Hemoglobin and Oxygen Saturation

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Questions and Answers

In pulse oximetry, what is the functional significance of using two different wavelengths of light (660nm and 940nm) when measuring arterial oxygen saturation?

  • To increase the intensity of light transmitted through the tissue, ensuring a stronger signal at the photodetector.
  • To differentiate between oxygenated and deoxygenated hemoglobin based on their unique light absorption characteristics at these wavelengths. (correct)
  • To minimize the effects of ambient light interference by averaging the readings from both wavelengths.
  • To measure the perfusion index accurately, where each wavelength corresponds to arterial and venous blood flow, respectively.

A patient with carbon monoxide poisoning may have a falsely elevated SpO2 reading. What is the underlying reason for this inaccuracy?

  • Carbon monoxide increases tissue perfusion, leading to an overestimation of arterial blood flow and oxygen saturation.
  • Carbon monoxide binds to hemoglobin with a higher affinity than oxygen, and carboxyhemoglobin absorbs light similarly to oxyhemoglobin at the wavelengths used in pulse oximetry. (correct)
  • The presence of carbon monoxide causes a shift in the Beer-Lambert Law's assumptions, affecting the calculation of SpO2.
  • Carbon monoxide interferes with the photodetector's ability to distinguish between arterial and venous blood, leading to signal misinterpretation.

How does a pulse oximeter differentiate between the absorbance of arterial blood and that of venous or capillary blood to accurately measure SpO2?

  • By using a specific wavelength of light that is only absorbed by arterial blood, thus ignoring the absorbance from other sources.
  • By physically separating arterial blood from venous and capillary blood within the tissue using a magnetic field.
  • By employing a complex algorithm that isolates the pulsatile component of the absorbance signal, corresponding to arterial blood, from the constant (DC) absorbance. (correct)
  • By calibrating the device for each individual based on their venous blood oxygen saturation levels to subtract the venous contribution from the measurement.

In the Beer-Lambert Law as applied to pulse oximetry, what does variations in the length of the absorbing medium (d) primarily account for, and how is this accounted for in SpO2 measurements?

<p><code>d</code> represents the combined thickness of skin, tissue, and blood, largely negated by the ratiometric approach of using two wavelengths and calculating the ratio (R). (D)</p> Signup and view all the answers

How might the presence of diagnostic dyes like methylene blue in the bloodstream affect pulse oximetry readings, and what is the underlying mechanism of this interference?

<p>Diagnostic dyes absorb light at or near the wavelengths used in pulse oximetry, leading to an underestimation of the light reaching the photodetector and skewing the SpO2 calculation. (B)</p> Signup and view all the answers

Why do pulse oximeters often extrapolate SpO2 values below 80%, and what are the limitations of relying on these extrapolated values for clinical decision-making?

<p>Accuracy below 80% is limited because the R-curve is based on data from hypoxic volunteers, and it's unethical to induce severe hypoxia for calibration, resulting in decreased reliability of extrapolated SpO2 values. (B)</p> Signup and view all the answers

How do increased levels of carbon dioxide and increased temperature relate to the oxygen-carrying ability of hemoglobin?

<p>Increased carbon dioxide and temperature both decrease the affinity of hemoglobin for oxygen, shifting the oxyhemoglobin dissociation curve to the right and facilitating oxygen release in tissues. (C)</p> Signup and view all the answers

What physiological response would be expected from the hemoglobin due to a decreased pH within the body?

<p>Decreased oxygen affinity, resulting in a rightward shift of the oxyhemoglobin dissociation curve. (C)</p> Signup and view all the answers

How does pulse oximetry contribute to the evaluation of the effects of exercise on Oâ‚‚ levels?

<p>It provides real-time monitoring of arterial oxygen saturation, which can indicate exercise-induced hypoxemia or desaturation. (D)</p> Signup and view all the answers

How is pulse oximetry utilized in determining the response to therapeutic interventions, particularly in the context of mechanical ventilation?

<p>By monitoring real-time arterial oxygen saturation levels to assess the efficacy of ventilator settings. (A)</p> Signup and view all the answers

Arterial saturation of oxygen is affected by the partial pressure of arterial oxygen (POâ‚‚). At low partial pressures, such as tissue capillary pressure of 40 mmHg, what change occurs at hemoglobin?

<p>Oxygen is released from hemoglobin, leading to a low SaOâ‚‚ of about 75%. (A)</p> Signup and view all the answers

How does the oxyhemoglobin dissociation curve illustrate the relationship between oxygen saturation and partial pressure?

<p>It illustrates a sigmoidal relationship where hemoglobin saturation increases rapidly at higher partial pressures and plateaus, but lower partial pressures correspond to changes in saturation. (D)</p> Signup and view all the answers

What criteria defines 'normal' blood oxygen levels and how is hypoxemia defined?

<p>Normal oxygen levels are 95-100 percent; hypoxemia is defined as oxygen levels below 90 percent. (A)</p> Signup and view all the answers

What is the potential significance of blood oxygen levels falling below 80 percent, and what is the appropriate clinical response?

<p>Levels below 80 percent may indicate compromised organ function, such as in the heart and brain, and should be promptly addressed with oxygen therapy or other interventions. (C)</p> Signup and view all the answers

Using the formula for calculating arterial saturation of oxygen (SaOâ‚‚), what is the ratio of oxyhemoglobin concentration to the total concentration of arterial hemoglobin available for reversible oxygen binding?

<p>SaOâ‚‚ = [HbOâ‚‚] / ([RHb] + [HbOâ‚‚]) (B)</p> Signup and view all the answers

How is beer lambert's law expressed mathematically?

<p>$I = I_oe^{-\varepsilon(\lambda)cd}$ (C)</p> Signup and view all the answers

What occurs when there is a decrease in the oxygen-carrying ability of hemoglobin, causing a shift to the right on the dissociation curve?

<p>A decrease in pH within the body. (D)</p> Signup and view all the answers

When oxygen reaches the tissues, what happens to it?

<p>It is released into tissue fluids. (B)</p> Signup and view all the answers

What is the main light absorber in human blood at wavelengths used in pulse oximetry?

<p>Hemoglobin (A)</p> Signup and view all the answers

What happens to the absorbing characteristics of human blood?

<p>They change with its chemical binding and the wavelength of incident light. (A)</p> Signup and view all the answers

Non-oxygenated hemoglobin partially absorbs light at what wavelength?

<p>660nm (B)</p> Signup and view all the answers

How should low blood oxygen levels (below 80%) be addressed?

<p>They should be addressed promptly. (D)</p> Signup and view all the answers

An arterial saturation of oxygen is considered to be affected by:

<p>The partial pressure of arterial oxygen (B)</p> Signup and view all the answers

Where do hemoglobin transport oxygen from?

<p>The Lungs (C)</p> Signup and view all the answers

How many oxygen molecules can each hemoglobin unit carry?

<p>4 (B)</p> Signup and view all the answers

What is the main component of red blood cells?

<p>Hemoglobin (C)</p> Signup and view all the answers

What is reduced hemoglobin also referred to as?

<p>RHb (A)</p> Signup and view all the answers

What is hemoglobin fully saturated with oxygen called?

<p>Oxyhemoglobin (A)</p> Signup and view all the answers

Flashcards

What is Hemoglobin?

A protein in red blood cells that carries oxygen.

Hemoglobin's Oxygen Capacity

Each unit has 4 heme groups, carrying one oxygen molecule each.

What is Oxyhemoglobin (HbO2)?

Hemoglobin fully saturated with oxygen.

What is Reduced Hemoglobin (RHb)?

Hemoglobin that is not fully saturated with oxygen.

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Hemoglobin's Main Job

Hemoglobin carries oxygen from lungs to tissues.

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What is Arterial Oxygen Saturation (SaO2)?

The ratio of oxyhemoglobin to total arterial hemoglobin.

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Why Measure Oxygen Saturation?

Used to determine response to interventions, evaluate exercise effects, and monitor disease.

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Normal Blood Oxygen Levels

Normal levels are considered 95-100%.

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Low Blood Oxygen

Levels below 90%, often from hypoxemia.

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SaO2 at Low PO2

A low SaO2 of about 75%.

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SaO2 at High PO2

A high SaO2 of about 95%.

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Decreased Hemoglobin Oxygen-Carrying Ability

The shift of the curve to the right.

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What is Beer-Lambert Law?

Light intensity decreases exponentially as it passes through a medium.

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What is Absorbance (A)?

Proportional to the concentration of the absorbing medium.

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Multiple Absorbing Media

Each contributes its part to total absorbance.

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What is Hemoglobin?

The main light absorber in blood used in pulse oximetry.

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Wavelength Absorption

RHb absorbs 660nm, HbO2 absorbs 940nm.

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What is Oxygen Saturation?

The percentage of total hemoglobin carrying oxygen.

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Arterial Blood in Cuvette

Send wavelengths through blood and measure absorbance.

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Pulse Oximetry Process

Two LEDs send signals; a detector receives absorbed signals.

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Oximeter Display

Processed into % saturation, pulse rate, plethysmography.

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Pulse Isolation

Isolate pulsatile arterial blood (AC) from constant venous blood (DC).

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Microprocessor Task

Determines AC/DC ratio at each wavelength.

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Oximeter Memory

Memory relates ratio (R) to arterial oxygen saturation.

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Inaccuracy Sources

Artifact, perfusion, COHb, MetHb, dyes, nail polish, ambient light.

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What is SpO2?

Arterial saturation of oxygen, estimated using pulse oximetry.

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Study Notes

Hemoglobin

  • Hemoglobin is a protein and the main component of red blood cells
  • Each hemoglobin unit has 4 heme groups, which can each carry one oxygen molecule
  • When fully saturated with oxygen (four bound oxygen molecules), hemoglobin is called oxyhemoglobin (HbO2)
  • When not fully saturated, it is called reduced hemoglobin (RHb)
  • Hemoglobin transports oxygen from the lungs (high PO2) to the tissues (low PO2)
  • Oxygen is released into tissues when oxygen reaches the tissues

Oxygen Saturation

  • Arterial saturation of oxygen (SaO2) is the ratio of oxyhemoglobin concentration to the total concentration of arterial hemoglobin available for reversible oxygen binding
  • SaO2 = [HbO2] / ([RHb] + [HbO2]) * 100%
  • Oxygen saturation measurements are made to:
    • Determine response to therapeutic intervention (e.g., mechanical ventilation)
    • Evaluate the effect of exercise on O2 levels
    • Monitor the progression of some diseases (anemia, thalassemia)

Blood Oxygen Levels

  • Normal blood oxygen levels in humans are 95-100%
  • A level below 90% is considered low, resulting in hypoxemia
  • Levels below 80% may compromise organ function, such as the brain and heart
  • Continued low oxygen levels may lead to respiratory or cardiac arrest
  • Oxygen therapy can assist in raising blood oxygen levels

Oxyhemoglobin Dissociation Curve

  • Arterial saturation of oxygen depends on the partial pressure of arterial oxygen (PO2)
  • At low partial pressures (e.g., tissue capillary pressure of 40 mmHg), oxygen is released from hemoglobin, leading to an SaO2 of about 75%
  • At high partial pressures (e.g., pulmonary capillary pressure of 95 mmHg), oxygen binds with hemoglobin, leading to a high SaO2 of about 95%
  • A decrease in the oxygen-carrying ability of hemoglobin (dissociation curve shifting to the right) is observed with an increase in carbon dioxide and temperature, as well as a decrease in pH within the body
  • The oxyhemoglobin dissociation curve illustrates the relationship of oxygen saturation and partial pressure

Beer-Lambert Law

  • The intensity of light passing through an absorbing medium decreases exponentially with the distance traveled
  • I = I0 * e^(-ε(λ)cd)
    • I = intensity of transmitted light
    • I0 = intensity of incident light
    • c = concentration of the absorbing medium
    • ε(λ) = molar extinction coefficient
    • d = length of the absorbing medium
  • Absorbance (A) is proportional to the concentration of the absorbing medium
  • A(λ) = -log(I/I0) = ε(λ) * d * c
  • Beer-Lambert law works even for multiple mediums and states that each absorber contributes its part to total absorbance
  • At(λ) = Σ εi(λ) * di * ci (summation from i=1 to n)

Basis for Oxygen Saturation Measurement

  • Hemoglobin is the main light absorber in human blood at wavelengths used in pulse oximetry
  • Absorbing characteristics change with its chemical binding and the wavelength of incident light
  • Non-oxygenated hemoglobin (RHb) partially absorbs light at 660nm, while oxygenated hemoglobin partially absorbs light at 940nm
  • Measuring the absorption of both wavelengths and determining their ratio gives the percentage of total hemoglobin carrying oxygen, which provides the "oxygen saturation"

Direct vs. Non-Invasive Measurement of Arterial Oxygen Saturation

  • Arterial blood can be placed in a glass cuvette of known dimension and transmit two wavelengths of light of known intensity separately through the sample for direct measurement
  • Detect each intensity and calculate the absorbance
  • The two equations and two unknowns are:
    • At(λ1) = εo(λ1) * do * co + εd(λ1) * dd * cd
    • At(λ2) = εo(λ2) * do * co + εd(λ2) * dd * cd
  • o = oxyhemoglobin subscript
  • d = deoxy-hemoglobin subscript
  • Can solve for co, cd, and SaO2
  • Then oxyhemoglobin and deoxyhemoglobin concentrations are used to calculate SaO2
  • In pulse oximetry, two LEDs each send a 660nm or 940nm signal through perfused tissue to a photodetector for non-invasive measurement
  • The photodetector receives the partially absorbed signals which are sent to a monitor, processed, and displayed as % saturation, pulse rate, and (optionally) a plethysmographic (volume) waveform

Non-invasive arterial oxygen saturation

  • In tissue, technical problems like scatter, reflection, and absorbance of light can confound readings
  • The system isolates absorbance of arterial blood from other tissue
  • This can be accomplished easily as arterial blood is pulsatile
  • The microprocessor selects absorbance of pulsatile arterial blood (AC) and filters the constant absorbance by nonpulsatile venous/capillary blood & tissue pigments (DC)
  • The microprocessor determines the AC component of absorbance at each wavelength and divides it by the corresponding DC component
  • Then it calculates a ratio (R) of the "pulse-added" absorbance from each component at the two frequencies:
    • R = [IAC(λ1) / IDC(λ1)] / [IAC(λ2) / IDC(λ2)]
  • An R-curve relates the ratio (R) to the arterial oxygen saturation within the oximeter memory
  • R is compared with the stored values and the oxygen saturation is determined and displayed

R-curve

  • Each manufacturer has a specific R-curve for their monitors
  • The components and algorithm used determines values
  • R-curve is obtained from experiments in which human volunteers were given increasingly hypoxic mixtures to breathe until saturation values of 80% were obtained
  • Since the microprocessor has no memory of values less than 80% (as it is unethical to make volunteers more hypoxic) accuracy cannot be ascertained below a value of 75-80%
  • Any saturation below that value would be an extrapolated one and hence inaccurate
  • The term SpO2 is arterial saturation of oxygen, i.e. estimated using pulse oximetry

Sources of Inaccuracy

  • Motion of the perfused tissue ("Motion Artifact")
  • Poor tissue perfusion due to cold temperature or vascular disease
  • Carboxyhemoglobin (COHb): typically high in smokers and may be misread as HbO2
  • Meth-hemoglobin (MetHb): from toxins may be misread as either HbO2 or RHb
  • Diagnostic dyes in bloodstream: methylene blue, cardio green, etc.
  • Nail polish
  • Ambient light that is rich in infrared (e.g., surgical lights)

System Description and Diagram

  • An LED driver powers on and off two LEDs
  • Typical red and infrared wavelengths used are 660 and 940 nm, respectively.
  • Light transmitted from an LED and through the finger is received by a photodiode
  • The photodiode current is amplified and undergoes data acquisition
  • Within the processor module, the AC and DC components in each waveform are used to calculate the ratio R
  • A lookup table determines the SpO2 value associated with this ratio
  • The final SpO2 value is transmitted to an LCD for display

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