Breathin’ and Breathin’...2 and PaO 2 •In general the partial pressure of oxygen in the blood...

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Just Keep Breathin’ and

Breathin’ and Breathin’:

Respiratory Monitors –

Monitoring OxygenationBrandon Wahler, DVM, MS, Practice Limited to Anesthesia


Goals

Describe the difference between oxygenation and

ventilation and how we can monitor each

Describe the way a pulse oximeter works

Describe the oxyhemoglobin dissociation curve and factors

that will influence it

Outside of SpO2 values, what other information can we

gather from the pulse oximeter?

What factors may cause abnormal pulse oximeter

readings?

What literature is present about the use of pulse oximeters in veterinary

medicine, and what role should the pulse oximeter play in our evaluation of a patient?


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


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.)


Role of Oxygen in the Body

https://quizlet.com/271984388/cellular-respiration-diagram/

The major player! Needs the NADH

Two part cycle – Generates a tiny amount of energy (ATP)

Produces H2O and CO2


Anaerobic Metabolism

https://myheart.net/lactic-acid-2/how-is-lactic-acid-produced/

Minimal generation of ATP

Produces lactic acid which must be cleared

This is the reason why lactate is measured to look at profusion –

inadequate oxygen delivery to tissues


Hemoglobin

• How do we get oxygen to the tissues?

https://openstax.org/books/anatomy-and-physiology/pages/18-3-erythrocytes

4 heme molecules = 4

oxygen molecules that can be bound

Each RBC can contain up to 300 million

hemoglobin molecules and

therefore bind 1.2 billion oxygen

molecules


Carrying Capacity for Oxygen

http://rc.rcjournal.com/content/62/6/645

The amount of oxygen that can be carried by hemoglobin is much greater than what is

dissolved in blood; this is why anemia is a big deal


Can You Keep Up?

http://www.learnpicu.com/respiratory/oxygenation-oxygen-transport

SaO2 = Saturated percentage of

hemoglobin in the arterial blood

Challenging to measure and

requires a centrally placed

catheter (central line) Challenging to prove, requires

samples before and after tissue; increased levels can be

indicative of shock

Challenging to prove; blood

pressure can be used as an indirect

measurement


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


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/app-notes/index.mvp/id/4671/


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


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/


Types of Pulse Oximeters

• Transmission Pulse Oximeters• “Classic” pulse oximeters

• Clamp onto tissue

• Light is passed through tissues

• Common in veterinary medicine

• Reflectance Pulse Oximeters• Flat probes

• Relies on light that is reflected due to differences in reflective indices of tissue


Pulse Oximeter Units

Probe is connected to the console by a cable, with most cables being specific to the console (cannot interchange

them)

Must make sure the probe is

appropriately sized, if too large then SpO2 is

falsely high

SpO2 values and pulse rates are normally

averaged (arrhythmias make them unreliable)

Pulse amplitude or plethysmograph

should be evaluated to determine accuracyMost pulse oximeters

provide an audible tone which changes with

saturation and have built in alarms to determine

low pulse rates, and saturation levels


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-oximeter


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)


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


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


When Do Problems Occur?

• Generally SpO2 values are between 97-100% • PaO2 = 85-110mmHg

• Technical hypoxemia is around 95%• PaO2 = 80mmHg

• Problems begin around 90-93% SpO2 values• PaO2 = 60mmHg

• Remember pulse oximeters are programmed for people

• AKA Dogs can have slightly lower pulse oximeter readings than people and have the same levels of oxygen, but large animals tend to be more hypoxemic at lower saturations

• Critical point ~93%• Large changes with only minimal changes in saturation

• Most accurate between 80-95%

Critical area because things begin to trend down quickly, oftentimes a

delay between reading and

patient levels


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


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

PaO2 = 21% x 5

PaO2 = 105mmHg

Or

PaO2 = 100% x 5

PaO2 = 500mmHg

Room Air

Intubated on 100% Oxygen

Easy, but not always realistic

No good way to discern why things are wrong


A-a Gradient Calculation

• Other ways to determine if the amount of oxygen in the blood is considered normal

• A-a (Alveolar-arterial) gradient = Alveolar oxygen content –arterial blood content

• Fancy math…

• Normally between 5-15

https://www.criticalcarepractitioner.co.uk/mechanical-ventilation-series-a-a-gradient/


Hypoxemia vs Hypoxia

Name Definition SpO2 a good measure? Categories

Hypoxemia Specific; low oxygen tension in the blood (PaO2

is low)

Yes Low FiO2, Hypoventilation, Venous Admixture

Hypoxia General; low oxygen to the tissues

Not necessarily Hypoxemia, Anemia, Stagnant, Histotoxic, Metabolic

Remember it is the partial pressure of oxygen in the arterial blood that determines how much oxygen is delivered to the tissues, but the concentration of oxygen on the hemoglobin is what buffers this as more and more oxygen is delivered during circulation…


Low Fio2

• Rarely an issue in veterinary medicine unless you are practicing on Mount Everest or you forget to turn on your oxygen tank/run out of oxygen in your tank

• Hypoxemia shows good response to oxygen therapy

• P (A-a) O2 is normal.

• PaCO2 is usually normal.

https://www.britannica.com/place/Mount-Everest

http://www.engaugeinc.net/oxygen-tank-violations


Diffusion Impairment

• Somewhat rare form of hypoxemia under anesthesia, but common in ER…

• Inflammation

• Fibrosis

• Decrease lung surface area for gas exchange

• Normally clues from previous diagnostics, physical examination, or history

• Not much can be done under anesthesia, but if not receiving supplemental oxygen then this will benefit them


• P (A-a) O2 is elevated

• PaCO2 is usually normal.

http://www.caninepulmonaryfibrosis.ulg.ac.be/about-ipf/

https://www.cliniciansbrief.com/article/common-pulmonary-diseases-dogs


Right-to-left Shunt

• Still a relatively rare cause of hypoxemia, but can be occasionally encountered

• Blood moves from the right side of the heart to the left side of the heart without going through the lungs

• Skips oxygenation step

• Primarily a concern in patients with cardiac disease (especially congenital disease)

• P (A-a) O2 is elevated

• Poor response to oxygen therapy

• PCO2 is normal.

http://drainameducci.blogspot.com/2012/03/tetralogy-of-fallot-tof.html


Hypoventilation

• Very common under anesthesia, and relatively common in medicine in general


• P(A-a)O2 is usually normal

• PaCO2 is high

• PaO2 and PaCO2 move in opposite direction to the same extent.

https://www.veterinarypracticenews.com/16483-2/2/

Impaired central driveDrug overdose: Opioids, benzodiazepines, alcoholBrainstem hemorrhage, infarctionPrimary alveolar hypoventilation

Spinal cord injuryNerve supplying respiratory muscleNeuromuscular junction: Myasthenia gravisRespiratory muscles: MyopathyDefects in chest wall


V/Q Mismatch

• Very common condition in both awake and anesthetized patients

• Responses are based on the type of mismatch and the degree

• Hypoxemia due to V/Q mismatch can be easily corrected by supplemental oxygen therapy

• Widened A-a oxygen gradient is another feature of V/Q mismatch.

• In Low V/Q relationships there is a high probability of hypoxemia and increased ETCO2-PaCO2 gradients are seen at high shunt fractions

• High V/Q relationships result in large ETCO2-PaCO2

gradients, but aren’t as likely to result in initial hypoxemia

https://www.pinterest.com/pin/517632550919924120/


Causes of Hypoxia - Hypoxemia

• Hypoxemic Hypoxia• See above

• Mostly oxygen responsive


Anemic Hypoxia

• Anemia results in inability to deliver adequate volumes of oxygen to the target tissue.

• Remember our equation:

• This is why anemic patients don’t benefit greatly from oxygen therapy

• Also includes dyshemoglobins(hemoglobin varieties that can’t bind oxygen or hemoglobin that binds to something else so oxygen can’t) https://www.researchgate.net/figure/Effects-of-hemoglobin-concentration-and-pH-CO2-2-3-DPG-and-temperature-on-blood-oxygen_fig1_258857315


Stagnant Hypoxia

• Stagnant blood flow reduces the ability to get oxygen to peripheral tissues

• Hypoxia is not due to an inability to get oxygen into the blood and bound to hemoglobin

• Improved with increases to cardiac output or relief of obstruction, and minimally helped with oxygen therapy

https://www.slideshare.net/drchintansinh/hypoxia-43375114


Histotoxic Hypoxia

• Histotoxic hypoxia is caused by a toxin or tissue adulterant that reduces the ability for the tissue to absorb or utilize the oxygen, even if it is being adequately delivered by the blood

• Cyanide toxicity

• Not oxygen responsive


Metabolic Hypoxia

• Metabolic hypoxia is likely secondary to inadequate oxygen compared to an increased demand, or to metabolic derangements at the cellular level which leads to an inability to use the oxygen that is available

https://www.topneurodocs.com/sepsis/


Hypoxia/hypoxemia vs CyanosisBlue is bad, but pink isn’t always good

Requires 5g/dL of deoxyhemoglobin to see cyanosis (some people are better at this than others)

If your patient is anemic, they may never become cyanotic until they are dead (PCV < 15%)

http://animalhospitalofcampbellcounty.com/pet-education-canine-anemia/


When to Use a Pulse Oximeter

• It’s never wrong to put one on…

• Commonly employed in the anesthetized and patients in anesthetic recovery

• Desaturation very common following induction and recovery of anesthesia

• Frequent desaturation in maintenance of anesthesia, but the changes aren’t normally as severe

• While does not tell you where the problem is occurring it clues you into an issue surrounding the patient’s lungs, the content of their blood, the tissue blood flow, or the anesthetic circuit/endotracheal tube


When to Use a Pulse Oximeter

• When transporting patients that are sedated/anesthetized around the hospital

• CPCR for respiratory arrest

• Seizure patients with risk of hypoxic cerebral damage

• Ventilator patients to determine lowest levels of oxygen needed

• Goal-directed fluid therapy


Endobronchial Intubation

• Used to help determine if bronchial intubation has occurred

• Oftentimes patients are hypoxemic and compliance of the lung during assisted ventilation is reduced

• Patients oftentimes seem to be “awake” while on inhalant, and there is no improvement in oxygenation or anesthetic depth with increases in assisted ventilation

• Carbon dioxide levels may vary…

https://www.openveterinaryjournal.com/OVJ-2017-11-213%20D.M.%20White%20et%20al.pdf


Unique Uses

• Determining peripheral circulation• Areas where the pulse oximeter has a

weak wave form or perfusion index may be indicative of poor circulation, vasoconstriction, or hypotension

• Sympathetic nervous blockade• Possible perfusion index is associated

with vasodilation following blockade with local anesthetics

• Determine which areas to amputate or do bypass surgery


Unique Uses

• Determining systolic blood pressure

• Locating arteries

• Monitoring for arrhythmia

• Monitoring for hypovolemia

https://www.masimo.com/technology/co-oximetry/pvi/?6qqiu-dp1gu-rj2ath8=&_v=2http://www.pef.uni-lj.si/eprolab/comlab/sttop/sttop-bm/bm-optical.htm

www.amperordirect.com/pc/help-pulse-oximeter/z-what-is-oximeter-plethysmograph.html


Literature

• Originally described as a method to get blood pressure in people• Cuff on arm proximal to the pulse oximeter• Inflate cuff slowly until the plethysmograph is gone• Deflate slowly until the plethysmograph comes back• Average the two numbers

• Similar to accuracy of other indirect methods compared to direct methods (i.e. not that great)

• Could be used to follow SAP trends


Inaccuracy of Pulse Oximetry

• Most accurate between 80-95% saturation

• Based on company algorithm

• Motion artifact

• Signal-to-noise ratio

• Skin pigment

• Thickness of tissue

• Smaller pulsatile blood pattern

• Electrical or optical interference

• Severe desaturation

• Severe anemia

• Severe vasoconstriction

• Cannot determine oxygen partial pressures >110mmHg

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Inaccuracy of Pulse Oximetry –Intrinsic Factors

• Carboxyhemoglobin

• Carbon monoxide binds to hemoglobin and displaces oxygen

• Falsely reads as oxyhemoglobin

• Saturation is read as falsely high

• Sulfahemoglobin

• Very rare

• Metoclopramide, Zonisamide, Sulfonamides

• Persistent saturation approximately 85%

• Methemobglobin

• Unable to bind oxygen

• Can be drug induced

• Makes the blood look a blue-brown color

• 85%

• Indocyanine Green and Methylene Blue

• Artificially lower pulse oximeter readings

https://www.pftforum.com/blog/cohb-and-pulse-oximetry/


Literature

• SaO2 tended to be reflected in SpO2 when >70%

• The position of the probe likely matters

• Probe type matters

• CO2 may influence how accurate the approximation of SpO2 to SaO2 (over estimates in hypercapnia)


Inaccuracy of Pulse Oximetry - People

• Increased risk of failure with increased ASA status (III, IV, V)

• Young and elderly patients

• Orthopedic/vascular/cardiac surgery/electrosurgery

• Hypothermia, hypotension, hypertension

• Duration of intraoperative procedure

• Chronic renal failure, low hematocrit


Ensuring Accurate Measures

• Make sure displayed pulse rate is the same as patient’s pulse

• Try giving oxygen• Oxygen responsive?

• Decreases on room air once taken off?

• Try different tissue beds • Poor blood flow

• Vasoconstriction

• Crushed capillaries


Tips and Tricks

• Choose the right probe for the tissue you are looking at

• Make sure to give it some time to get an average measurement

• If there isn’t a waveform or a good index of perfusion…don’t believe it

• If the measurement isn’t believable then move it to different spots

• Highest number is likely real

• Water isn’t useful

• Likely just moving it

• Maybe vasodilating the arterial bedshttps://mammothmemory.net/physics/refraction/refraction--water-or-glass-to-air/refraction-water-or-glass-to-air.html


Literature

• 1175 closed malpractice claims were examined

• 31.5% of negative outcomes could be prevented by additional monitoring devices

• The monitors determined to be most helpful were pulse oximeters plus capnography

• Together these could have prevented 93% of mishaps


Literature

• Very important paper written about anesthetic-related death in small animal practice

• Certain things found to increase mortality in cats undergoing anesthesia

• Cats were found to have a decrease in mortality when their pulse was monitored and the use of a pulse oximeter was used for monitoring


Summary

• Describe the difference between oxygenation and ventilation and how we can monitor each• Oxygenation is the ability to monitor the amount of oxygen present in the blood that occurs as a

result of respiration. Ventilation is a measurement of the amount of carbon dioxide in the blood during the same process. Oxygenation can be monitored in several ways including pulse oximetry and arterial blood gases. Ventilation is monitored with capnography (discussed later) and arterial blood gases

• Describe the way a pulse oximeter works• The pulse oximeter uses a non-invasive light source to determine the amount of oxyhemoglobin

and deoxyhemoglobin and uses a computer to determine what percentage of the hemoglobin is saturated with oxygen

• Describe the oxyhemoglobin dissociation curve and factors that will influence it• The oxyhemoglobin dissociation curve is a mathematical model to describe the amount of oxygen

in the blood compared to the saturation of available hemoglobin (PaO2 versus SaO2). The curve is sigmoidal in shape and has a critical point at approximately 93% SaO2 where the curve becomes linear and starts to decline. Using this relationship we can use the pulse oximeter to approximate the amount of oxygen in the blood. Chronic changes in pH, temperature, species, and others can influence this curve and hemoglobin’s affinity for oxygen


Summary

• Outside of SpO2 values, what other information can we gather from the pulse oximeter? • The pulse oximeter can be used to indirectly determine things such as: blood pressure, tissue

viability and perfusion, arrhythmias, accurate heart rates, and location of arteries. It also has potential roles in ability to determine if patients are fluid responsive, in determining if there is reduction in sympathetic tone following local blocks, and determining if a patient is endobronchial intubated

• What factors may cause abnormal pulse oximeter readings?• Many things may influence pulse oximeter readings, but common causes include: motion,

hypothermia, dyshemoglobinemia, hypercapnia, tissue thickness, and tissue pigment

• What literature is present about the use of pulse oximeters in veterinary medicine, and what role should the pulse oximeter play in our evaluation of a patient? • The role of pulse oximeters in veterinary medicine is somewhat sparse. There is evidence to say

they are fairly accurate, but are bound by species limitations. There are several studies in human medicine that have shown that pulse oximeters can, and should, be used in anesthetic cases to reduce morbidity and mortality, and that it is one of the most important monitors (especially when used in conjunction with capnography). There is some evidence from veterinary medicine that pulse oximeters may reduce anesthetic mortality in cats.


Questions?

https://www.machinedesign.com/community/article/21836908/what-questions-should-you-ask-during-the-product-lifecycle


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Breathin’ and Breathin’...2 and PaO 2 •In general the partial pressure of oxygen in the blood...

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