Pulse Oximetry Flashcards
How many hemoglobin molecules in one RBC?
There are ≈300 million hemoglobin molecules inside each red blood cell (RBC)
Each of the 4 subunits can bind one oxygen molecule, which means that each hemoglobin can bine 4 oxygen molecules, which means that each RBC can contain over 1 billion O2 molecules!
What is hematocrit? Normal male/female values?
Hematocrit refers to the PERCENTAGE of blood that is red blood cells
–Normal male Hct ≈ 45%
–Normal female Hct ≈ 39%
What is in blood?
- Plasma
- White blood cells/thrombocytes
- Red blood cells (RBCs)
What does hemoglobin and hematocrit look like on a lab test?
On lab results, hematocrit is a percentage, and hemoglobin is a concentration
–The hematocrit is usually ≈ 3x the Hb
How can you define anemia?
Anemia can be defined as a decrease in hemoglobin and/or a decrease in hematocrit
What are causes of anemia?
1. Blood loss
–Because hemoglobin molecules are lost
2. Fluid administration
–Because it dilutes the hemoglobin concentration
3. Lysed RBCs
–Because hemoglobin is destroyed
4. Decreased production of RBCs
–Happens with decreased erythropoietin (EPO) production, which is common in patients with renal insufficiency/failure
What are PAO2, PaO2, & SaO2
PAO2 refers to the partial pressure of oxygen in the ALVEOLI
PaO2 refers to the partial pressure of oxygen in the ARTERIES
SaO2 refers to the percentage of hemoglobin that is saturated with oxygen
What determines PA02?
- FiO2
High FiO2 = High PAO2; low FiO2 = Low PAO2 - Barometric pressure
- High elevation (low barometric pressure) = lower PAO2
- Low elevation (high barometric pressure) = higher PAO2
What is normal PA02?
–70-100mmHg on room air
≈5x the FiO2 in healthy patients
What decreases PA02?
PAO2 decreases with age
How does PA02 affect Pa02?
The higher the PAO2, the higher the rate of diffusion into the blood, and the higher the PaO2
What is the A-a gradient? What are normal values?
The difference in PAO2 and PaO2 is referred to as the A-a gradient
A normal A-a gradient is:
5-15mmHg on room air
10-110mmHg on 100% FiO2
What can increase (widen) the A-a gradient?
If patients have lung disease, oxygen will not have a problem getting into the alveoli (PAO2 shouldn’t change much), but the amount of oxygen that is able to get into the blood will be decreased (because of impaired alveolar capillary diffusion), which means PaO2 will be decreased
How are Pa02 and Sa02 related?
SaO2 is PRIMARILY determined by PaO2
–The analogy is that hemoglobin is like a sponge
It soaks up oxygen from the blood until it is maximally saturated (100%)
PaO2 of 60mmHg =
SaO2 of 90%
“Below a PaO2 of 60mmHg, cerebral blood flow increases rapidly.”
PaO2 of 27mmHg =
SaO2 of 50% (in adults)
SaO2 can ALSO (but to a lesser degree) be affected by these 7 other things:
- pH
- CO2
- Temperature
- Anemia
- 2,3 DPG
- Carboxyhemoglobin levels
- Methemoglobin levels
All of these factors alter the way hemoglobin “soaks up” oxygen
How does supplementary 02 affect Sa02?
- Supplementary oxygen increases PAO2
- The increased PAO2 leads to greater diffusion of oxygen into the blood, leading to increased PaO2
- The increased PaO2 allows hemoglobin to soak up more oxygen, which leads to an increased SaO2
What does a right shift on a Hb02 curve show you?
A right shift of this curve means that a higher PaO2 is required in order to achieve the same SaO2
-This means that the hemoglobin isn’t holding on to the oxygen as well, which means that there is better oxygen delivery to the tissues
What can cause a right shift?
- Acidosis
–Increased CO2 (hypoventilation) and/or decreased pH - Increased 2,3-DPG
- Anemia
- Hyperthermia
What can cause a left shift on a Hb02 curve?
A left shift of this curve means that it doesn’t require as high of a PaO2 in order to achieve the same SaO2
This means that the hemoglobin is more affinitive for oxygen (is holding onto it more tightly), which means that there will be worse oxygen delivery to the tissues
What can cause a left shift (worse 02 delivery)?
- Alkalosis
- Decreased CO2 (hyperventilation) and/or increased pH - Decreased 2,3 DPG
- Hypothermia
- Methemoglobinemia (MetHb)
- Carboxyhemoglobinemia (COHb)
What does P50 on a Hb02 curve tell you?
The “P50” on the HbO2 curve represents that PaO2 which will cause an SaO2 of 50%
Hemoglobin Pa02 of a sickle cell anemia patient (HbS)
31mmHg
Hemoglobin Pa02 of a pregnant mother hemoglobin at term
30mmH20
Fetal Pa02 Hemoglobin
19mmHg
Hemoglobin Pa02 in packed red blood cells (PRBCs) at the end of storage
18mmHg
This is caused by depleted 2,3 DPG
How would you get a direct measurement of Sa02?
In order to get a directmeasurement of SaO2, we have to draw an arterial blood sample (arterial blood gas/ABG)
What does pulse oximetry “indirectly” measure?
A pulse oximeter can measure both SpO2 AND heart rate (HR)
How does pulse oximetry work?
The amount of light absorbed by oxygenated and deoxygenated blood is used to determine the oxygen saturation
Explain pulse oximetry through the absorption of red and infrared light
Oxygenated (oxidized) Hb (horizontal red line) absorbs more infrared light (vertical blue line; 940nm), which allows more red light (vertical red line; 660nm) to pass through
De-oxygenated (reduced) Hb (horizontal blue line) absorbs more red light (vertical red line; 660nm), which allows more infrared light (vertical blue line; 940nm) to pass through
Causes Of Low SpO2, Despite Normal SaO2
- Decreased cardiac output/hypotension/low perfusion states
- Vasoconstriction
- Cold patients
- Misplaced probe - Motion, or shivering
- Diagnostic IV dye
- Dark nail polish
- Manual BP cuff inflation
How accurate is the Sp02 waveform?
SpO2 monitor is 95% accurate when oxygen saturation is above 70% (that’s pretty good!)
- Below 70%, there is a ±4% error
- Below 50%, there is a ±15% error
Causes Of Low SaO2, Despite Normal SpO2
- Carbon monoxide (CO) poisoning
- Cyanide (CN-) toxicity
Clinical Implications Of Carbon Monoxide Poisoning
The skin will typically be “pink red” (cyanosis is unusual)
- The patient’s SpO2 reading is normal or elevated
- The patient’s SaO2 is decreased
- The patient’s PaO2 doesn’t change
Carbon monoxide (CO) binds hemoglobin, displaces oxygen from it (which lowers the SaO2), and forms carboxyhemoglobin (COHb). CO is 230X more affinitive for hemoglobin.
How would you diagnose CO poisoning?
In order to diagnose carbon monoxide poisoning, we would need to draw an arterial blood gas (which can tell us oxygen saturation, carboxyhemoglobin saturation, and the percentage of methemoglobin), or we could use a “special” pulse oximeter called a “co-oximeter,” which can detect carbon monoxide
- Normal PaO2/SpO2 despite hypoxia (low SaO2)
- Typically no cyanosis because the hemoglobin is still bound (even though it’s not bound to oxygen)
Treatment of Carbon Monoxide (CO) Poisoning
Simply administer 100% oxygen
–This works because carbon monoxide is competitively bound to hemoglobin (and is thus easily removed from hemoglobin with high concentrations of oxygen)
Causes Of Carboxyhemoglobin (COHb) Formation
- Smoking or smoke inhalation
- Dried out (desiccated) CO2 absorbent
Possible Causes of Cyanide Poisoning (CNHb)
- High doses of nitroprusside (Nipride)
- Smoke inhalation
- Inhaling chemicals, etc
Treatment for cyanide poisoning
Cyanide ions (CN-) bind to Hb and displace oxygen, which:
–Creates cyanohemoglobin (CNHb)
–Causes a decrease in SaO2 (hypoxia)
- Sodium Nitrate
- Sodium Thiosulfate
Methemoglobinemia (MetHb)
On normal hemoglobin, iron has a +2 charge
In patients with methemoglobinemia, iron is oxidized (loses an electron) and ends up with a +3 charge
Possible Causes Of Methemoglobinemia (MetHb)
- High doses of Nitroprusside (Nipride)
- High doses of Nitroglycerin (NTG)
- Local anesthetic spray (such as benzocaine or cetacaine) in the pharynx
- Factory/mine/pesticide/chemical workers inhaling certain chemicals
Clinical Implications Of Methemoglobin (MetHb)
- SaO2 decreases (hypoxia)
- SpO2 reads 85%(regardless of what the true SaO2 is)
- PaO2 doesn’t change
- The patient develops cyanosis
Diagnosis & Treating Methemoglobinemia
Clinical Signs
- Normal PaO2
- SpO2 of 85% with intense cyanosis
Diagnosis
- Draw an arterial blood gas (ABG)
- Use a “co-oximeter”
Treatment
–Methylene blue
