Flow-Volume loop showing successful FVC maneuver. Positive values represent expiration, negative values represent inspiration. At the start of the test both flow and volume are equal to zero (representing the volume in the spirometer rather than the lung). The trace moves clockwise for expiration followed by inspiration. After the starting point the curve rapidly mounts to a peak (the peak expiratory flow). (Note the FEV1 value is arbitrary in this graph and just shown for illustrative purposes; these values must be calculated as part of the procedure).
Total lung capacity: the volume in the lungs at maximal inflation, the sum of VC and RV.
TV
Tidal volume: that volume of air moved into or out of the lungs in 1 breath (TV indicates a subdivision of the lung; when tidal volume is precisely measured, as in gas exchange calculation, the symbol TV or VT is used.)
RV
Residual volume: the volume of air remaining in the lungs after a maximal exhalation
ERV
Expiratory reserve volume: the maximal volume of air that can be exhaled from the end-expiratory position
IRV
Inspiratory reserve volume: the maximal volume that can be inhaled from the end-inspiratory level
IC
Inspiratory capacity: the sum of IRV and TV
IVC
Inspiratory vital capacity: the maximum volume of air inhaled from the point of maximum expiration
VC
Vital capacity: the volume of air breathed out after the deepest inhalation.
VT
Tidal volume: that volume of air moved into or out of the lungs during quiet breathing (VT indicates a subdivision of the lung; when tidal volume is precisely measured, as in gas exchange calculation, the symbol TV or VT is used.)
FRC
Functional residual capacity: the volume in the lungs at the end-expiratory position
RV/TLC%
Residual volume expressed as percent of TLC
VA
Alveolar gas volume
VL
Actual volume of the lung including the volume of the conducting airway.
FVC
Forced vital capacity: the determination of the vital capacity from a maximally forced expiratory effort
FEVt
Forced expiratory volume (time): a generic term indicating the volume of air exhaled under forced conditions in the first t seconds
FEV1
Volume that has been exhaled at the end of the first second of forced expiration
FEFx
Forced expiratory flow related to some portion of the FVC curve; modifiers refer to amount of FVC already exhaled
FEFmax
The maximum instantaneous flow achieved during a FVC maneuver
FIF
Forced inspiratory flow: (Specific measurement of the forced inspiratory curve is denoted by nomenclature analogous to that for the forced expiratory curve. For example, maximum inspiratory flow is denoted FIFmax. Unless otherwise specified, volume qualifiers indicate the volume inspired from RV at the point of measurement.)
PEF
Peak expiratory flow: The highest forced expiratory flow measured with a peak flow meter
MVV
Maximal voluntary ventilation: volume of air expired in a specified period during repetitive maximal effort
Spirometry (meaning the measuring of breath) is the most common of the pulmonary function tests (PFTs). It measures lung function, specifically the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled. Spirometry is helpful in assessing breathing patterns that identify conditions such as asthma, pulmonary fibrosis, cystic fibrosis, and COPD. It is also helpful as part of a system of health surveillance, in which breathing patterns are measured over time.[1]
Spirometry generates pneumotachographs, which are charts that plot the volume and flow of air coming in and out of the lungs from one inhalation and one exhalation.
A modern USB PC-based spirometer.Device for spirometry. The patient places his or her lips around the blue mouthpiece. The teeth go between the nubs and the shield, and the lips go over the shield. A nose clip guarantees that breath will flow only through the mouth.Screen for spirometry readouts at right. The chamber can also be used for body plethysmography.
Spirometer
The spirometry test is performed using a device called a spirometer,[2] which comes in several different varieties. Most spirometers display the following graphs, called spirograms:
a volume-time curve, showing volume (litres) along the Y-axis and time (seconds) along the X-axis
a flow-volume loop, which graphically depicts the rate of airflow on the Y-axis and the total volume inspired or expired on the X-axis
Procedure
The basic forced volume vital capacity (FVC) test varies slightly depending on the equipment used. It can be in the form of either closed or open circuit. Regardless of differences in testing procedure providers are recommended to follow the ATS/ERS Standardisation of Spirometry. The standard procedure ensures an accurate and objectively collected set of data, based on a common reference, to reduce incompatibility of the results when shared across differing medical groups.
The patient is asked to put on soft nose clips to prevent air escape and a breathing sensor in their mouth forming an air tight seal. Guided by a technician, the patient is given step by step instructions to take an abrupt maximum effort inhale, followed by a maximum effort exhale lasting for a target of at least 6 seconds. When assessing possible upper airway obstruction, the technician will direct the patient to make an additional rapid inhalation to complete the round. The timing of the second inhale can vary between persons depending on the length of the proceeding exhale. In some cases each round of test will be proceeded by a period of normal, gentle breathing for additional data.
Limitations
Clinically useful results are highly dependent on patient cooperation and effort and must be repeated for a minimum of three times to ensure reproducibility with a general limit of ten attempts. Given variable rates of effort, the results can only be underestimated given an effort output greater than 100% is not possible.[citation needed]
Due to the need for patient cooperation and an ability to understand and follow instructions, spirometry can typically only be done in cooperative children when they at least 5 years old[3][4] or adults without physical or mental impairment preventing effective diagnostic results. In addition, General anesthesia and various forms of sedation are not compatible with the testing process.
Another limitation is that persons with intermittent or mild asthma can present normal spirometry values between acute exacerbation, reducing spirometry's effectiveness as a diagnostic tool in these circumstances.[citation needed]
Supplemental diagnostics
Spirometry can also be part of a bronchial challenge test, used to determine bronchial hyperresponsiveness to either rigorous exercise, inhalation of cold/dry air, or with a pharmaceutical agent such as methacholine or histamine.
To assess the reversibility of a particular condition, a bronchodilator can be administered before performing another round of tests for comparison. This is commonly referred to as a reversibility test, or a post bronchodilator test (Post BD), and is an important part in diagnosing asthma versus COPD.
to detect respiratory disease in patients presenting with symptoms of breathlessness, and to distinguish respiratory from cardiac disease as the cause[8]
to measure bronchial responsiveness in patients suspected of having asthma[8]
The most common parameters measured in spirometry are vital capacity (VC), forced vital capacity (FVC), forced expiratory volume (FEV) at timed intervals of 0.5, 1.0 (FEV1), 2.0, and 3.0 seconds, forced expiratory flow 25–75% (FEF 25–75) and maximal voluntary ventilation (MVV),[10] also known as Maximum breathing capacity.[11] Other tests may be performed in certain situations.
Results are usually given in both raw data (litres, litres per second) and percent predicted—the test result as a percent of the "predicted values" for the patients of similar characteristics (height, age, sex, and sometimes race and weight). The interpretation of the results can vary depending on the physician and the source of the predicted values. Generally speaking, results nearest to 100% predicted are the most normal, and results over 80% are often considered normal. Multiple publications of predicted values have been published and may be calculated based on age, sex, weight and ethnicity. However, review by a doctor is necessary for accurate diagnosis of any individual situation.
A bronchodilator is also given in certain circumstances and a pre/post graph comparison is done to assess the effectiveness of the bronchodilator. See the example printout.
Functional residual capacity (FRC) cannot be measured via spirometry, but it can be measured with a plethysmograph or dilution tests (for example, helium dilution test).
Average values for forced vital capacity (FVC), forced expiratory volume in 1 second (FEV1) and forced expiratory flow 25–75% (FEF25–75%), according to a study in the United States 2007 of 3,600 subjects aged 4–80 years. Y-axis is expressed in litres for FVC and FEV1, and in litres/second for FEF25–75%.
Forced vital capacity (FVC)
Forced vital capacity (FVC) is the volume of air that can forcibly be blown out after full inspiration,[13] measured in liters. FVC is the most basic maneuver in spirometry tests.
Forced expiratory volume in 1 second (FEV1)
FEV1 is the volume of air that can forcibly be blown out in first 1-second, after full inspiration.[13] Average values for FEV1 in healthy people depend mainly on sex and age, according to the diagram. Values of between 80% and 120% of the average value are considered normal.[14] Predicted normal values for FEV1 can be calculated and depend on age, sex, height, mass and ethnicity as well as the research study that they are based on.
FEV1/FVC ratio
FEV1/FVC is the ratio of FEV1 to FVC. In healthy adults this should be approximately 70–80% (declining with age).[15] In obstructive diseases (asthma, COPD, chronic bronchitis, emphysema) FEV1 is diminished because of increased airway resistance to expiratory flow; the FVC may be decreased as well, due to the premature closure of airway in expiration, just not in the same proportion as FEV1 (for instance, both FEV1 and FVC are reduced, but the former is more affected because of the increased airway resistance). This generates a reduced value (<70%, often ~45%). In restrictive diseases (such as pulmonary fibrosis) the FEV1 and FVC are both reduced proportionally and the value may be normal or even increased as a result of decreased lung compliance.
A derived value of FEV1 is FEV1% predicted (FEV1%), which is defined as FEV1 of the patient divided by the average FEV1 in the population for any person of the same age, height, gender, and race.[medical citation needed]
Forced expiratory flow (FEF)
Forced expiratory flow (FEF) is the flow (or speed) of air coming out of the lung during the middle portion of a forced expiration. It can be given at discrete times, generally defined by what fraction of the forced vital capacity (FVC) has been exhaled. The usual discrete intervals are 25%, 50% and 75% (FEF25, FEF50 and FEF75), or 25% and 50% of FVC that has been exhaled. It can also be given as a mean of the flow during an interval, also generally delimited by when specific fractions remain of FVC, usually 25–75% (FEF25–75%). Average ranges in the healthy population depend mainly on sex and age, with FEF25–75% shown in diagram at left. Values ranging from 50 to 60% and up to 130% of the average are considered normal.[14] Predicted normal values for FEF can be calculated and depend on age, sex, height, mass and ethnicity as well as the research study that they are based on.
MMEF or MEF stands for maximal (mid-)expiratory flow and is the peak of expiratory flow as taken from the flow-volume curve and measured in liters per second. It should theoretically be identical to peak expiratory flow (PEF), which is, however, generally measured by a peak flow meter and given in liters per minute.[16]
Recent research suggests that FEF25-75% or FEF25-50% may be a more sensitive parameter than FEV1 in the detection of obstructive small airway disease.[17][18] However, in the absence of concomitant changes in the standard markers, discrepancies in mid-range expiratory flow may not be specific enough to be useful, and current practice guidelines recommend continuing to use FEV1, VC, and FEV1/VC as indicators of obstructive disease.[19][20]
More rarely, forced expiratory flow may be given at intervals defined by how much remains of total lung capacity. In such cases, it is usually designated as e.g. FEF70%TLC, FEF60%TLC and FEF50%TLC.[16]
Forced inspiratory flow 25–75% or 25–50%
Forced inspiratory flow 25–75% or 25–50% (FIF 25–75% or 25–50%) is similar to FEF 25–75% or 25–50% except the measurement is taken during inspiration.[medical citation needed]
Peak expiratory flow (PEF)
Normal values for peak expiratory flow (PEF), shown on EU scale.
Peak expiratory flow (PEF) is the maximal flow (or speed) achieved during the maximally forced expiration initiated at full inspiration, measured in liters per minute or in liters per second.
Diffusing capacity (or DLCO) is the carbon monoxide uptake from a single inspiration in a standard time (usually 10 seconds). During the test the person inhales a test gas mixture that consisting of regular air that includes an inert tracer gas and CO, less than one percent. Since hemoglobin has a greater affinity to CO than oxygen the breath-hold time can be only 10 seconds, which is a sufficient amount of time for this transfer of CO to occur. Since the inhaled amount of CO is known, the exhaled CO is subtracted to determine the amount transferred during the breath-hold time. The tracer gas is analyzed simultaneously with CO to determine the distribution of the test gas mixture. This test will pick up diffusion impairments, for instance in pulmonary fibrosis.[22] This must be corrected for anemia (a low hemoglobin concentration will reduce DLCO) and pulmonary hemorrhage (excess RBC's in the interstitium or alveoli can absorb CO and artificially increase the DLCO capacity). Atmospheric pressure and/or altitude will also affect measured DLCO, and so a correction factor is needed to adjust for standard pressure.
Maximum voluntary ventilation (MVV)
Maximum voluntary ventilation (MVV) is a measure of the maximum amount of air that can be inhaled and exhaled within one minute. For the comfort of the patient this is done over a 15-second time period before being extrapolated to a value for one minute expressed as liters/minute. Average values for males and females are 140–180 and 80–120 liters per minute respectively.[medical citation needed]
Static lung compliance (Cst)
When estimating static lung compliance, volume measurements by the spirometer needs to be complemented by pressure transducers in order to simultaneously measure the transpulmonary pressure. When having drawn a curve with the relations between changes in volume to changes in transpulmonary pressure, Cst is the slope of the curve during any given volume, or, mathematically, ΔV/ΔP.[23] Static lung compliance is perhaps the most sensitive parameter for the detection of abnormal pulmonary mechanics.[24] It is considered normal if it is 60% to 140% of the average value in the population for any person of similar age, sex and body composition.[14]
In those with acute respiratory failure on mechanical ventilation, "the static compliance of the total respiratory system is conventionally obtained by dividing the tidal volume by the difference between the 'plateau' pressure measured at the airway opening (PaO) during an occlusion at end-inspiration and positive end-expiratory pressure (PEEP) set by the ventilator".[25]
Measurement
Approximate value
Male
Female
Forced vital capacity (FVC)
4.8 L
3.7 L
Tidal volume (Vt)
500 mL
390 mL
Total lung capacity (TLC)
6.0 L
4.7 L
Others
Forced Expiratory Time (FET) Forced Expiratory Time (FET) measures the length of the expiration in seconds.
Slow vital capacity (SVC) Slow vital capacity (SVC) is the maximum volume of air that can be exhaled slowly after slow maximum inhalation.
Maximal pressure (Pmax and Pi)
Spirometer - ERV in cc (cm3) average Age 20
Male
Female
4320
3387
Pmax is the asymptotically maximal pressure that can be developed by the respiratory muscles at any lung volume and Pi is the maximum inspiratory pressure that can be developed at specific lung volumes.[26] This measurement also requires pressure transducers in addition. It is considered normal if it is 60% to 140% of the average value in the population for any person of similar age, sex and body composition.[14] A derived parameter is the coefficient of retraction (CR) which is Pmax/TLC .[16]
Mean transit time (MTT) Mean transit time is the area under the flow-volume curve divided by the forced vital capacity.[27]
Maximal inspiratory pressure (MIP) MIP, also known as negative inspiratory force (NIF), is the maximum pressure that can be generated against an occluded airway beginning at functional residual capacity (FRC). It is a marker of respiratory muscle function and strength.[28] Represented by centimeters of water pressure (cmH2O) and measured with a manometer. Maximum inspiratory pressure is an important and noninvasive index of diaphragm strength and an independent tool for diagnosing many illnesses.[29] Typical maximum inspiratory pressures in adult males can be estimated from the equation, MIP = 142 - (1.03 x Age) cmH2O, where age is in years.[30]
Turbine/Stator Rotor (normally incorrectly referred to as a turbine. Actually a rotating vane which spins because of the air flow generated by the subject. The revolutions of the vane are counted as they break a light beam)
Lung volumes and lung capacities refer to the volume of air in the lungs at different phases of the respiratory cycle.
A spirometer is an apparatus for measuring the volume of air inspired and expired by the lungs. A spirometer measures ventilation, the movement of air into and out of the lungs. The spirogram will identify two different types of abnormal ventilation patterns, obstructive and restrictive. There are various types of spirometers that use a number of different methods for measurement.
Exhalation is the flow of the breath out of an organism. In animals, it is the movement of air from the lungs out of the airways, to the external environment during breathing. This happens due to elastic properties of the lungs, as well as the internal intercostal muscles which lower the rib cage and decrease thoracic volume. As the thoracic diaphragm relaxes during exhalation it causes the tissue it has depressed to rise superiorly and put pressure on the lungs to expel the air. During forced exhalation, as when blowing out a candle, expiratory muscles including the abdominal muscles and internal intercostal muscles generate abdominal and thoracic pressure, which forces air out of the lungs.
A plethysmograph is an instrument for measuring changes in volume within an organ or whole body. The word is derived from the Greek "plethysmos", and "graphein".
Interstitial lung disease (ILD), or diffuse parenchymal lung disease (DPLD), is a group of respiratory diseases affecting the interstitium and space around the alveoli of the lungs. It concerns alveolar epithelium, pulmonary capillary endothelium, basement membrane, and perivascular and perilymphatic tissues. It may occur when an injury to the lungs triggers an abnormal healing response. Ordinarily, the body generates just the right amount of tissue to repair damage, but in interstitial lung disease, the repair process is disrupted, and the tissue around the air sacs (alveoli) becomes scarred and thickened. This makes it more difficult for oxygen to pass into the bloodstream. The disease presents itself with the following symptoms: shortness of breath, nonproductive coughing, fatigue, and weight loss, which tend to develop slowly, over several months. The average rate of survival for someone with this disease is between three and five years. The term ILD is used to distinguish these diseases from obstructive airways diseases.
The peak expiratory flow (PEF), also called peak expiratory flow rate (PEFR) and peak flow measurement, is a person's maximum speed of expiration, as measured with a peak flow meter, a small, hand-held device used to monitor a person's ability to breathe out air. It measures the airflow through the bronchi and thus the degree of obstruction in the airways. Peak expiratory flow is typically measured in units of liters per minute (L/min).
Vital capacity (VC) is the maximum amount of air a person can expel from the lungs after a maximum inhalation. It is equal to the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume. It is approximately equal to Forced Vital Capacity (FVC).
Airway obstruction is a blockage of respiration in the airway that hinders the free flow of air. Airway obstructions can occur either in the upper airway (UPA) or lower airway (LOA). The upper airway consists of the nose, throat, and larynx. The lower airway comprises the trachea, bronchi, and bronchioles.
Bronchiolitis obliterans (BO), also known as obliterative bronchiolitis, constrictive bronchiolitis and popcorn lung, is a disease that results in obstruction of the smallest airways of the lungs (bronchioles) due to inflammation. Symptoms include a dry cough, shortness of breath, wheezing and feeling tired. These symptoms generally get worse over weeks to months. It is not related to cryptogenic organizing pneumonia, previously known as bronchiolitis obliterans organizing pneumonia.
Air trapping, also called gas trapping, is an abnormal retention of air in the lungs where it is difficult to exhale completely. It is observed in obstructive lung diseases such as asthma, bronchiolitis obliterans syndrome and chronic obstructive pulmonary diseases such as emphysema and chronic bronchitis.
An incentive spirometer is a handheld medical device used to help patients improve the functioning of their lungs. By training patients to take slow and deep breaths, this simplified spirometer facilitates lung expansion and strengthening. Patients inhale through a mouthpiece, which causes a piston inside the device to rise. This visual feedback helps them monitor their inspiratory effort. Incentive spirometers are commonly used after surgery or certain illnesses to prevent pulmonary complications.
Lung compliance, or pulmonary compliance, is a measure of the lung's ability to stretch and expand. In clinical practice it is separated into two different measurements, static compliance and dynamic compliance. Static lung compliance is the change in volume for any given applied pressure. Dynamic lung compliance is the compliance of the lung at any given time during actual movement of air.
Occupational asthma is new onset asthma or the recurrence of previously quiescent asthma directly caused by exposure to an agent at workplace. It is an occupational lung disease and a type of work-related asthma. Agents that can induce occupational asthma can be grouped into sensitizers and irritants.
Obstructive lung disease is a category of respiratory disease characterized by airway obstruction. Many obstructive diseases of the lung result from narrowing (obstruction) of the smaller bronchi and larger bronchioles, often because of excessive contraction of the smooth muscle itself. It is generally characterized by inflamed and easily collapsible airways, obstruction to airflow, problems exhaling, and frequent medical clinic visits and hospitalizations. Types of obstructive lung disease include asthma, bronchiectasis, bronchitis and chronic obstructive pulmonary disease (COPD). Although COPD shares similar characteristics with all other obstructive lung diseases, such as the signs of coughing and wheezing, they are distinct conditions in terms of disease onset, frequency of symptoms, and reversibility of airway obstruction. Cystic fibrosis is also sometimes included in obstructive pulmonary disease.
Restrictive lung diseases are a category of extrapulmonary, pleural, or parenchymal respiratory diseases that restrict lung expansion, resulting in a decreased lung volume, an increased work of breathing, and inadequate ventilation and/or oxygenation. Pulmonary function test demonstrates a decrease in the forced vital capacity.
Pulmonary function testing (PFT) is a complete evaluation of the respiratory system including patient history, physical examinations, and tests of pulmonary function. The primary purpose of pulmonary function testing is to identify the severity of pulmonary impairment. Pulmonary function testing has diagnostic and therapeutic roles and helps clinicians answer some general questions about patients with lung disease. PFTs are normally performed by a pulmonary function technologist, respiratory therapist, respiratory physiologist, physiotherapist, pulmonologist, or general practitioner.
The FEV1/FVC ratio, also called modified Tiffeneau-Pinelli index, is a calculated ratio used in the diagnosis of obstructive and restrictive lung disease. It represents the proportion of a person's vital capacity that they are able to expire in the first second of forced expiration (FEV1) to the full, forced vital capacity (FVC). FEV1/FVC ratio was first proposed by E.A. Haensler in 1950. The FEV1/FVC index should not be confused with the FEV1/VC index as they are different, although both are intended for diagnosing airway obstruction. Current recommendations for diagnosing pulmonary function recommend using the modified Tiffeneau-Pinelli index. This index is recommended to be represented as a decimal fraction with two digits after the decimal point.
The post bronchodilator test, also commonly referred to as a reversibility test, is a test that utilizes spirometry to assess possible reversibility of bronchoconstriction in diseases such as asthma.
A respiratory pressure meter measures the maximum inspiratory and expiratory pressures that a patient can generate at either the mouth (MIP and MEP) or inspiratory pressure a patient can generate through their nose via a sniff maneuver (SNIP). These measurements require patient cooperation and are known as volitional tests of respiratory muscle strength. Handheld devices displaying the measurement achieved in centimetres of water pressure (cmH2O) and the pressure trace created, allow quick patient testing away from the traditional pulmonary laboratory and are useful for ward-based, out-patient and preoperative assessment, as well as for use by pulmonologists and physiotherapists.
Asthma-Chronic Obstructive Pulmonary Disease (COPD) Overlap (ACO), also known as Asthma-COPD Overlap Syndrome (ACOS), is a chronic inflammatory, obstructive airway disease in which features of both asthma and COPD predominate. Asthma and COPD were once thought of as distinct entities; however, in some, there are clinical features of both asthma and COPD with significant overlap in pathophysiology and symptom profile. It is unclear whether ACO is a separate disease entity or a clinical subtype of asthma and COPD. The pathogenesis of ACO is poorly understood, but it is thought to involve both type 2 inflammation as well as type 1 inflammation. The incidence and prevalence of ACO are not well known. The risk factors for ACO are also incompletely understood, but tobacco smoke is known to be a major risk factor.
1 2 3 4 LUNGFUNKTION — Practice compendium for semester 6. Department of Medical Sciences, Clinical Physiology, Academic Hospital, Uppsala, Sweden. Retrieved 2010.
↑ Clinic, the Cleveland (2010). Current clinical medicine 2010 (2nded.). Philadelphia, Pa.: Saunders. p.8. ISBN978-1416066439.
1 2 3 Interpretation model — compendium at Uppsala Academic Hospital. By H. Hedenström. 2009-02-04
↑ Ciprandi, Giorgio; Cirillo, Ignazio (1 February 2011). "Forced expiratory flow between 25% and 75% of vital capacity may be a marker of bronchial impairment in allergic rhinitis". Journal of Allergy and Clinical Immunology. 127 (2): 549, discussion 550–1. doi:10.1016/j.jaci.2010.10.053. PMID21281879.
↑ George, Ronald B. (2005). Chest medicine: essentials of pulmonary and critical care medicine. Lippincott Williams & Wilkins. p.96. ISBN978-0-7817-5273-2.
↑ Sud, A.; Gupta, D.; Wanchu, A.; Jindal, S. K.; Bambery, P. (2001). "Static lung compliance as an index of early pulmonary disease in systemic sclerosis". Clinical Rheumatology. 20 (3): 177–180. doi:10.1007/s100670170060. PMID11434468. S2CID19170708.
↑ Rossi, A.; Gottfried, S. B.; Zocchi, L.; Higgs, B. D.; Lennox, S.; Calverley, P. M.; Begin, P.; Grassino, A.; Milic-Emili, J. (May 1985). "Measurement of static compliance of the total respiratory system in patients with acute respiratory failure during mechanical ventilation. The effect of intrinsic positive end-expiratory pressure". The American Review of Respiratory Disease. 131 (5): 672–677. doi:10.1164/arrd.1985.131.5.672 (inactive 1 November 2024). PMID4003913.{{cite journal}}: CS1 maint: DOI inactive as of November 2024 (link)
↑ Page 352 in: Irwin, Richard (2008). Procedures, techniques, and minimally invasive monitoring in intensive care medicine. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. ISBN978-0-7817-7862-6.
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