Modules Sample
nh095: Patient Care Equipment Techonology , (Pulse Oximetry)
nh195: Advanced Medical Equipment Systems & Technology Management , (Radiation Therapy)
Pulse Oximetry
Principles of operation
The oxygenation of the blood is a key physiological parameter in patient care management. Oxygen enters the body through the lungs and is diffused into the blood across the alveoli. The oxygen is carried in the blood in two forms. 98-99% is attached to hemoglobin which is termed the oxygen saturation or SaO2 or SpO2. A small percentage is dissolved in the arterial plasma, and this is called the PaO2.
The oxygen dissociation curve is a graphic relationship between hemoglobin oxygen saturation and the partial pressure of oxygen in the blood. There are a number of conditions (changes in pH or temperature) and diseases (anemia) that alter the relationship. Hypoxemia is insufficient oxygenation of the arterial blood. On average, SaO2 measurements below 90% are of concern. This condition can result in reduced energy and cognition, tissue damage, brain damage and death.
Non-invasive, continuous measurement of oxygenation reduces the use of invasive methods to obtain blood samples for analysis. An number of hospital patients are at risk for hypoxemia including surgical/post-surgical, pain medication, sleep apnea, cardiopulmonary, neonatal, burns, and other critical care patients.
The original development of oximetry - the measurement of oxygen in the blood - was for high altitude aviation where several large, laboratory systems were developed in the 1930-1950 era. Hewlett Packard developed an eight wavelength ear oximeter in the mid-1960's to identify dyshemoglobins or red blood cells that impede the delivery of oxygen to the tissue cells. The ear oximeter was a cart mounted unit, but required the patient to wear a large ear piece during the measurements. It wasn't until Takuo Aoyagi of Nihon Kohden in Japan invented the pulse oximeter in 1974 that a practical measurement device was conceived. Early US companies with designs similar to Aoyagi included Nellcor and Biox. In the mid-1990's, Masimo came out with signal processing technologies to reduce movement artifact. Today, most US systems either use the Masimo or Nellcor technology in their systems.
A pulse oximeter is a device that provides a noninvasive and continuous measurement of the percent of oxygenated hemoglobin. Because of the measurement technique, it also provides the pulse rate. The basis for pulse oximetry is the Beer-Lambert Law of optical absorption which states that the concentration of a substance can be determined by it's absorption of light. In pulse oximetry, a differential measurement of spectrophotometeric absorption is used based on the absorption curves of oxygenated hemoglobin (HbO2) and un-oxygenated hemoglobin (Hb) at different wavelengths of light. Two wavelengths of light generated by a light emitting diode (LED) are used in conjunction with a photodetector to make the measurement across a vascular bed - typically the finger. A LED emitting in the 660 nm (red) range is a wavelength highly absorbed by un-oxygenated hemoglobin whereas a second LED emitting 920 nm (infra-red) light is highly absorbed by oxygenated hemoglobin. The LEDs are sequentially pulsed on and off, and the photodetector is synchronized to make simultaneous reading for the red and infra-red light transmission. This measurement is a ratio of the pulsatile component of the red light absorption divided the non-pulsatile, static component which is then divided by by the pulsatile component of the infrared light absorption again divided by its static component times a constant used for calibration. The term pulse oximeter was coined due to the relationship between the reading and the pulse component when arterial blood is flowing - this is the only signal that changes with time. The "static" absorbers such as bone, soft tissue and the venous blood are cancelled out. A description with a graphical presentation of pulse oximetery can be found at the Oximetry.org site.
For a resting patient under normal conditions, the accuracy of pulse oximeters is about +/- 2% in the typical range of clinical interest - a SaO2 value of 80% - 100%. The accuracy decreases rapidly as the oxygen saturation goes below 80%. Device accuracy also suffers based on a number of patient, environmental, technical and other issues which occur in unusual situations.
The photo shows the finger probe attached to a physiological monitor with pulse oximeter capabilities.
Pulse oximeters display oxygen saturation, pulse rate, pulse strength, low battery, and alarms. There may be a waveform display also. A block diagram of the major components of a pulse oximeter is shown below.
(insert block diagram)
Since first generation devices, technical advances which have been made to improve pulse oximetry include:
● Calibration resistors and chips embedded into the sensor to compensate for LED differences
● The use of ECG synchronization techniques to anticipate arterial pulses by simultaneously recording the electrocardiograph.
● Various motion sensing improvements to help with reducing artifact and low pulsatile flow situations. An example of a new development can be found at Masimo SET
●Specialty sensors for high altitude climbers, resuscitation situations and cyanotic babies.
● Smart alarm systems for pulse oximeters.
● A reduction in size, cost and power use. Researchers at MIT have developed a ring sized pulse oximeter.
● Fingertip pulse oximeters with wireless conection via Bluetooth technology are made by NONIN
Today, anesthesia standards require pulse oximetry on all anesthetized patients. It is commonly used in the hospital in the continuous mode for critical applications and intermittently for less critical patients. Pulse oximetry is frequently incorporated into vital signs monitors measuring heart rate, blood pressure, and temperature.
The pulse oximeter is not the only non-invasive method for determining oxygen saturation. There is also the reflectance pulse oximeter and a method involving a temporal probe.
Clinical Application
The most common pulse oximeter probe is a reusable finger probe placed on the tip of the index finger.
Other reusable sensors are ear probes. There are also single use, disposable probes which may be adhesive-style sensors. Probes are available for neonates, children and adults. Sensors on appendages - finger or toe - are tranittance sensor. There are also reflectance probes which may be used on the forehead. A good visual description of probe types can be found at Nellcor Education. Also, for magnetic resonance imaging application, special equipment and probes are required. In general, application of the system should include:
● Using of the proper sensor for the application
● Applying the sensors according to the directions
● Checking and reapplying the sensor if necessary
● Understanding patient factors including excessive motion, poor perfusion, anemia, venous pulsations, dysfunctional hemoglobins, nail polish, skin pigmentation, intravascular dyes, and edema
● Understanding the effects of light and electrical interference
● Insuring that the sensor is not put on the same side of the body as an NIBP cuff as it will cut off the blood flow to the sensor and cause it to alarm
Device Safety
Pulse oximeters are relatively safe devices with a few safety issues:
● Infection especially with reusable sensors
● Possible heating and minor burns to sensitive skin due to the red/infra-red LEDs
● Routine electrical safety concerns
Common problems and solutions
Pulse oximetry measurements problems and solutions may include:
| PROBLEM | CAUSE | SOLUTION |
| Blood gas reading (CO oximetry) from clinical laboratory is different than pulse oximetry reading | 1. Values have changed due to time between blood drawing and pulse oximeter readings 2. Blood gas sampling technique not correct 3. SaO2 calculated from PaO2 by analyzer 4. Dysfunctional hemoglobins such as carboxyhemoglobin or methemoglobin 5. Intracardiac shunting such as congenital heart condition results in differing SaO2 reading in different parts of the body 6. Dye injected into the body for another medical test |
1. Use blood gas analyzer in close time proximity 2. Check blood drawing and transport technique 3. Check analyzer 4. Do not rely on pulse oximeter 5. Be consistent in measurement location 6. Wait till the dye disperses before relying on pulse oximetery data |
| Reading intermittent | 1. Nail polish on finger measuring O2 sat 2. Skin pigmentation 3. Ambient light interference 4. Electrical/electromagnetic interference |
1. Remove nail polish or chose a different site 2. If lower pigmentation site available, use it 3. Shield probe from interfering light source or remove light source 4. Remove interfering source |
| Reading intermittent with baseline wander | 1. Motion artifact 2. Wrong probe for application site used |
1. Reduce motion if possible (e.g. blanket over shivering patient); use a newer pulse oximeter with motion artifact reduction 2. Use correct probe |
| Reading intermittent; waveform low | Poor perfusion | Find a site with better perfusion or use a newer pulse oximeter with more senstivity |
| Reading periodically goes away and device alarms | NIBP cuff on same side of patient as finger probe | Change probe to finger on side without NIBP monitoring |
| Readings low, waveform good | 1. Dysoxyhemoglobins present in the blood 2. Venous pulsations from tricuspid valve failure |
1. Use a different instrument to measure saturations 2. Use a different instrument to measure saturations |
| Readings high, waveform good | Anemia | Check red blood cell count |
| Unit on; No reading or waveform; red LED light on | 1. Defective cable or probe/cable unit 2. Defective pulse oximeter |
1. Replace the cable or probe/cable unit 2. Replace pulse oximeter |
| Unit on; No readings of waveforms; red LED light is off | 1. Defective probe 2. Defective cable or probe cable unit 3. Defective pulse oximeter |
1. Replace probe 2. Replace cable or probe/cable unit 3. Replace pulse oximeter |
Information on situations when the pulse oximeter may not be accurate can be found at Pulse Oximetry, a publication of World Anaesthesia Online.
Inspection, testing and preventative maintenance
Below is an inspection procedure and form for a pulse oximeter:
Pulse Oximeter Inspection Procedure and Form
Technology Management
Pulse oximeters were introduced in the 1970's and have gone through a number of significant changes to reach the current generation. Advanced signal processing to reduce motion artifact and read low perfusion situations is characteristic of today's generation of machines. Also, individual sensor calibration is part of the newest generation. For pulse oximeters which are part of a monitoring system, there should be integration of functions such as ECG/pulse oximetry to improve measurement, alarms and system performance. Pulse oximeters have shrunk in size, so extreme portability is now available for applications requiring a small package.
The life expectancy for a pulse oximeter is on average seven years. Technological changes may advance the replacement schedule as would portable use where devices are subject to physical damage. The pulse oximeters which are part of a physiological monitoring system are likely to be replaced when the physiological monitor is replaced. The life expectance for the monitoring system is 7-10 years.
Pulse oximetry units require the use of non-durable cables and, for reusable probes, non-durable cable/probe combinations. These will typically be replaced within 6 months to a year for a busy machine. Disposable probes are replaced after each patient use.
Device Lab
Go to the links below and answer the question posed related to the device simulations
Radiation Therapy
Topics
Principles of operation | Clinical application | Device safety | Common problems and solutions | Inspection, testing, and preventative maintenance | Technology management
Radiation therapy is used to treat cancer. External beam radiation is used to treat about half of all cancer patients with tumors. While the treatment aim is to cure the disease, radiation therapy is also used on a palliative basis to relieve the patient of the symptoms, e.g. pain, but not cure the disease. Radiation therapy techniques include external beam treatments from linear accelerators (also called linacs), cobalt units, and image-guided surgery systems. Also under the classification of radiation therapy are brachytherapy systems which administer a destructive radioisotope directly to cancerous tissue. It is typically used in conjunction with external beam radiation. As a part of all radiation therapy systems is a radiotherapy planning system consisting of a device to register the tumor - radiographic or CT system, and a high performance computer and software. These and other radiation therapy treatments are found at the CancerNet patient education site. Radiation therapy was first used in the 1920's. Linacs were first installed in 1952 in England and at Stanford University Medical Center in 1956 by Varian. Review the history of radiation therapy at the RTanswers webpage.
Linear accelerator showing beam from AAPM
The medical linear accelerator outputs a well defined beam of intense x-ray radiation to treat deep seated tumors. There are four major subsystems:
- Modulator - produces high voltage DC pulses
- Electron gun - injects electrons into the accelerator guide
- RF power source/amplifier - uses a magnetron or klystron to supply high frequency and high energy
- Accelerator guide - a waveguide to direct the beam
The linac also includes a drive stand, gantry, patient-support assembly, and control systems. Below is a simplified block diagram of a linear accelerator.
Low energy linear accelerators use a magnetron to produce 4-6 megavolt beams of electrons or bremsstrahlung x-rays. Bremsstrahlung refers a change in the electron velocity when it collides with another object producing electromagnetic radiation. High energy linacs are equipped with a klystron to generate 15-25 megavolt beams. The klystron was invented by the Varian brothers in 1937. The original application of this device was in radar units. The microwave power from klystron directs the energy to a waveguide and uses a circulator to prevent any reflected microwaves from returning to the klystron. A water cooling system is needed to provide thermal stability. The radiation production process is conducted in a vacuum. The gantry includes the electron gun to produce electrons and the accelerator guide which slow up the waves to synchronize them with the flowing bunches of electrons. The electrons are then directed to a target to produce x-rays - similar process to a radiographic x-ray systems but at much higher voltage - or scattering foil for electron production. Today, the output beam to the patient is controlled by a multi-leaf collimator which has over 100 lead leaves controlled by the treatment computer.
Cobalt therapy has a Cobalt-60 radiation source. The radiation energy is lower than that from a linear accelerator - 1.17 and 1.33 megavolts. The source is sealed in lead shielded container in the treatment head. Collimators and other limiting devices are used to direct the beam to the patient.
The radiation therapy planning system is used to match the output of the linear accelerator or cobalt unit to kill the cancer cells while preserving healthy cells. The planning system calculates the number of beams to enter the patient, the beam type and energy, and the distribution typically in a graphic form. 3D techniques called conformal radiotherapy uses CT and/or MRI image scans. Intensity modulated radiation therapy (IMRT) involves varying the beam intensity through the use of a multi-leaf collimator and advanced software. Tomotherapy is an IMRT technique where the gantry rotates around the patient in conjunction with multi-leaf collimator changes. Go to the Varian treatment techniques website and review the various approaches to treatment.
From AAPM
Brachytherapy systems with remote afterloading are not external beam systems. Rather this technique involves administering iridium-192 or other isotope to the patient in high enough doses to kill the tumor. The afterloader is the storage compartment for the radio isotope. The radioisotope advances through tubes to the patient treatment site. The time and dose is computer controlled using a treatment planning system and physician intervention. As with external beam radiation therapy, the operator is shielded from the radiation.
Facilities designed to accommodate linacs, cobalt units, and brachytherapy devices require significant shielding for the "vault" where the equipment is housed. Room size varies from 500-600 square feet for external beam units and wall thickness can be more than one meter. Design of these facilities in the US is governed by the National Council on Radiation Protection (NCRP) report No. 151. This is optional reading.
Review Dartmouth's Thayer School of Engineering lectures on Radiation Therapy I sections on cobalt therapy and linear accelerators, and Radiation Therapy II sections on beam shaping, CT-guided radiation therapy - tomotherapy, and brachytherapy. The rest of the materials in these two presentations are optional.
As cancer can be found at various body locations, a variety of penetrating energies are needed. About 2/3rds will need low energy treatments, a quarter medium to high energy and the rest, high energy treatment (from ECRI). High energy linacs and proton accelerators are used for deep tumors in the thorax, abdomen, and pelvis. Low-energy linacs and cobalt systems primary application is bone cancer, and tumors of the head, neck and breast. Brachytherapy is a key treatment in lip, tongue, vaginal and rectal cancers along with soft tissue sarcomas and endobrachial tumors. Brachytherapy is used in conjunction with external beam therapy for deep seated tumors.
View the video on Radiation Therapy from the UCL department of medical physics to get an inside look into a radiation therapy department and equipment. This video covers the radiation therapy planning, cobalt and linear accelerators.
Review the patient site of the American College of Radiology/RSNA to understand clinical applications and procedure types from the patient's perspective.
In the United States, the National Council on Radiation Protection & Measurements (NCRP) formulates and disseminates information, guidance, and recommendation on radiation protection and measurements. It was chartered by the US Congress in 1964.
The FDA Product Classification Radiation therapy database lists primarily International Electrotechnical Commission (IEC) standards be followed for products being considered for the US market.
There have been serious consequences due to improper use of radiation therapy, equipment failure, or mistakes in treatment planning. Review the International Commission on Radiation Protection document containing case histories of accidental exposures, clinical consequences, and recommendations for prevention of these serious accidents.
The Radiation Oncology Safety Information System's database shows most problems are due to human error. The complexity of new techniques may amplify problems such as inputting the incorrect data into a treatment planning system because of the complex pattern of radiation treatment. Review the recent report in the FDA Maude Database - ADAC Pinnacle3 Radiation Therapy Planning System. Mechanical failures and software programming errors can also lead to serious problems.
Equipment repair and preventative maintenance has been reported to vary with the complexity of the system and typically have an "immediate" priority. A review of the cost benefit for increased preventative maintenance beyond the manufacturers recommendation was not justified based on increased downtime. This information is based on a report in British Journal of Radiology article - Treatment Machine Maintenance and Breakdown.
A review of repairs on linear accelerators at a local teaching hospital over the past eight months showed the most common problem areas were: pendent, beam stopper, computer, table, com port, and multi-leaf collimator. On-site 3rd party staff were responsible for solving these problems with manufacturer assistance for the most serious issues.
Specialized radiotherapy service is available from several 3rd party independent service providers such as Acceletronics and Oncology Services International.
Inspection, Testing and Preventative Maintenance
Hospitals with radiation therapy services will have a medical physicist on staff or will contract with a service. They are involved in both treatment planning assistance and quality assurance of systems.
Linear accelerator specification, acceptance testing, and commission process is described in this paper by Palta. Quality control must be ongoing for installed equipment. The Canadian cancer agencies issued Standards for Quality Control - Medical Linear Accelerators.
Linear accelerators and cobalt therapy units have been available for many years. Technological enhancements have included IMRT, 3-D conformal, intra-operative, image guided, dynamic radiotherapy. Stereotactic radiosurgery to treat intracranial tumors and malformation is available as the Gamma Knife and the CyberKnife. These devices use multiple precisely focused Cobalt 60 sources controlled by computer. The CyberKnife also has robotic control. An ECRI article on procurement trends showed the interest in the Gamma Knife and CT based radiation therapy (discussed in the Dartmouth lectures) is slipping, CyberKnife is holding steady, while conventional linacs are increasing in interest primarily due to image guided radiation therapy (IGRT) capabilities. In general, the differentiation between radiotherapy and radiosurgery is becoming less clear so hospitals have to carefully plan acquisitions.
A new treatment is proton therapy. Review the Loma Linda University site - they are the nation's first proton therapy center. Protons are best at reaching deep seated tumors such as prostate cancer.
A description of equipment specifications for a high energy linear accelerator and treatment planning system is found at the Ministry of Health website of India. This is not dissimilar to a US specification.
The life cycle for radiation therapy is variable based on upgradability. The range is
generally 7-15 years. The high cost of the systems preclude more frequent replacement. Low energy systems are still the most widely prescribed leading to longer retention of
these units. The multi-leaf collimator is a critical upgrade and systems which cannot be upgraded to this feature will be replaced in the near future. Treatment planning computers
software has to be foolproof so it is generally not upgraded as quickly as other
software. Hardware upgrades are more common.
