Respiratory Monitoring Flashcards
Formula of the arterial content of O2
CaO2 = (SaO2 x Hgb x 1.34) + (0.0031 x PaO2)
- (CaO2, in mL of O2 per 100 mL of blood [hemoglobin—Hb], mL/100 mL)
** 1.34 mL/g is the O2 binding capacity of Hb (i.e., Hüfner constant, theoretically equal to 1.39 mL/g but experimental range between 1.31 and 1.37 mL/g because of the presence of small amounts of other Hb species)
*** 0.0031 is the solubility of O2 in blood (mL/100 mL/ mm Hg)
Five species of Hb are found in adult blood: …
oxygenated Hb (O2Hb);
deoxygenated Hb (deO2Hb);
carboxyhemoglobin (COHb);
methemoglobin (MetHb);
sulfhemoglobin (SHb)
Functional O2 saturation (SaO2) refers to …
the amount of O2Hb as a fraction of the total amount of O2Hb and deO2Hb
SaO2 = (O2Hb) / (O2Hb + Hb) x 100%
The O2Hb fraction or fractional saturation is defined as
the amount of O2Hb as a fraction of the total amount of Hb
FO2Hb = (O2Hb) / (O2Hb + Hb + COHb + MetHb + SHb) x 100%
Describe the factors that affect the oxyhemoglobin dissociation curve
Left shift:
Alkalosis
Hypocarbia
Hypothermia
Decreased 2,3-DPG
COHb
Fetal Hb
Right shift:
Acidosis
Hypercarbia
Hyperthermia
Increased 2,3-DPG
Oximetry is the measurement of the O2 saturation of Hb. It is an application of the … law, which relates …
Beer–Lambert
the transmission of light through a solution to the concentration of the solute in the solution
The concentration of a single solute in solution can be calculated by measuring the amount of light transmitted through the solution as long as the other variables are known. For a solution containing multiple solutes, the calculation of the concentrations of the different solutes requires that light absorption be measured at a number of different wavelengths that are at least …
equal to the number of solutes
Light absorption by tissue can be divided into a time-varying (pulsatile) component, historically referred to as “AC” (from “…”), and a steady (nonpulsatile) component, referred to as “DC” (“…”). In conventional pulse oximetry, the ratio (R) of AC and DC light absorption at two different wavelengths is calculated
alternating current
direct current
The most commonly used wavelengths of light in pulse oxymeters are …
At … , there is greater light absorption by deO2Hb than by O2Hb.
At … , there is greater light absorption by O2Hb than by deO2Hb
660 nm and 940 nm
660 nm
940 nm
In pulse oxymeters, how is the ratio (R) obtained? How is the relation between the ration and the arterial O2 saturation?
R = (AC 660 / DC 660) / (AC 940 / DC 940)
The ratio R is then empirically related to O2 saturation based on a calibration curve internal to each pulse oximeter. A ratio of 1 corresponds to a O2 saturation of ~85%
Along with measuring O2 saturation, the pulse oximeter can also be used as a photoplethysmograph. Because the absorption of light is proportional to the …, changes in the … are reflected in the pulse oximeter trace.
Variations in the … (ΔPOP) have been shown to predict fluid responsiveness in mechanically ventilated patients
An index derived from the percent difference between the maximum and minimum amplitudes of the plethysmographic waveform during a respiratory cycle (…) has been incorporated into a commercially available pulse oximeter and used to quantify ΔPOP and predict …
amount of blood between the transmitter and photodetector
blood volume
amplitude of the pulse oximetry plethysmographic waveform
PVI, Pleth Variability Index
fluid responsiveness
Describe how the following conditions affect de SpO2 in relation to SaO2
Hypotension
Anemia
Polycythemia
Motion
Low SaO2
Methemoglobinemia
Carboxyhemoglobinemia
Hypotension: ↓
Anemia: ↓
Polycythemia: No significant effect
Motion: ↓
Low SaO2: Variable
Methemoglobinemia: ↓/↑ (SpO2 approaches 85%)
Carboxyhemoglobinemia: ↑
Describe how the following conditions affect de SpO2 in relation to SaO2
Sulfhemoglobin
Hemoglobin F
Hemoglobin H
Hemoglobin K
Hemoglobin S
Sulfhemoglobin: No significant effect
Hemoglobin F: No significant effect
Hemoglobin H: No significant effect
Hemoglobin K: ↓
Hemoglobin S: No significant effect
Describe how the following conditions affect de SpO2 in relation to SaO2
Methylene blue
Indigo carmine
Indocyanine green
Isosulfan blue
Fluorescein
Nail polish
Acrylic fingernails
Henna Red
Methylene blue: ↓
Indigo carmine: ↓
Indocyanine green: ↓
Isosulfan blue: No significant effect/↓
Fluorescein: No significant effect
Nail polish: Black, dark blue, purple ↓
Acrylic fingernails: No significant effect
Henna: Red – No significant effect / ↓
Describe how the following conditions affect de SpO2 in relation to SaO2
Skin pigmentation
Jaundice
Ambient light
Sensor contact
IABP
Skin pigmentation: At SaO2 >80%, no significant effect / At SaO2 <80%, ↑
Jaundice: No significant effect
Ambient light: No significant effect
Sensor contact: ↓
IABP (intra-aortic balloon pump): ↑
The calibration of pulse oximeters is based on curves obtained in normal individuals under experimental conditions with SaO2 as low as …%. As such, pulse oximeters have limited accuracy for SaO2 values less than …%. Moreover, systematic errors in SpO2 tend to increase as SaO2 falls below …’%.
70
70
90
Significantly erroneous reductions in SpO2 may be observed for systolic blood pressures lower …
than 80 mm Hg
Venous pulsations may result in the detection of venous O2Hb saturation by the pulse oximeter, resulting in artifactual reduction of the presumed arterial SpO2 being measured. Venous pulsations can be due to …
excessively tight placement of adhesive finger probes, severe tricuspid regurgitation, probe placement in dependent positions (e.g., forehead during Trendelenburg position), and possibly in distributive shock when vasodilation may result in physiologic arteriovenous shunting
SHb absorbs red light (660nm) … [less/more] than deoxyhemoglobin (HHb) or MetHb, and likely as much near the infrared spectrum. This results in SpO2 values … in severe cases of sulfhemoglobinemia
more
close to 85%
With normal SaO2, anemia has little effect on SpO2. However, in the presence of hypoxia, SpO2 readings … [overestimate/underestimate] SaO2 in anemic patients with true hypoxemia
underestimate
- anemia = less hb = less light absorption. With hypoxia, even hewer O2Hb. This creates less contrast between O2Hb and deO2Hb = oximeter starts “guessing”, and tends to underestimate true satiration.
Pulse oximeters are sufficiently accurate in adult patients with sickle cell disease, as well as in the presence of fetal Hb. However, caution is warranted when using pulse
oximetry in patients with sickle cell disease, because heme metabolism may result in …
Some studies also suggest that SpO2 may … [over/underestimate] SaO2 during vaso-occlusive crises. Another relevant point is that in patients with sickle cell disease, the affinity of O2 for Hb is … under normoxic conditions but becomes … during hypoxia.
elevated COHb
overestimate
normal
low
A relatively uncommon cause for reduced SpO2 readings is the presence of congenital variants of Hb. Some variants, such as Hb …, Hb …, and Hb …, have a reduced affinity for O2, and changes in SpO2 appropriately reflect changes in SaO2. Other variants, such as Hb …, Hb …, Hb …, Hb ,=.., and Hb …, have altered absorption spectra (closer to HHb) that result in low SpO2 readings in the setting of normal SaO2.
Bassett
Rothschild
Canabiere
Lansing
Bonn
Koln
Cheverly
Hammersmith
Which of the following dyes is associated with a lower SpO2 measurment? Why?
Methylene blue
Indigo carmine
indocyanine green
Methylene blue leads to a transient, marked decrease in SpO2 down to 65% due to its peak light absorption at 668nm, which is very close to that of HHb. Indigo carmine and indocyanine green also artificially decrease SpO2 measurements, although to a lesser
extent than methylene blue, because they do not substantially absorb red light
Although all colors of nail polish can reduce the calculated value of SpO2, black, purple, and dark blue colors have the greatest effect. Nonetheless, the error generally
remains within …%
2
… is the O2 saturation of blood at the proximal pulmonary artery.
Normal values range between… .
Values close to … are associated with tissue hypoxia, anaerobic metabolism, and lactate production
Mixed venous oxygen saturation (SvO2)
65% and 80%
40%
Formulas for global oxygen delivery (DO2), global oxygen uptake (VO2) and O2 extraction ratio, ERO2
DO2 = Q x CaO2
VO2 = Q x (CaO2 - CvO2) x 10
ERO2 = VO2/DO2
The saturation of blood at the level of a central venous catheter placed in the superior vena cava (ScvO2) reflects the balance between O2 supply and O2 demand in…
Under normal physiologic conditions, ScvO2 is typically … than SvO2, primarily because of the …
However, during hemodynamic instability, as circulation is redistributed to …, the relationship between SvO2 and SCVO2 may …, and the difference between the two may increase significantly
the brain and upper extremities
2% to 5% less
higher O2 content of splanchnic and renal venous blood
the upper body
reverse
Causes of SvO2 and ScvO2 reduction
Increase of VO2:
* Pain
* Shivering
* Agitation
* Hyperthermia
Reduction of DO2
* Heart failure
* Hypovolemia
* Hypoxia
* Anemia
* COHb
Causes of SvO2 and ScvO2 increase
Reduction of VO2
* Sedation
* Anesthesia
* Analgesia
* Warming
* Respiratory support
Increase of DO2
* Inotropic
agent
* IV fluid
* Transfusion
* O2 therapy
