Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (CO 2) in the respiratory gases. Its main development has been as a monitoring tool for use during anesthesia and intensive care. It is usually presented as a graph of CO 2 (measured in kilopascals, "kPa" or millimeters of mercury, "mmHg") plotted against time, or, less commonly, but more usefully, expired volume (known as volumetric capnography). The plot may also show the inspired CO 2, which is of interest when rebreathing systems are being used. When the measurement is taken at the end of a breath (exhaling), it is called "end tidal" CO 2 (PETCO2).[1]
The capnogram is a direct monitor of the inhaled and exhaled concentration or partial pressure of CO 2, and an indirect monitor of the CO 2 partial pressure in the arterial blood. In healthy individuals, the difference between arterial blood and expired gas CO 2 partial pressures is very small (normal difference 4-5 mmHg). 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 which can be an indication of new pathology or change in the cardiovascular-ventilation system.[2][3]
Medical Use
Oxygenation and capnography, although related, remain distinct elements in the physiology of respiration. Ventilation refers to the mechanical process of which the lungs expand and exchange volumes of gasses, however respiration further describes the exchange of gasses (mainly CO 2 and O 2) at the level of the alveoli. The process of respiration can be divided into two main functions: elimination of CO 2 waste and replenishing tissues with fresh O 2. Oxygenation (typically measured via pulse oximetry) measures the latter portion of this system. Capnography measures the elimination of CO 2 which may be of greater clinical usefulness than oxygenation status.[4]
During the normal cycle of respiration, a single breath can be divided into two phases: inspiration and expiration. At the beginning of inspiration, the lungs expand and CO 2 free gasses fill the lungs. As the alveoli are filled with this new gas, the concentration of CO 2 that fills the alveoli is dependent on the ventilation of the alveoli and the perfusion (blood flow) that is delivering the CO 2 for exchange. Once expiration begins to occur, the lung volume decreases as air is forced out the respiratory tract. The volume of CO 2 that is exhaled at the end of exhalation is generated as a by product of metabolism from tissue throughout the body. The delivery of CO 2 to the alveoli for exhalation is dependent on an intact cardiovascular system to ensure adequate blood flow from the tissue to the alveoli. If cardiac output (the amount of blood that is pumped out of the heart) is decreased, the ability to transport CO 2 is also decreased which is reflected in a decreased expired amount of CO 2. The relationship of cardiac output and end tidal CO 2 is linear, such that as cardiac output increases or decreases, the amount of CO 2 is also adjusted in the same manner. Therefore the monitoring of end tidal CO 2 can provide vital information on the integrity of the cardiovascular system, specifically how well the heart is able to pump blood.[5]
The amount of CO 2 that is measured during each breath requires an intact cardiovascular system to delivery the CO 2 to the alveoli which is the functional unit of the lungs. During phase I of expiration, the CO 2 transported to the lungs gas occupies a given space that is not involved in gas exchange, called dead space. Phase II of expiration is when the CO 2 within the lungs is forced up the respiratory tract on its way to leave the body, which causes mixing of the air from the dead space with the air in the functional alveoli responsible for gas exchange. Phase III is the final portion of expiration which reflects CO 2 only from the alveoli and not the dead space. These three phases are important to understand in clinical scenarios since a change in the shape and absolute values can indicate respiratory and/or cardiovascular compromise.[6]
Intraoperative Complications (ie. air embolism, thromboembolism, etc.)
CPR use in ACLS (Advanced Cardiovascular Life Support)
Procedural Sedation Monitoring
Anesthesia
Capnograph
During anesthesia, there is interplay between two components: the patient and the anesthesia administration device (which is usually a breathing circuit and a ventilator). The critical connection between the two components is either an endotracheal tube or a mask, and CO 2 is typically monitored at this junction. Capnography directly reflects the elimination of CO 2 by the lungs to the anesthesia device. Indirectly, it reflects the production of CO 2 by tissues and the circulatory transport of CO 2 to the lungs.[7]
When expired CO 2 is related to expired volume rather than time, the area beneath the curve represents the volume of CO 2 in the breath, and thus over the course of a minute, this method can yield the CO 2 per minute elimination, an important measure of metabolism. Sudden changes in CO 2 elimination during lung or heart surgery usually imply important changes in cardiorespiratory function.[8]
Capnography has been shown to be more effective than clinical judgement alone in the early detection of adverse respiratory events such as hypoventilation, esophageal 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.[9][10]
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 anesthesia mishaps according to an ASA (American Society of Anesthesiologists) closed claim study.[11]
Emergency medical services
Capnography is increasingly being used by EMS personnel 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 or a blind insertion airway device. 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 the patient's death if it goes undetected. [12]
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.[13] The American Heart Association (AHA) affirmed the importance of using capnography to verify tube placement in their 2005 CPR and Emergency Cardiovascular Care Guidelines.[14]
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 CO 2 (PETCO2, 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 return of spontaneous circulation, the first indication is often a sudden rise in the PETCO2 as the rush of circulation washes untransported CO 2 from the tissues. Likewise, a sudden drop in PETCO2 may indicate the patient has lost pulses and CPR may need to be initiated.[15]
Paramedics are also now beginning to monitor the PETCO2 status of nonintubated patients by using a special nasal cannula that collects the carbon dioxide. A high PETCO2 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. Low PETCO2 readings on patients may indicate hyperventilation.[16]
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 by providing paramedics with an early warning system into a patient's respiratory status. When compared to oxygenation which is measured by pulse oximetry, there are several disadvantages that capnography can help address to provide a more accurate reflection of cardiovascular integrity. One shortcoming of measuring pulse oximetry alone is that administration of supplemental oxygen (ie. via nasal cannula) can delay desaturation in a patient if they stopped breathing, therefore delaying medical intervention. Capnography provides a rapid way to directly assess ventilation status and indirectly assess cardiac function. Clinical studies are expected to uncover further uses of capnography in asthma, congestive heart failure, diabetes, circulatory shock, pulmonary embolus, acidosis, and other conditions, with potential implications for the prehospital use of capnography.[17]
Registered nurses
Registered nurses, but more so RRTs (respiratory therapists), in critical care settings may use capnography to determine if a nasogastric tube, which is used for feeding, has been placed in the trachea as opposed to the esophagus.[18] Usually a patient will cough or gag if the tube is misplaced, but most patients in critical care settings are sedated or comatose. If a nasogastric tube is accidentally placed in the trachea instead of the esophagus, the tube feedings will go into the lungs, which is a life-threatening situation. If the monitor displays typical CO 2 waveforms then placement should be confirmed.[19]
Diagnostic usage
Capnography provides information about CO 2 production, pulmonary (lung) perfusion, alveolar ventilation, respiratory patterns, and elimination of CO 2 from the anesthesia 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.[20]
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 CO 2 and arterial blood CO 2. Capnography can also be used to measure carbon dioxide production, a measure of metabolism. Increased CO 2 production is seen during fever and shivering. Reduced production is seen during anesthesia and hypothermia.[21]
Capnographs work on the principle that CO 2 is a polyatomic gas and therefore absorbs infrared radiation. A beam of infrared light is passed across the gas sample to fall on a sensor. The presence of CO 2 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 infrared absorption via the phenomenon of collision broadening.[22] This must be corrected for measuring the CO 2 in human breath by measuring its infrared absorptive power. This was established as a reliable technique by John Tyndall in 1864, though 19th and early 20th century devices were too cumbersome for everyday clinical use.[23] Today, technologies have since improved and are able to measure the values of CO 2 near instantaneously and has become a standard practice in medical settings. There are currently two main types of CO 2 sensors that are used in clinical practice: main-stream sensors and side-stream sensors. Both effectively serve the same function to quantify the amount of CO 2 that is being exhaled in each breath.
Capnogram model
The capnogram waveform provides information about various respiratory and cardiac parameters. The capnogram double-exponential model attempts to quantitatively explain the relationship between respiratory parameters and the exhalatory segment of a capnogram waveform.[24] According to the model, each exhalatory segment of capnogram waveform follows the analytical expression:
where
represents the partial pressure of carbon dioxide measured by the capnogram as a function of time since the beginning of exhalation.
represents the alveolar partial pressure of carbon dioxide.
↑ Nunn, J; Hill, D (May 1960). "Respiratory dead space and arterial to end-tidal carbon dioxide tension difference in anesthetized man". J Appl Physiol. 15: 383–389. doi:10.1152/jappl.1960.15.3.383. PMID14427915.
↑ Williams, Emma; Dassios, Theodore; Greenough, Anne (October 2021). "Carbon dioxide monitoring in the newborn". Pediatr Pulmonol. 56 (10): 3148–3156. doi:10.1002/ppul.25605. PMID34365738. S2CID236960627.
↑ Davis, Daniel; Dunford, James; Ochs, Mel; Park, Kenneth; Hoyt, David (April 2004). "The use of quantitative end-tidal capnometry to avoid inadvertent severe hyperventilation in patients with head injury after paramedic rapid sequence intubation". J Trauma. 56 (4): 808–814. doi:10.1097/01.TA.0000100217.05066.87. PMID15187747.
↑ Potter, Patricia Ann, and Anne Griffin Perry. "Nutrition." Essentials for nursing practice. Eighth ed. St. Louis: Elsevier, 2015. 940. Print.
↑ Roubenoff, Ronenn; Ravich, William (April 1998). "Pneumothorax due to nasogastric feeding tubes. Report of four cases, review of the literature, and recommendations for prevention". Arch Intern Med. 149 (1): 184–188. doi:10.1001/archinte.1989.00390010156022. PMID2492185.
↑ Yaron, Michael; Padyk, Paul; Hutsinpiller, Molly; Cairns, Charles (October 1996). "Utility of the expiratory capnogram in the assessment of bronchospasm". Ann Emerg Med. 28 (4): 403–407. doi:10.1016/S0196-0644(96)70005-7. PMID8839525.
The respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals the respiratory surface is internalized as linings of the lungs. Gas exchange in the lungs occurs in millions of small air sacs; in mammals and reptiles these are called alveoli, and in birds they are known as atria. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood. These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea, which branches in the middle of the chest into the two main bronchi. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds the bronchioles are termed parabronchi. It is the bronchioles, or parabronchi that generally open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration.
Tracheal intubation, usually simply referred to as intubation, is the placement of a flexible plastic tube into the trachea (windpipe) to maintain an open airway or to serve as a conduit through which to administer certain drugs. It is frequently performed in critically injured, ill, or anesthetized patients to facilitate ventilation of the lungs, including mechanical ventilation, and to prevent the possibility of asphyxiation or airway obstruction.
Respiratory failure results from inadequate gas exchange by the respiratory system, meaning that the arterial oxygen, carbon dioxide, or both cannot be kept at normal levels. A drop in the oxygen carried in the blood is known as hypoxemia; a rise in arterial carbon dioxide levels is called hypercapnia. Respiratory failure is classified as either Type 1 or Type 2, based on whether there is a high carbon dioxide level, and can be acute or chronic. In clinical trials, the definition of respiratory failure usually includes increased respiratory rate, abnormal blood gases, and evidence of increased work of breathing. Respiratory failure causes an altered mental status due to ischemia in the brain.
Apnea, BrE: apnoea, is the temporal cessation of breathing. During apnea, there is no movement of the muscles of inhalation, and the volume of the lungs initially remains unchanged. Depending on how blocked the airways are, there may or may not be a flow of gas between the lungs and the environment. If there is sufficient flow, gas exchange within the lungs and cellular respiration would not be severely affected. Voluntarily doing this is called holding one's breath. Apnea may first be diagnosed in childhood, and it is recommended to consult an ENT specialist, allergist or sleep physician to discuss symptoms when noticed; malformation and/or malfunctioning of the upper airways may be observed by an orthodontist.
Dead space is the volume of air that is inhaled that does not take part in the gas exchange, because it either remains in the conducting airways or reaches alveoli that are not perfused or poorly perfused. It means that not all the air in each breath is available for the exchange of oxygen and carbon dioxide. Mammals breathe in and out of their lungs, wasting that part of the inhalation which remains in the conducting airways where no gas exchange can occur.
Mechanical ventilation, assisted ventilation or intermittent mandatory ventilation (IMV) is the medical term for using a machine called a ventilator to fully or partially provide artificial ventilation. Mechanical ventilation helps move air into and out of the lungs, with the main goal of helping the delivery of oxygen and removal of carbon dioxide. Mechanical ventilation is used for many reasons, including to protect the airway due to mechanical or neurologic cause, to ensure adequate oxygenation, or to remove excess carbon dioxide from the lungs. Various healthcare providers are involved with the use of mechanical ventilation and people who require ventilators are typically monitored in an intensive care unit.
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment.
Acute respiratory distress syndrome (ARDS) is a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), and bluish skin coloration (cyanosis). For those who survive, a decreased quality of life is common.
In physiology, respiration is the movement of oxygen from the outside environment to the cells within tissues, and the removal of carbon dioxide in the opposite direction that's to the environment.
Respiratory arrest is a medical condition caused by apnea or respiratory dysfunction severe enough that it will not sustain the body. Prolonged apnea refers to a patient who has stopped breathing for a long period of time. If the heart muscle contraction is intact, the condition is known as respiratory arrest. An abrupt stop of pulmonary gas exchange lasting for more than five minutes may permanently damage vital organs, especially the brain. Lack of oxygen to the brain causes loss of consciousness. Brain injury is likely if respiratory arrest goes untreated for more than three minutes, and death is almost certain if more than five minutes.
Pulse oximetry is a noninvasive method for monitoring a person's blood oxygen saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2% accuracy of the more accurate reading of arterial oxygen saturation (SaO2) from arterial blood gas analysis. But the two are correlated well enough that the safe, convenient, noninvasive, inexpensive pulse oximetry method is valuable for measuring oxygen saturation in clinical use.
In anaesthesia and advanced airway management, rapid sequence induction (RSI) – also referred to as rapid sequence intubation or as rapid sequence induction and intubation (RSII) or as crash induction – is a special process for endotracheal intubation that is used where the patient is at a high risk of pulmonary aspiration. It differs from other techniques for inducing general anesthesia in that several extra precautions are taken to minimize the time between giving the induction drugs and securing the tube, during which period the patient's airway is essentially unprotected.
A bag valve mask (BVM), sometimes known by the proprietary name Ambu bag or generically as a manual resuscitator or "self-inflating bag", is a hand-held device commonly used to provide positive pressure ventilation to patients who are not breathing or not breathing adequately. The device is a required part of resuscitation kits for trained professionals in out-of-hospital settings (such as ambulance crews) and is also frequently used in hospitals as part of standard equipment found on a crash cart, in emergency rooms or other critical care settings. Underscoring the frequency and prominence of BVM use in the United States, the American Heart Association (AHA) Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiac Care recommend that "all healthcare providers should be familiar with the use of the bag-mask device." Manual resuscitators are also used within the hospital for temporary ventilation of patients dependent on mechanical ventilators when the mechanical ventilator needs to be examined for possible malfunction or when ventilator-dependent patients are transported within the hospital. Two principal types of manual resuscitators exist; one version is self-filling with air, although additional oxygen (O2) can be added but is not necessary for the device to function. The other principal type of manual resuscitator (flow-inflation) is heavily used in non-emergency applications in the operating room to ventilate patients during anesthesia induction and recovery.
Hypoxemia is an abnormally low level of oxygen in the blood. More specifically, it is oxygen deficiency in arterial blood. Hypoxemia has many causes, and often causes hypoxia as the blood is not supplying enough oxygen to the tissues of the body.
Masimo Corporation is an Irvine, California-based global medical technology company that develops, manufactures, and markets non-invasive patient monitoring technologies, hospital automation solutions, home monitoring devices, ventilation solutions, and consumer products. The company's core measurement technologies are pulse oximetry, alongside advanced Pulse CO-Oximetry measurements, brain function monitoring, regional oximetry, acoustic respiration rate monitoring, capnography, nasal high-flow respiratory support therapy, patient position and activity tracking, and neuromodulation technology for the reduction of symptoms associated with opioid withdrawal.
Modes of mechanical ventilation are one of the most important aspects of the usage of mechanical ventilation. The mode refers to the method of inspiratory support. In general, mode selection is based on clinician familiarity and institutional preferences, since there is a paucity of evidence indicating that the mode affects clinical outcome. The most frequently used forms of volume-limited mechanical ventilation are intermittent mandatory ventilation (IMV) and continuous mandatory ventilation (CMV). There have been substantial changes in the nomenclature of mechanical ventilation over the years, but more recently it has become standardized by many respirology and pulmonology groups. Writing a mode is most proper in all capital letters with a dash between the control variable and the strategy.
Work of breathing (WOB) is the energy expended to inhale and exhale a breathing gas. It is usually expressed as work per unit volume, for example, joules/litre, or as a work rate (power), such as joules/min or equivalent units, as it is not particularly useful without a reference to volume or time. It can be calculated in terms of the pulmonary pressure multiplied by the change in pulmonary volume, or in terms of the oxygen consumption attributable to breathing.
Respiratory compromise describes a deterioration in respiratory function with a high likelihood of rapid progression to respiratory failure and death. Respiratory failure occurs when inadequate gas exchange by the respiratory system occurs, with a low oxygen level or a high carbon dioxide level.
Colorimetric capnography or colorimetric capnometry is a technique of detecting CO2 in exhaled gas using a color changing device. Such devices usually incorporate pH paper which is designed to change colour with the acidity of CO2. These devices are placed in the breathing system, portable, do not require electricity, change reversibly (breath-by-breath), and have a small amount of dead space. They are commonly used for neonatal intubations in an emergency to confirm placement of an endotracheal tube, and rule out accidental oesophageal intubation.
Ventilation-perfusion coupling is the relationship between ventilation and perfusion processes, which take place in the respiratory and cardiovascular systems. Ventilation is the movement of gas during breathing, and perfusion is the process of pulmonary blood circulation, which delivers oxygen to body tissues. Anatomically, the lung structure, alveolar organization, and alveolar capillaries contribute to the physiological mechanism of ventilation and perfusion. Ventilation-perfusion coupling maintains a constant ratio near 0.8 on average, while the regional variation exists within the lungs due to gravity. When the ratio gets above or below 0.8, it is considered abnormal ventilation-perfusion coupling, also known as a ventilation-perfusion mismatch. Lung diseases, cardiac shunts, and smoking can cause a ventilation-perfusion mismatch that results in significant symptoms and diseases, which can be treated through treatments like bronchodilators and oxygen therapy.
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