Capnography And Pulse Oximetry


Article Author:
Nirzari Pandya


Article Editor:
Sandeep Sharma


Editors In Chief:
Jeanie Skibiski
Kathrin Allen
Brian Cornelius


Managing Editors:
Avais Raja
Orawan Chaigasame
Khalid Alsayouri
Kyle Blair
Radia Jamil
Erin Hughes
Patrick Le
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Hassam Zulfiqar
Navid Mahabadi
Hussain Sajjad
Steve Bhimji
Muhammad Hashmi
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Abbey Smiley
Sarosh Vaqar
Mark Pellegrini
James Hughes
Beenish Sohail
Hajira Basit
Phillip Hynes
Sandeep Sekhon


Updated:
4/23/2019 9:56:20 PM

Introduction

Oxygen, discovered by Joseph Priestley, is the most important gas for human survival. In 1771, Priestley observed that a mouse in a sealed jar would eventually collapse. He placed a sprig of mint inside and the subject revived, which led him to discover that plants give off a gas that he named it as "dephlogisticated air." This discovery enabled future scientist, Antoine Lavoisier to soon discover that this "dephlogisticated air" is indeed oxygen. It also helped future scientists to understand the importance and the working of cellular respiration and photosynthesis, both essential to life on Earth.[1]

The primary purpose of the respiratory system is to take in oxygen and give off carbon dioxide. Oxygen is necessary for cellular metabolism; it acts as the last acceptor of an electron in the electron transport chain in mitochondria. Without oxygen, the human body metabolizes anaerobically, an unstable stage. If this continues for some time, cells die. It becomes essential to monitor the levels of oxygen in cases of cardio-respiratory illnesses by measuring the amount of hemoglobin saturation by pulse oximetry. Pulse oximetry is a noninvasive means by which to monitor a person's oxygen saturation.

The products of cellular metabolism are water and carbon dioxide. Carbon dioxide is an acid, and when it accumulates, it causes respiratory acidosis, which can lead to lowering of the pH and can have a profound effect on cellular homeostasis and can also lead to cellular death. It becomes essential to monitor the levels of carbon dioxide during procedural sedation through capnography. Capnography is a method of monitoring the concentration or partial pressure of carbon dioxide in the respiratory gases.[2]

Function

There are multiple current clinical uses of pulse oximetry in primary care. In stable underlying lung disease patients, pulse oximetry is helpful for the following[3]:

  • To establish a baseline value.
  • Monitoring the patients with exercise-related dyspnea.
  • A screening tool to identify patients with underlying lung disease (with SpO2 less than 92%)
  • Stable underlying lung disease or recovering from an exacerbation; a SpO2 88% or less qualifies for oxygen therapy
  • Titrating the rate of flow of oxygen in patients on long-term oxygen therapy - target is resting and ambulatory pulse oxymetry around 88 to 92 %

During COPD or asthma exacerbations, pulse oximetry is helpful to[4]:

  • Evaluate patients with severe disease (FEV1 less than 50% predicted), cyanosis, or cor pulmonale for possible respiratory insufficiency/failure
  • Assess patients with acutely worsening symptoms, especially shortness of breath (SOB)
  • Determine how severe is the exacerbation based on pulse oximetry and partial pressure of oxygen in an arterial blood gas - based on this physician can determine whether the patient is treatable on an outpatient basis or needs in-hospital treatment
  • Titrating oxygen therapy during exacerbation of any underlying lung disease - COPD, asthma, interstitial lung diseases, IPF, etc.
  • Asthma 

Clinical uses of Capnography[5][6]:

  • Confirmation of tracheal intubation
  • Assessing tracheal tube and tracheostomy patency and position
  • Monitor adequate ventilator support
  • During percutaneous tracheostomy placement
  • Monitoring patients with raised intracranial pressure
  • Monitoring response to treatment of bronchospasm
  • Estimation of cardiac output
  • Use during cardiac arrest
  • Measures kinetics of CO2 elimination on a breath-by-breath basis

Issues of Concern

Pulse oximeters have some limitations. They can only employ light at two wavelengths. Thus the devices can only distinguish between Hb and OHb. When carboxyhemoglobin and methemoglobin are also present, there are two additional wavelengths required for differentiation. In the presence of elevated carboxyhemoglobin level, oximetry overestimates the true saturation of oxygen as carboxyhemoglobin binds with a higher affinity than oxygen. In the case of carbon monoxide poisoning, the absorbance spectrum of CO is very similar to Hb, which results in a falsely high level of oxygen (overestimation of oxygen saturation).[7]

Clinical Significance

Oxygen that reaches the trachea is calculated as the percent of oxygen in the air, i.e., that is 0.21 times the atmospheric partial pressure. This number comes to 160 mm Hg. The amount of oxygen that reaches our alveoli gets calculated via the alveolar gas equation.

  • PAO2 = (Patm - PH2O) FiO2 - PaCO2/RQ[8]

Patm is the atmospheric pressure (at sea level 760 mm Hg), PH2O is partial pressure of water (approximately 45 mm Hg). FiO2 is the fraction of inspired oxygen. PaCO2 is the partial pressure of carbon dioxide in alveoli (in normal physiological conditions around 40 to 45 mmHg). RQ is the respiratory quotient. The value of the RQ can vary depending upon the type of diet and metabolic state. RQ is different for carbohydrates, fats, and proteins (average value is around 0.82 for the human diet).

The calculated partial pressure of oxygen, from the alveolar gas equation that reaches our alveoli, is 100 mm Hg. However, not all of the 100 mmHg diffuses out and dissolves in the arteries to reach the peripheral tissues. The A-a gradient explains this discrepancy. It is calculated by the difference between the partial pressure of oxygen in arteries and partial pressure of oxygen in alveoli (PAO2 - PaO2). The A-a gradient indicates the effectiveness of gas exchange across the alveolocapillary membrane. This gradient widens when there is a pathology. The normal range of A-a gradient is between 12 to 15 mm Hg. If this number is greater than the upper limit of that range, it indicates the presence of a pathology interfering with the gas exchange. Hypoxemia is the primary cause of changes in the A-a gradient. Calculation of the A-a gradient helps distinguish the fundamental pathogenic causes of hypoxemia.

Five Causes of Hypoxemia

  • Diffusion limitation
  • Hypoventilation
  • Decreased PiO2
  • Low V'A/Q'
  • Shunt[9]

Oxygen diffuses through the alveolar-capillary membrane and into the blood. It is carried in the blood bound to hemoglobin and transported to the tissues;. 98.5% of all oxygen is bound to hemoglobin, and 1.5% dissolves in plasma. The hemoglobin molecule is composed of 4 subunits.  Hydrophilic or charged amino acids (for example, Asp, Glu, Lys, Arg) form ionic bonds and hold the four subunits of heme in a quaternary structure. Oxygen can only bind when the hemoglobin switches from a tense to a relaxed form. One oxygen molecule binding increases the affinity of other oxygen molecules to bind to the heme molecule, thus promoting cooperative binding which produces a sigmoidal dissociation curve.[10]

Oxygen is delivered to the tissues while carbon dioxide is the metabolite delivered back to the lungs. Carbon dioxide is the result of cellular metabolism in the mitochondria through the Kreb cycle. The amount of carbon dioxide produced metabolically depends on the relative amounts of carbohydrate, fat, and protein metabolized. Carbon dioxide is transported in blood from tissues to the lung in 3 ways: (1) dissolved in solution, (2) buffered with water as carbonic acid, or (3) bound to hemoglobin. Increased skeletal muscle activity results in localized increases in partial pressure of carbon dioxide and reduces the blood pH. Carbonic anhydrase converts gaseous carbon dioxide to carbonic acid that in turn releases hydrogen ions. The decrease in pH causes a lower affinity of hemoglobin for oxygen, thus causing a rightward shift in the oxygen-hemoglobin dissociation curve.[11]

Hemoglobin molecule transports oxygen from lungs to peripheral tissues and picks up carbon dioxide metabolized from the peripheral tissues to the lungs. Oxygen binding to hemoglobin promotes the release of carbon dioxide, known as the Haldane effect. It is the result of 2 effects of oxygen binding on hemoglobin. First, the binding oxygen to hemoglobin reduces the affinity of the protein for carbon dioxide in the form of carbaminohemoglobin. Second, binding of oxygen to hemoglobin makes it more acidic thus resulting in the release of hydrogen ions. The higher concentration of hydrogen ions pushes equilibrium between bicarbonate and carbon dioxide in the direction of carbon dioxide, thus enhancing its elimination.[8]

Carbon dioxide homeostasis regulation is by the pulmonary and the renal system. The partial pressure of PCO2 in arterial blood is directly proportional to the CO2 that is generated by metabolic processes and inversely related to the rate of CO2 elimination via alveolar ventilation. Mathematically, alveolar ventilation can be derived as follows:

  • PaCO2 = 0.863 x VCO2/ VA
  • VA = VE - VD
  • VE = RR x TV
  • TV = RR x dead-space volume

VCO2 is the metabolic production of CO2, VA is alveolar ventilation, VE is minute ventilation, VD is dead space ventilation, RR is the respiratory rate, and TV is tidal volume.[12]

The partial pressure of alveolar CO2 is calculated by simply rearranging the alveolar equation. The alveolar partial pressure of CO2 is proportional to the body’s metabolic rate and is inversely proportional to the alveolar ventilation rate. We can understand this principle in the context of exercise. During exercise, our body’s metabolic rate of CO2 productions was to double, then the alveolar partial pressure of CO2 would also double if alveolar ventilation remains the same. Conversely, if the rate of alveolar ventilation doubles then the alveolar partial pressure of carbon dioxide would become half, given a constant metabolic CO2 production.

Ventilation is physiologically controlled by respiratory rate and tidal volume. Increase or decrease in either of the two changes ventilation. In cases of hypoxia, the body tends to hyperventilate, and thus the partial pressure of CO2 reduces as more gets expelled from the body. The causes of hypoxia can be divided into the following categories[13]:

  • Central: Head injuries, stroke, hyperthyroidism, anxiety, pain, fear, stress, drugs, medication induced and toxin-induced
  • Pulmonary causes: Embolism, pneumothorax, pneumonia, acute asthma or COPD exacerbations
  • Iatrogenic causes primarily occur in intubated patients on mechanical ventilation

While ventilation is measured best by looking at CO2 partial pressures, oxygenation is measured by looking at oxygen partial pressures. In the clinical setting, the measurement of the amount of oxygen saturation in blood is via pulse oximeters. They provide a warning about the presence of hypoxemia to patients. The basis of pulse oximetry is on two principles: (1) presence of a pulsatile signal that is generated by the arterial blood in the finger and (2) the different wavelengths generated by oxyhemoglobin and reduced hemoglobin.

However, oxygenation and ventilation are 2 separate things. While pulse oximeter provides accurate measurement of oxygen saturation, it does not provide any information about alveolar ventilation. Oxygen saturation is considered a moderately later sign of less-than-desired ventilation; hence, despite providing visual quantification of oxygen saturation, pulse oximetry does not provide real-time assessment of alveolar ventilation. The best example of this principle is seen in conditions of sedation, which depresses the central respiratory drive to arterial carbon dioxide tension, resulting in arterial hypoxemia accompanied by alveolar hypoventilation. Although pulse oximetry is currently the standard of care for determining accurate oxygen saturation, it lacks the potential to provide early warning to detect hypoventilation, apnea or airway obstruction in patients.

Oxygenation involves inhaling O2, diffusing it through the alveolocapillary membrane into the blood that will supply peripheral tissues. Ventilation consists of the exchange of inspired and expired gases from the lungs, thereby involving the exchange of both oxygen and CO2. To measure expired CO2, it is essential to have sufficient circulation to facilitate transport of CO2 to the lungs and out through the mouth. Evidence-based literature has found that capnography is known to be a better method at evaluation of ventilation in patients, with higher sensitivity at detecting apneic episodes than pulse oximetry. It monitors the end tidal volume of carbon dioxide, which is more sensitive to alveolar hypoventilation than SpO2.

The waveform of capnography divides into 4 phases. Phase I, II, and III occur during expiration, while phase IV occurs during inspiration. Phase I occurs during exhalation of air from anatomical dead space which contains no CO2. Phase II represents a steep upward slop indicating CO2 from the alveoli reaching the upper airway that is detectable in exhaled air. Phase III indicates the rick alveolar gas full of CO2 that now constitutes the majority of exhaled air. End of phase III is the end of exhalation, end-tidal CO2 volume, which contains the highest amount of CO2. Normal EtCO2 is 35 to 45 mm Hg. Phase IV is when inhalation begins, and the level of CO2 begins to drop rapidly.[14]

Capnography can measure ventilation via measuring EtCO2. However, it does not provide a measure of any form of oxygenation. Conversely, pulse oximetry measures arterial oxygen saturation but is unable to provide a measure of alveolar hypo or hyperventilation. Capnography will diagnose hypoventilation long before the latter results in hypoxia, and this is especially the case in patients receiving supplemental oxygen in ICU and anesthesia. In hypoventilation, tall (high PCO2) low-frequency waves manifest with a well-defined alveolar plateau. In hyperventilation, short (low PCO2) high-frequency waves with a well-defined alveolar plateau are seen.[15]

In conditions like an obstructed airway, weakened respiratory muscle, or lung dysfunctions, abnormalities can be detected immediately from the CO2 waveform and PCO2. In pulse oximetry, since oxygen saturation reaches 100% during sedation and general anesthesia, changes in respiration are not detected unless the SpO2 falls below 100% in spite of the fall in PaO for some reason. For example, in the case when respiration stops while maintaining the SpO2 at 100%, the fall in SpO2 below 100% only occurs after 4 to 5 minutes. With capnography, CO2 waveform ceases as soon as apnea occurs. Additionally, in cases of deep sedation where there is an increased risk of airway obstruction, CO2 monitoring provides early detection of airway obstruction.[16]

In conclusion, pulse oximetry and capnography individually offer two useful non-invasive techniques to facilitate respiratory monitoring in clinical settings. A proper grasp of the benefits and limitations of each of the techniques can significantly aid in patient management and safety.[17]

Enhancing Healthcare Team Outcomes

an interprofessional approach is essential for a patient, especially when undergoing conscious sedation for a procedure like a bronchoscopy, endoscopy, cardiac stent placement, etc. Patients under sedation for these procedures generally hypo-ventilate and are usually placed on high levels of oxygen to prevent desaturations. The patient care can be compromised if the anesthesia team (CRNA or anesthesiologist) does not monitor the pulse oximetry and capnography simultaneously. 


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Capnography And Pulse Oximetry - Questions

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A 52-year-old patient is brought into the emergency department after a gunshot wound trauma. He is taken to the operating room for surgery and intubated with an endotracheal tube. Temperature is 38.1 C, blood pressure is 130/90mmHg, heart rate is 80b/min, PaO2 is 98%, and PaCO2 is 40 mmHg. After 3 minutes, PaO2 starts to decline rapidly. Which of the following best represents the capnography waveform for this patient?

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    Contributed by Nirzari Pandya
Attributed To: Contributed by Nirzari Pandya



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Capnography has an essential role to play in cardiopulmonary resuscitation and emergency cardiovascular care in the pre- and post-hospital environment. What are some indications for its use? Select all that apply.



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A 62-year-old male comes to the emergency department with complaints of left-sided chest pain radiating to his jaw and left arm. The pain started 2 hours back, gets worse with activity, and is relieved with rest. Associated symptoms include shortness of breath, nausea, and excessive sweating. Vital signs upon arrival were 160/92 mmHg, pulse oximetry 94% on room air, heart rate 110/minute, temperature 97.2 F. Physical examination reveals that he is in pain and distress, lungs are clear, and he is tachycardic. The rest of the examination is unremarkable. While EKG and other labs are ordered he goes into the ventricular fibrillation, a code blue is called, and a high-quality CPR is started. During the CPR he was intubated. Which of the following value of end-tidal CO2 on capnography is expected with high-quality CPR and after the restoration of spontaneous circulation is achieved?



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A 56-year-old female patient presents to the emergency department with shortness of breath on exertion. Her temperature is 37 C, heart rate of 124 beats/min, blood pressure is 130/92 mmHg, and respiratory rate 32 breaths per minute. She is a frequent flyer to Japan for business purposes. She smokes one pack of cigarettes per day and drinks occasionally. Physical examination is normal. A chest x-ray is obtained and is normal. Which of the following represents the most accurate pulse oximetry and capnography findings in this patient?



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A 62-year-old obese man with a past medical history of COPD and obstructive sleep apnea undergoes carotid endarterectomy. On the second postoperative day, his temperature is 37.7 C, blood pressure is 120/90 mmHg, the pulse is 90/min, and respirations are 14/min. Oxygen saturation is 89% on room air. His arterial blood gas results show that his arterial pH is 7.28, PaO2 is 60 mmHg, PaCO2 is 61mmHg and his HCO3 is 28 mEq/L. Which of the following is most likely capnography waveform for this patient?

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A 62-year-old male presented to the hospital with extreme shortness of breath. Vital signs revealed a blood pressure of 160/80mmHg, respiratory rate of 32b/min, afebrile and pulse of 120/min. On physical examination, he was found to use accessory muscles, was extremely short of breath, and lung auscultation revealed extensive exploratory wheezing. Laboratory work-up was mostly normal except the arterial blood gas, which showed a pH of 7.28, CO2 of 60mmHg and partial pressure of oxygen 48mmHg. He was placed on 8 liters nasal cannula oxygen which improved his pulse oximetry. He was admitted to the telemetry floor. Two hours later a rapid response is called, and the patient was found to be obtunded. He was immediately intubated and transferred to ICU. After intubation, a capnography and pulse oximetry are taken. What is the expected tracing value of carbon dioxide on the capnography tracings?



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Capnography And Pulse Oximetry - References

References

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Hara N,Tanaka T,Minami T, [Anesthetic circle system failure caused by a plastic film--a case report]. Masui. The Japanese journal of anesthesiology. 2006 Feb     [PubMed]
Ortega R,Connor C,Kim S,Djang R,Patel K, Monitoring ventilation with capnography. The New England journal of medicine. 2012 Nov 8     [PubMed]
Cacho G,Pérez-Calle JL,Barbado A,Lledó JL,Ojea R,Fernández-Rodríguez CM, Capnography is superior to pulse oximetry for the detection of respiratory depression during colonoscopy. Revista espanola de enfermedades digestivas : organo oficial de la Sociedad Espanola de Patologia Digestiva. 2010 Feb     [PubMed]
Gallagher JJ, Capnography Monitoring During Procedural Sedation and Analgesia. AACN advanced critical care. 2018 Winter     [PubMed]

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