to Grow 150 Percent by 2013", HomeCareMag.com (Paramus, New Jersey: Penton Media Inc.), 2007-08-20, https://homecaremag.com/news/pulse-oximetry-market/index.html, retrieved on 2009-01-19 
9. ^ "Key Portable Medical Device Vendors Worldwide", China Portable Medical Devices Report (Beijing: ResearchInChina), December 2008 
10. ^ Pigna, Kris (2009-06-02). "Satoru Iwata Announces Wii Vitality Sensor". 1UP.com. https://www.1up.com/do/newsStory?cId=3174528. Retrieved on 2009-06-02. 
11. ^ "Nintendo Introduces New Social Entertainment Experiences at E3 Expo". Nintendo of America. 2009-06-02. https://e3.nintendo.com/pressrelease/. Retrieved on 2009-06-02. 

 External links

• Principles of Pulse Oximetry Technology
• How Pulse Oximetry Works

Pulse oximetry

A modern pulse oximeter that also provides pulse co-oximetry.

Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's hemoglobin.

A sensor is placed on a thin part of the patient's anatomy, usually a fingertip or earlobe, or in the case of a neonate, across a foot, and a light containing both red and infrared wavelengths is passed from one side to the other. Changing absorbance of each of the two wavelengths is measured, allowing determination of the absorbances due to the pulsing arteria blood alone, excluding venous blood, skin, bone, muscle, fat, and (in most cases) fingernail polish.^[1] Based upon the ratio of changing absorbance of the red and infrared light caused by the difference in color between oxygen-bound (bright red) and oxygen unbound (dark red or blue, in severe cases) blood hemoglobin, a measure of oxygenation (the per cent of hemoglobin molecules bound with oxygen molecules) can be made.

Contents   1 Indication 2 History 3 Limitations 4 See also 5 References

 Indication

Pulse oximetry data is necessary whenever a patient's oxygenation is unstable, including intensive care, critical care, and emergency department areas of a hospital. Data can also be obtained from pilots in unpressurized aircraft and for assessment of any patient's oxygenation in primary care.^{[citation needed]} A patient's need for oxygen is the most essential element to life; no human life thrives in the absence of oxygen (cellular or gross). Although pulse oximetry is used to monitor oxygenation, it cannot determine the metabolism of oxygen, or the amount of oxygen being used by a patient. For this purpose, it is necessary to also measure carbon dioxide (CO₂) levels. It is possible that it can also be used to detect abnormalities in ventilation. However, the use of pulse oximetry to detect hypoventilation is impaired with the use of supplemental oxygen, as it is only when patients breathe room air that abnormalities in respiratory function can be detected reliably with its use. Therefore, the routine administration of supplemental oxygen may be unwarranted if the patient is able to maintain adequate oxygenation in room air, since it can result in hypoventilation going undetected.^{[citation needed]}

 History

In 1935 Matthes developed the first 2-wavelength ear O₂ saturation meter with red and green filters, later switched to red and infrared filters. This was the first device to measure O₂ saturation.^{[citation needed]}

In 1949 Wood added a pressure capsule to squeeze blood out of ear to obtain zero setting in an effort to obtain absolute O₂ saturation value when blood was readmitted. The concept is similar to today's conventional pulse oximetry but suffered due to unstable photocells and light sources. This method is not used clinically. In 1964 Shaw assembled the first absolute reading ear oximeter by using eight wavelengths of light. Commercialized by Hewlett Packard, its use was limited to pulmonary functions and sleep laboratories due to cost and size.^{[citation needed]}

Pulse oximetry was developed in 1972, by Takuo Aoyagi, a bioengineer, at Nihon Kohden using the ratio of red to infrared light absorption of pulsating components at the measuring site. Susumu Nakajima, a surgeon, and his associates first tested the device in patients, reporting it in 1975.^[2] It was commercialized by Biox in 1981 and Nellcor in 1983. Biox was founded in 1979, and introduced the first pulse oximeter to commercial distribution in 1981. Biox initially focused on respiratory care, but when the company discovered that their pulse oximeters were being used in operating rooms to monitor oxygen levels, Biox expanded its marketing resources to focus on operating rooms in late 1982. A competitor, Nellcor (now part of Covidien, Ltd.), Incorporated in 1982, and began to compete with Biox for the US operating room market in 1983. Prior to its introduction, a patient's oxygenation was determined by a painful arterial blood gas, a single point measure which typically took a minimum of 20–30 minutes processing by a laboratory. (In the absence of oxygenation, damage to the brain starts in 5 minutes with brain death in another 10–15 minutes). In the US alone, approximately $2 billion was spent annually on this measurement. With the introduction of pulse oximetry, a non-invasive, continuous measure of patient's oxygenation was possible, revolutionizing the practice of anesthesia and greatly improving patient safety. Prior to its introduction, studies in anesthesia journals estimated US patient mortality as a consequence of undetected hypoxemia at 2,000 to 10,000 deaths per year, with no known estimate of patient morbidity.^{[citation needed]}

By 1987, the standard of care for the administration of a general anesthetic in the US included pulse oximetry. From the operating room, the use of pulse oximetry rapidly spread throughout the hospital, first in the recovery room, and then into the various intensive care units. Pulse oximetry was of particular value in the neonatal unit where the patients do not thrive with inadequate oxygenation, but also can be blinded with too much oxygen. Furthermore, obtaining an arterial blood gas from a neonatal patient is extremely difficult.^{[citation needed]}

In 1996, Masimo, a California-based company, introduced the first pulse oximeter able to provide accurate measurements during periods of patient motion or low peripheral perfusion, long thought to be limitations of pulse oximetry technology that could not be overcome. ^[3] The ability to provide accurate measurements under these difficult clinical conditions meant pulse oximetry could be used outside the operating room, where patients were generally well perfused and not moving, allowing for adoption in neonatal intensive care units, ambulances, and other challenging settings.^[4]

By 2008, the accuracy and capability of Pulse Oximetry had continued to increase, and had allowed for the adoption of the term High Resolution Pulse Oximetry (HRPO).^[5][6][7] One area of particular interest in the area of Pulse Oximetry, is the use of Pulse Oximetry in conducting portable and in-home sleep apnea screening and testing.^[5][8]

 Limitations

This is a measure solely of oxygenation, not of ventilation, and is not a substitute for blood gases checked in a laboratory as it gives no indication of base deficit, carbon dioxide levels, blood pH, or bicarbonate HCO3- concentration. The metabolism of oxygen can be readily measured by monitoring expired CO₂. Saturation figures also give no information about blood oxygen content. Most of the oxygen in the blood is carried by hemoglobin. In severe anemia, the blood will carry less total oxygen, despite the hemoglobin being 100% saturated.

Falsely low readings may be caused by hypoperfusion of the extremity being used for monitoring (often due to the part being cold or from vasoconstriction secondary to the use of vasopressor agents); incorrect sensor application; highly calloused skin; and movement (such as shivering), especially during hypoperfusion. To ensure accuracy, the sensor should return a steady pulse and/or pulse waveform. Falsely high or falsely low readings will occur when hemoglobin is bound to something other than oxygen. In cases of carbon monoxide poisoning, the falsely high reading may delay the recognition of hypoxemia (low blood oxygen level). Methemoglobinemia characteristically causes pulse oximetry readings in the mid-80s. Cyanide poisoning can also give a high reading because it reduces oxygen extraction from arterial blood (the reading is not false, as arterial blood oxygen is indeed high in early cyanide poisoning).

Pulse oximetry only reads the percentage of bound hemoglobin. It can be bound to other gasses such as carbon monoxide and still read high even though the patient is hypoxemic. The only noninvasive methodology that allows for the continuous and noninvasive measurement of the dyshemoglobins is a pulse co-oximeter. Pulse CO-Oximetry was invented in 2005 by Masimo and currently allows clinicians to measure total hemoglobin levels in addition to carboxyhemoglobin, methemoglobin and PVI, which initial clinical studies have shown may provide clinicians with a new method for noninvasive and automatic assessment of patient fluid volume status.^[9][10][11] Appropriate fluid levels are vital to reducing postoperative risks and improving patient outcomes as fluid volumes that are too low (under hydration) or too high (over hydration) have been shown to decrease wound healing, increase risk of infection and cardiac complications.^[12]

 See also

• Oxygen sensor
• Oxygen saturation
• Pulse oximeter
• Capnography, measuring of carbon dioxide (CO₂) in the respiratory gases
• Sleep Apnea

 References

1. ^ Brand TM, Brand ME, Jay GD. Enamel nail polish does not interfere with pulse oximetry among normoxic volunteers J Clin Monit Comput. 2002 Feb;17(2):93-6.
2. ^ John W. Severinghaus, MD,and Yoshiyuki Honda, MD: HISTORY OF BLOOD GAS ANALYSIS. VII. PULSE OXlMETRY"History of Pulse Oximetry"
3. ^ Barker SJ, Shah NK (October 1996). "Effects of motion on the performance of pulse oximeters in volunteers" (^{[dead link]}). Anesthesiology 85 (4): 774–81. doi:10.1097/00000542-199610000-00012. PMID 8873547. https://www.anesthesiology.org/pt/re/anes/fulltext.00000542-199610000-00012.htm;jsessionid=LgzLhCRl1xxQ2j5NJZsjccHQy1wGXpGSsSmWsGGDSMxDnfKYhShL!-1123973585!181195628!8091!-1. Retrieved on 2008-08-11. 
4. ^ Giuliano KK, Higgins TL (January 2005). "New-generation pulse oximetry in the care of critically ill patients". Am. J. Crit. Care 14 (1): 26–37; quiz 38–9. PMID 15608106. https://ajcc.aacnjournals.org/cgi/content/full/14/1/26. Retrieved on 2008-08-11. 
5. ^ ^{a b} https://www.sleepreviewmag.com/issues/articles/2008-04_10.asp
6. ^ https://www.maxtecinc.com/assets/docs/pulsox/ml187.p300iDataSheet.pdf
7. ^ https://www.anesthesiology.org/pt/re/anes/fulltext.00000542-200809000-00004.htm
8. ^ https://www.post-gazette.com/pg/08009/847751-114.stm
9. ^ Keller G, Cassar E, Desebbe O, Lehot JJ, Cannesson M (2008). "Ability of pleth variability index to detect hemodynamic changes induced by passive leg raising in spontaneously breathing volunteers". Crit Care 12 (2): R37. doi:10.1186/cc6822. PMID 18325089. 
10. ^ Cannesson M, Delannoy B, Morand A, et al. (April 2008). "Does the Pleth variability index indicate the respiratory-induced variation in the plethysmogram and arterial pressure waveforms?". Anesth. Analg. 106 (4): 1189–94, table of contents. doi:10.1213/ane.0b013e318167ab1f. PMID 18349191. 
11. ^ Cannesson M, Desebbe O, Rosamel P, et al. (August 2008). "Pleth variability index to monitor the respiratory variations in the pulse oximeter plethysmographic waveform amplitude and predict fluid responsiveness in the operating theatre". Br J Anaesth 101 (2): 200–6. doi:10.1093/bja/aen133. PMID 18522935. 
12. ^ Ishii M, Ohno K (March 1977). "Comparisons of body fluid volumes, plasma renin activity, hemodynamics and pressor responsiveness between juvenile and aged patients with essential hypertension". Jpn. Circ. J. 41 (3): 237–46. PMID 870721.

Photoplethysmograph

Representative PPG taken from an ear pulse oximeter. Variation in amplitude are from Respiratory Induced Variation.

A photoplethysmograph (PPG) is an optically obtained plethysmograph, a volumetric measurement of an organ. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption (Shelley and Shelley, 2001). A conventional pulse oximeter monitors the perfusion of blood to the dermis and subcutaneous tissue of the skin.

Diagram of the layers of human skin

With each cardiac cycle the heart pumps blood to the periphery. Even though this pressure pulse is somewhat damped by the time it reaches the skin, it is enough to distend the arteries and arterioles in the subcutaneous tissue. If the pulse oximeter is attached without compressing the skin, a pressure pulse can also be seen from the venous plexus, as a small secondary peak.

The change in volume caused by the pressure pulse is detected by illuminating the skin with the light from a Light Emitting Diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode. Each cardiac cycle appears as a peak, as seen in the figure. Because blood flow to the skin can be modulated by multiple other physiological systems, the PPG can also be used to monitor breathing, hypovolemia, etc. Additionally, the shape of the PPG waveform differs from subject to subject, and varies with the location and manner in which the pulse oximeter is attached.

Contents [hide] [hide] 1 Sites for measuring PPG 2 Uses 2.1 Monitoring Heart Rate and Cardiac Cycle 2.2 Monitoring Respiration 2.3 Monitoring Depth of Anesthesia 2.4 Monitoring Hypo- and Hyper-volemia 3 References 4 External links 5 See also

 Sites for measuring PPG

While pulse oximeters are a commonly used medical device the PPG derived from them is rarely displayed, and is nominally only processed to determine heart rate. PPGs can be obtained from transmissive absorption (as at the finger tip) or reflective (as on the forehead).

In outpatient setting pulse oximeters are commonly worn on the finger and ear. However, in cases of shock, hypothermia, etc. blood flow to the periphery can be reduced, resulting in a PPG without a discernible cardiac pulse. In this case, a PPG can be obtained from a pulse oximeter on the head, with the most common sites being the ear, nasal septum, and forehead.

PPGs can also be obtained from the vagina and esophagus.

 Uses

Premature Ventricular Contraction (PVC) can be seen in the PPG just as in the EKG and the Blood Pressure (BP).

Venous pulsations can be clearly seen in this PPG.

 

 Monitoring Heart Rate and Cardiac Cycle

Because the skin is so richly perfused, it is relatively easy to detect the pulsatile component of the cardiac cycle. The DC component of the signal is attributable to the bulk absorption of the skin tissue, while the AC component is directly attributable to variation in blood volume in the skin caused by the pressure pulse of the cardiac cycle.

The height of AC component of the photoplethysmogram is proportional to the pulse pressure, the difference between the systolic and diastolic pressure in the arteries. As seen in the figure showing Premature Ventricular Contractions (PVCs) the PPG pulse for the cardiac cycle with the PVC results in lower amplitude blood pressure and a PPG. Ventricular Tachycardia and Ventricular Fibrillation can also be detected.

 Monitoring Respiration

The effects of Sodium Nitroprusside(Nipride) a peripheral vasodilator on the finger PPG of a sedated subject. As expected the PPG amplitude increases after infusion and additionally the Respiratory Induced Variation (RIV) becomes enhanced.

Respiration effects the cardiac cycle by varying the intrapleural pressure, the pressure between the thoracic wall and the lungs. Since the heart resides in the thoracic cavity between the lungs, the partial pressure of inhaling and exhaling greatly influence the pressure on the vena cava and the filling of the right atrium. This effect is often referred to as normal sinus arrhythmia.

During inspiration, intrapleural pressure decreases by up to 4 mm Hg which distends the right atrium, allowing for faster filling from the vena cava, increasing ventricular preload, and increasing the stroke volume. Conversely during expiration, the heart is compressed, decreasing cardiac efficiency and reducing stroke volume. However, the overall net effect of respiration is to act as pump for the cardiovascular system. When the frequency and depth of respiration increases, the venous return increase leading to increased cardiac output. (Shelley, et al., 2006)

 Monitoring Depth of Anesthesia

Effects of an incision on a subject under general anesthesia on the photoplethysmograph (PPG) and blood pressure (BP).

Anesthesiologist must often judge subjectively whether a patient is sufficiently anesthetized for surgery. As seen in the figure if a patient is not sufficiently anesthetized the sympathetic nervous system response to an incision can generate an immediate response in the amplitude of the PPG.

 Monitoring Hypo- and Hyper-volemia

Shamir, Eidelman, et al. studied the interaction between inspiration and removal of 10% of a patient’s blood volume for blood banking before surgery (Shamir, Eidelman et al. 1999). They found that blood loss could be detected both from the photoplethysmogram from a pulse oximeter and an arterial catheter. Patients showed a decrease in the cardiac pulse amplitude caused by reduced cardiac preload during exhalation when the heart is being compressed.

 References

1. M. Shamir, L. A. Eidelman, Y. Floman, L. Kaplan, and R. Pi-zov, Pulse Oximetry Plethysmographic Waveform During Changes in Blood Volume, Br. J. Anaesth., vol. 82, pp. 178-181, 1999.
2. K. Shelley and S. Shelley, Pulse Oximeter Waveform: Photoelectric Plethysmography,in Clinical Monitoring, Carol Lake, R. Hines, and C. Blitt, Eds.: W.B. Saunders Company, 2001, pp. 420-428.
3. K. H. Shelley, D. H. Jablonka, A. A. Awad, R. G. Stout, H. Rezkanna, and D. G. Silverman, What Is the Best Site for Measuring the Effect of Ventilation on the Pulse Oximeter Waveform? Anesth Analg, vol. 103, pp. 372-377, 2006.

 External links

A student project: building a device for collecting PPGs

 See also

Hemodynamics

CO-oximeter

A CO-oximeter is a device for detecting hypoxia, a medical condition relating to oxygen deficiency at tissue level.

The device measures absorption at several wavelengths to distinguish oxyhemoglobin from carboxyhemoglobin and determine the oxyhemoglobin saturation: the percentage of oxygenated Hb compared to the total amount of hemoglobin (Hb), including carboxy-Hb, met-Hb, oxy-Hb, and reduced Hb. When a patient presents with carbon monoxide poisoning (CO), the CO-oximeter will detect this Hb and will report the oxyhemoglobin saturation as markedly reduced.

Measurement

Traditionally, this measurement is made from arterial blood processed in a blood gas analyzer with a CO-oximeter.^[1][2] More recently, pulse CO-oximeters have made it possible to estimate carboxyhemoglobin with non-invasive technology similar to a Pulse oximeter^[3] The use of a pulse oximeter is not effective in the diagnosis of CO poisoning as patients suffering from carbon monoxide poisoning may have a normal oxygen saturation reading on a pulse oximeter.^[4]

References

1. ^ Rodkey FL, Hill TA, Pitts LL, Robertson RF (August 1979). "Spectrophotometric measurement of carboxyhemoglobin and methemoglobin in blood". Clinical Chemistry 25 (8): 1388–93. PMID 455674. https://www.clinchem.org/cgi/pmidlookup?view=long&pmid=455674. Retrieved on 2009-07-17. 
2. ^ Rees PJ, Chilvers C, Clark TJ (January 1980). "Evaluation of methods used to estimate inhaled dose of carbon monoxide". Thorax 35 (1): 47–51. PMID 7361284. PMC: 471219. https://thorax.bmj.com/cgi/pmidlookup?view=long&pmid=7361284. Retrieved on 2009-07-17. 
3. ^ Coulange M, Barthelemy A, Hug F, Thierry AL, De Haro L (2008). "Reliability of new pulse CO-oximeter in victims of carbon monoxide poisoning". Undersea & Hyperbaric Medicine 35 (2): 107–11. PMID 18500075. https://archive.rubicon-foundation.org/8084. Retrieved on 2009-07-17. 
4. ^ Vegfors M, Lennmarken C (May 1991). "Carboxyhaemoglobinaemia and pulse oximetry". British Journal of Anaesthesia 66 (5): 625–6. PMID 2031826. https://bja.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=2031826. 

 

Capnography

Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (CO₂) in the respiratory gases. Its main development has been as a monitoring tool for use during anaesthesia and intensive care. It is usually presented as a graph of expiratory CO₂ plotted against time, or, less commonly, but more usefully, expired volume. The plot may also show the inspired CO₂, which is of interest when rebreathing systems are being used.

The capnogram is a direct monitor of the inhaled and exhaled concentration or partial pressure of CO₂, and an indirect monitor of the CO₂ partial pressure in the arterial blood. In healthy individuals, the difference between arterial blood and expired gas CO₂ partial pressures is very small, and is probably zero in children. In the presence of most forms of lung disease, and some forms of congenital heart disease (the cyanotic lesions) the difference between arterial blood and expired gas increases and can exceed 1 kPa.

During anaesthesia, there is interplay between two components: the patient and the anaesthesia administration device (which is usually a breathing circuit and a ventilator or respirator). The critical connection between the two components is either an endotracheal tube or a mask, and CO₂ is typically monitored at this junction. Capnography directly reflects the elimination of CO₂ by the lungs to the anaesthesia device. Indirectly, it reflects the production of CO₂ by tissues and the circulatory transport of CO₂ to the lungs.

When expired CO₂ is related to expired volume rather than time, the area beneath the curve represents the volume of CO₂ in the breath, and thus over the course of a minute, this method can yield the CO₂ minute elimination, an important measure of metabolism. Sudden changes in CO₂ elimination during lung or heart surgery usually imply important changes in cardiorespiratory function.

Capnographs usually work on the principle that CO₂ absorbs infra-red radiation. A beam of infra-red light is passed across the gas sample to fall on to a sensor. The presence of CO₂ in the gas leads to a reduction in the amount of light falling on the sensor, which changes the voltage in a circuit. The analysis is rapid and accurate, but the presence of nitrous oxide in the gas mix changes the infra-red absorption via the phenomenon of collision broadening. This must be corrected for.

Contents   1 Diagnostic usage 2 Use in anaesthesia 3 Capnography in emergency medical services 4 External links

 Diagnostic usage

Capnography provides information about CO₂ production, pulmonary (lung) perfusion, alveolar ventilation, respiratory patterns, and elimination of CO₂ from the anaesthesia breathing circuit and ventilator. The shape of the curve is affected by some forms of lung disease; in general there are obstructive conditions such as bronchitis, emphysema and asthma, in which the mixing of gases within the lung is affected.

Conditions such as pulmonary embolism and congenital heart disease, which affect perfusion of the lung, do not, in themselves, affect the shape of the curve, but greatly affect the relationship between expired CO₂ and arterial blood CO₂. Capnography can also be used to measure carbon dioxide production, a measure of metabolism. Increased CO₂ production is seen during fever and shivering. Reduced production is seen during anaesthesia and hypothermia.

 Use in anaesthesia

Capnography has been shown to be more effective than clinical judgement alone in the early detection of adverse respiratory events such as hypoventilation, oesophageal intubation and circuit disconnection; thus allowing patient injury to be prevented. During procedures done under sedation, capnography provides more useful information, e.g. on the frequency and regularity of ventilation, than pulse oximetry.

Capnography provides a rapid and reliable method to detect life-threatening conditions (malposition of tracheal tubes, unsuspected ventilatory failure, circulatory failure and defective breathing circuits) and to circumvent potentially irreversible patient injury.

Capnography and pulse oximetry together could have helped in the prevention of 93% of avoidable anaesthesia mishaps according to an ASA (American Society of Anesthesiologists) closed claim study.

 Capnography in emergency medical services

Capnography is increasingly being used by paramedics to aid in their assessment and treatment of patients in the prehospital environment. These uses include verifying and monitoring the position of an endotracheal tube. A properly positioned tube in the trachea guards the patient's airway and enables the paramedic to breathe for the patient. A misplaced tube in the esophagus can lead to death.

A study in the March 2005 Annals of Emergency Medicine, comparing field intubations that used continuous capnography to confirm intubations versus non-use showed zero unrecognized misplaced intubations in the monitoring group versus 23% misplaced tubes in the unmonitored group. The American Heart Association (AHA) affirmed the importance of using capnography to verify tube placement in their 2005 CPR and ECG Guidelines.

The AHA also notes in their new guidelines that capnography, which indirectly measures cardiac output, can also be used to monitor the effectiveness of CPR and as an early indication of return of spontaneous circulation (ROSC). Studies have shown that when a person doing CPR tires, the patient's end-tidal CO₂ (ETCO2, the level of carbon dioxide released at the end of expiration) falls, and then rises when a fresh rescuer takes over. Other studies have shown when a patient experiences ROSC the first indication is often a sudden rise in the ETCO2 as the rush of circulation washes untransported CO2 from the tissues. Likewise, a sudden drop in ETCO2 may indicate the patient has lost pulses and CPR may need to be initiated.

Paramedics are also now beginning to monitor the ETCO2 status of nonintubated patients by using a special nasal cannula that collects the carbon dioxide. A high ETCO2 reading in a patient with altered mental status or severe difficulty breathing may indicate hypoventilation and a possible need for the patient to be intubated.

Capnography, because it provides a breath by breath measurement of a patient's ventilation, can quickly reveal a worsening trend in a patient's condition providing paramedics with an early warning system into a patient's respiratory status. As more clinical studies are conducted into the uses of capnography in asthma, congestive heart failure, diabetes, circulatory shock, pulmonary embolus, acidosis, and other conditions, the prehospital use of capnography will greatly expand.

 External links

• American Society of Anesthesiologists
• Textbook: Capnography: Clinical Aspects
• Capnography.com
• Capnography for Paramedics