Monitoring Oxygenation Brandon Wahler, DVM, MS, Practice Limited to Anesthesia
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Just Keep Breathin’ and Breathin’ and Breathin’: Respiratory Monitors – Monitoring Oxygenation Brandon Wahler, DVM, MS, Practice Limited to Anesthesia Copyright © 2020 Care Center. All rights reserved. Goals Describe the difference Describe the oxyhemoglobin between oxygenation and Describe the way a pulse dissociation curve and factors ventilation and how we can oximeter works that will influence it monitor each What literature is present Outside of SpO values, what about the use of pulse 2 What factors may cause other information can we oximeters in veterinary abnormal pulse oximeter gather from the pulse medicine, and what role readings? oximeter? should the pulse oximeter play in our evaluation of a patient? Copyright © 2020 Care Center. All rights reserved. Take My Breath Away… • Remember that while respiration is technically just breathing in and out… http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/henry.html http://www.chemistry.wustl.edu/~edudev/LabTutorials/CourseTutorials/Tutorials/Hemoglobin/conformation.htm https://www.ck12.org/book/skills-for-a-healthy-you/section/3.1/ http://people.eku.edu/ritchisong/301notes6.htm https://www.researchgate.net/figure/Schematic-presentation-of-tissue-oxygen-metabolism-at-the-cellular-and-intramitochondrial_fig4_51460198 Copyright © 2020 Care Center. All rights reserved. Ventilation vs. Oxygenation • Must be specific when describing the problems the patient is having! • Is the problem with Oxygenation (O2 problems that are measured by pulse oximetry or an arterial blood gas) or is the problem with Ventilation (CO2 problems measured by ETCO2/Capnography and arterial or venous blood gas measurements)? • These problems are oftentimes influenced by one another, but are independent issues • You may have problems that are deal with the respiratory system that are not necessarily issues with either (tachypnea, bradypnea, etc.) Copyright © 2020 Care Center. All rights reserved. Role of Oxygen in the Body The major player! Needs the NADH Produces H2O and CO2 https://quizlet.com/271984388/cellular-respiration-diagram/ Two part cycle – Generates a tiny amount of energy (ATP) Copyright © 2020 Care Center. All rights reserved. Anaerobic Metabolism Minimal generation of ATP This is the reason why lactate is Produces lactic acid measured to look at profusion – which must be cleared inadequate oxygen delivery to tissues https://myheart.net/lactic-acid-2/how-is-lactic-acid-produced/ Copyright © 2020 Care Center. All rights reserved. Hemoglobin • How do we get oxygen to the tissues? 4 heme Each RBC can contain molecules = 4 up to 300 million oxygen hemoglobin molecules that molecules and can be bound therefore bind 1.2 billion oxygen molecules https://openstax.org/books/anatomy-and-physiology/pages/18-3-erythrocytes Copyright © 2020 Care Center. All rights reserved. Carrying Capacity for Oxygen The amount of oxygen that can be carried by hemoglobin is much greater than what is dissolved in blood; this is why http://rc.rcjournal.com/content/62/6/645 anemia is a big deal Copyright © 2020 Care Center. All rights reserved. Can You Keep Up? SaO2 = Saturated percentage of hemoglobin in the arterial blood Challenging to Challenging to measure and prove; blood requires a pressure can be centrally used as an indirect placed measurement catheter (central line) Challenging to prove, requires samples before and after tissue; increased levels can be indicative of shock http://www.learnpicu.com/respiratory/oxygenation-oxygen-transport Copyright © 2020 Care Center. All rights reserved. Measuring FiO2 • The first step is understanding how much oxygen we are breathing in • Normally around 21% (0.21) oxygen in atmospheric air (the majority is nitrogen) • Under anesthesia or in cases where supplemental oxygen is being supplied this is oftentimes higher: 30-100% • FiO2 can be measured with different sensors • Oftentimes are stand-alone sensors to measure supplemental oxygen % • Not normally measured in veterinary anesthesia in general practice (assumed to be 100%) • Required in human medicine • Can be useful for detecting leaks and hypoventilation Copyright © 2020 Care Center. All rights reserved. Measuring Oxygen to the Tissues • We can indirectly measure this with pulse oximetry • Measures SpO2 which is the hemoglobin saturation of pulsatile blood flow in peripheral tissues • Different from SaO2 (requires a special machine) • Assumed to be similar to SaO2 in healthy patients https://www.maximintegrated.com/en/appCopyright-notes/index.mvp/id/4671/ © 2020 Care Center. All rights reserved. How Does It Work? • Differential absorption between red and infrared spectrum • Deoxyhemoglobin absorbs more light in the red band than oxyhemoglobin • Oxyhemoglobin absorbs more light in the infrared spectrum than deoxyhemoglobin • The pulse oximeter computes the difference and presents the percentage of hemoglobin that is oxygenated based on an algorithm Copyright © 2020 Care Center. All rights reserved. The pulse oximeter pulses the LEDs (red and infrared) on and off hundreds of times per second which allows it to look at peaks and troughs. The trough represents light transmitted through the capillary beds and venous blood The peak represents the light as it travels through this tissue and arterial blood The sensor turns “off” to look for background light and subtracts this from the other measurements http://newblog.tunstallhealthcare.com.au/connected-health/what-is-a-pulse-oximeter/ Copyright © 2020 Care Center. All rights reserved. Types of Pulse Oximeters • Transmission Pulse Oximeters • Reflectance Pulse Oximeters • “Classic” pulse oximeters • Flat probes • Clamp onto tissue • Relies on light that is reflected due • Light is passed through tissues to differences in reflective indices • Common in veterinary medicine of tissue Copyright © 2020 Care Center. All rights reserved. Probe is connected to SpO2 values and pulse the console by a cable, rates are normally with most cables being averaged (arrhythmias specific to the console make them unreliable) (cannot interchange them) Pulse amplitude or plethysmograph should be evaluated to Most pulse oximeters determine accuracy provide an audible tone which changes with saturation and have built in alarms to determine Must make sure the low pulse rates, and probe is saturation levels appropriately sized, if too large then SpO2 is falsely high Pulse Oximeter Units Copyright © 2020 Care Center. All rights reserved. Benefits of Pulse Oximetry • Known to be accurate (highest accuracy between 80- 95%) • Accuracy does not change with time • Not impacted by anesthetic gases or vapors • Fast response times • Noninvasive • Continuous measurements • Measurement of peripheral pulse quality • Light weight and compact • User-friendly • Reusable, affordable https://getwellue.com/products/pediatric-finger-pulse-oximeterCopyright © 2020 Care Center. All rights reserved. Literature • Pulse oximeter was ranked first in this study to determine monitor detected incidents • Front-line monitor for endobronchial intubation • Good backup monitor for monitors that would normally be used to detect incidents (capnography, ECG, etc.) • Found to be useful in circuit leak or disconnection • Of 1256 general anesthesia incidents 82% would have been detected by pulse oximetry alone if they had been allowed to play out (60% before damage to organs) Copyright © 2020 Care Center. All rights reserved. Literature • Looked at pulse oximetry of transmittance and reflectance pulse oximeters in 10 anesthetized dogs who they made hypoxemic and hypotensive • Found to provide accurate SpO2 measurements over a large range of MAP. • The monitors overestimate SaO2 when arterial saturation <70% • This is a common issue in most pulse oximeters Copyright © 2020 Care Center. All rights reserved. Oxyhemoglobin Dissociation Curve • Hemoglobin must be able to load and unload oxygen • Predictable based on relationship between oxygen tension and oxygen binding • Allows us to use SpO2 to predict how much oxygen is present at the tissues • Hypoxemia PaO2<80mmHg • Severe Hypoxemia PaO2<60mmHg • Hyperoxemia PaO2>110mmHg Copyright © 2020 Care Center. All rights reserved. When Do Problems Occur? • Generally SpO2 values are between 97-100% • PaO2 = 85-110mmHg Critical area • Technical hypoxemia is around 95% • because things PaO2 = 80mmHg begin to trend • Problems begin around 90-93% SpO2 values down quickly, • PaO2 = 60mmHg oftentimes a • Remember pulse oximeters are programmed for delay between people reading and • AKA Dogs can have slightly lower pulse oximeter readings than people and have the same levels of oxygen, but large animals patient levels tend to be more hypoxemic at lower saturations • Critical point ~93% • Large changes with only minimal changes in saturation • Most accurate between 80-95% Copyright © 2020 Care Center. All rights reserved. Shifting Oxyhemoglobin Dissociation Curve ↑ C – carbon dioxide ↑ A – acidosis ↑ D – 2,3-DPG (anemia) ↑ E – Exercise ↑ T – Temperature ↑ S – Species Face Right More likely to offload oxygen Copyright © 2020 Care Center. All rights reserved. FiO2 and PaO2 • In general the partial pressure of oxygen in the blood should be approximately 5 times the concentration of oxygen you are breathing in PaO = 21% x 5 2 Room Air Easy, but not always PaO2 = 105mmHg realistic Or No good way to discern why things are wrong PaO2 = 100% x 5 Intubated on 100% Oxygen PaO2 = 500mmHg Copyright © 2020 Care Center. All rights reserved. • Other ways to determine if the amount of oxygen in the blood is considered