Chp. 15: Monitoring Oxygenation Flashcards
Hypoxia
Condition in which the supply of oxygen to tissues fails to ensure normal cellular metabolism
Hypoxemia
State of low arterial oxygen partial pressure
What are frequent causes of tissue hypoxia?
Hypoxemia, anemia, histotoxicity, reduced forward blood flow
Causes of hypoxemia
Low FiO2, hypoventilation, V/Q mismatch, physiologic shunt, diffusion impairment
Alveolar Gas Equation
PAO2 (mmHg) = FIO2 (PB – PH2O) – PaCO2 / R)
R = ratio of CO2 elimination over O2 uptake, typically estimated at 0.8
CaO2 Equation
CaO2 (mL/dL) = (1.34 x Hb X SaO2) + (0.0031 x PaO2)
First term represents amount of oxygen bound to hemoglobin.
Second term represents quantity of oxygen dissolved in plasma.
Oxygen Deliver (DO2) Equation
DO2 (mL/min) = CO x CaO2
Oxygen Consumption (VO2) Equation
VO2 (mL/min) = CO x C(a-v)O2
Oxygen Extraction Ratio (O2ER) Equation
O2ER = VO2/DO2
Causes of increased O2ER
Anything that increases oxygen consumption (seizures, hyperthermia, sepsis) or decreases oxygen delivery (hypoxemia and anemia). Low O2ER is usually due to decreased oxygen consumption caused by anesthesia, mechanical ventilation, and hypothermia.
Cyanosis
Indicates the presence of deoxygenated blood in the observed tissue. A minimal concentration of deoxygenated hemoglobin (5g/dL) is required to cause a visually detectable cyanosis.
Why might an anemic animal be hypoxemia without showing cyanosis?
Decreased hemoglobin concentration would be saturated with oxygen even in the presence of considerably reduced partial pressures of oxygen in the blood.
True or False: Drug induced peripheral vasoconstriction can cause cyanosis.
True. When peripheral blood flow slows down due to vasoconstriction, oxygen can diffuse into tissues over a longer time (oxygen extraction increases), leaving more deoxygenated hemoglobin in the capillary bed.
What is a normal A-a gradient?
Less than 20mmHg
What is a normal PaO2/FiO2 ratio?
400-500 at sea level
What are the main disadvantages of using the PaO2/FiO2 ratio to guid clinical decision?
Dependence on barometric pressure and its inability to distinguish alveolar hypoventilation from other causes of hypoxemia. Does not indicate CaO2 and does not provide information on oxygen delivery to tissues
What is the greatest limitation of arterial blood gas analysis?
Inability to detect regional or local changes in tissue oxygenation.
Mixed venous blood
Blood in the pulmonary artery, representative of venous blood from all organs and tissues.
What is the normal PvO2 (partial pressure of oxygen in mixed venous blood) while breathing room air? What saturation does this correspond to?
40mmHg, corresponding to a saturation of ~70%
What is PvO2 a reflection of?
Metabolic activity and oxygen consumption
Is transcutaneous blood gas measurement accurate in veterinary patients?
No
What is measured by a CO-oximeter?
All forms of hemoglobin, including carboxyhemoglobin, methemoglobin, and sulfhemoglobin
What do CO-oximeters report?
Fractional hemoglobin saturation (FO2Hb), or the percent of total hemoglobin that is saturated by oxygen:
FO2Hb = O2Hb/O2Hb + HHb + COHb + MetHb + SulfHb
What do pulse oximeters report?
Functional hemoglobin saturation (SO2), or the fraction of effective hemoglobin that is saturated by oxygen:
SO2 = O2Hb/O2Hb + HHb
Why is SO2 slightly higher than FO2Hb?
Small amounts of COHb and MetHb exist in normal individuals
When is CO-oximetry used and how does it work?
CO-oximetry is used for blood gas analysis. It utilizes spectrometry with multiple wavelengths of light. The light is fractionated and filtered and specific wavelengths directed through the blood sample onto photodiode sensors. Analysis is according to the Beer-Lambert law.
What is SvO2 in a healthy patient breathing room air?
65-80%
What does a low SvO2 mean?
DO2 is not meeting metabolic oxygen demands.
Blood obtained from where is a reasonable surrogate of SvO2 measured from the PA?
Cranial vena cava
SjvO2
- Jugular venous saturation, used mostly to monitor brain oxygenation
- Acts as an indirect marker of cerebral metabolic rate of oxygen (CMRO2)
What is the therapeutic target for SjvO2? What causes an increase or decrease in this value?
- Greater than 50%
- Increase: Increased oxygen supply or decreased oxygen demand such as reduced cerebral metabolism, alteration in oxygen diffusion or extraction due to neuronal infarction or inflammation, cerebral hyperemia
- Decrease: Decreased oxygen supply or increased oxygen demand such as generalized hypoxemia, local or systemic hypo perfusion, increased cerebral metabolism or oxygen extraction resulting from pyrexia or seizures
What is the principle underlying pulse oximetry?
O2Hb and HHb hemoglobin differentially absorb red and near-infrared light.
O2Hb absorbs greater amounts of near-infrared and lower amounts of red light than HHb (O2Hb appears bright red because it scatters more red light than HHb).
What two wavelengths of light are emitted by pulse oximeters?
red light at 660nm
near-infrared light at 940nm
How does a pulse oximeter work?
Red and near-infrared light are emitted from diodes on one arm of the probe and the light transmitted through the tissues is detected by a photodiode on the opposite arm. Relative amounts of the two lights absorbed are used to determine proportion of O2Hb based on the Beer-Lambert Law.
The signal is divided into an AC component (greater light absorption during pulsatile arterial flow) and a DC component (baseline light absorption by venous blood and tissue).
R = [AC/DC (660nm)] / [AC/DC (940nm)]
What information can be gained from plethysmograph analysis?
- Pulse width correlates with SVR
- Systolic amplitude has been related to SV and found directly proportional to local vascular distensibility over a wide range of cardiac output values
- Position of dicrotic notch is used as an indicator of SVR
- PVI is an indicator of fluid responsiveness
What causes erroneous SpO2 readings in anesthetized animals?
Poor peripheral perfusion resulting from vasoactive drugs, hypotension, and hypothermia. Dysrhytmias, arterial compression caused by BP cuffs, and peripheral vascular disease can also impact readings.
What is the effect of venous pulsations on SpO2 readings?
Falsely low SpO2 readings because they cause venous O2Hb saturation to be added to signals coming from arterial blood.
What is the possible range of PaO2 values when SpO2 is >/= 97%?
90-600mmHg
What value of oxygen reserve index (ORI) may provide advanced indication of declining PaO2 when SpO2 is still >98%?
0.24
Why is infrared analysis not used for determining inspired and expired O2 concentrations?
Oxygen is a nonpolar molecule
How do oxygen analyzers work?
They employ paramagnetic analysis, which relies on the fact that unpaired electrons in the outer shell of an oxygen atom cause it to be attracted to the strongest part of an inhomogeneous magnet field, whereas all other respiratory gases move to the weakest part of it. Sampling a reference chambers separated by a pressure transducer are subjected to a switched electromagnetic field and the resulting pressure difference is converted to a voltage that is directly proportional to the concentration of oxygen in the sample gas.
What are the fundamental differences between near-infrared spectroscopy and Raman techniques?
1) Nature of molecular transitions (change in molecular polarizability for Raman, dipole moment change for NIRS)
2) type of monochromatic light used (near-infrared for Raman, infrared for NIRS)
3) strength of absorbance and transmittance (weaker for Raman)
What is Laser Doppler Flowmetry and what are its potential applications?
Measures the amount of Doppler-shifted laser light reflected at a superficial level to determine microcirculatory flux of erythrocytes.
Applications include qualitative evaluation of cutaneous, renal, and ocular/retinal blood flow and assessment of microcirculatory status of potentially unhealthy tissues during intestinal, orthopedic, or plastic surgeries.
What is the basis of mitochondrial redox state assessment?
Impaired microvascular oxygen supply reduces oxidative phosphorylation and causes an increase in cellular NADH, which can be monitored and used as a reliable indicator of oxygen tissue supply and consumption.
