Martin ABG Flashcards
Blood gas (def)
Any element or compound that is gas under ordinary conditions
Also dissolved got some extent in our blood
Inert gases in blood
Nitrogen, krypton, helium (last two in trace amounts)
Cause no clinical problems –> not measured as part of ABG
Carbon Monoxide
Gas, measured in its combined form with hgb as % carboxyhgb (%COhgb)
Measurement is only an indirect guide to %COHb
T/F: not all blood gases are routinely measured
True
T/F: not all blood gas measurements are of true blood gases
True
Co-oximeter
Can measure, on a small portion of the arterial blood sample, hgb content (in grams per deciliter) and values related to the hgb binding: SaO2, %COHb, percent methemoglobin (%MetHb)
–> can calculate arterial oxygen content
Co-oximetry
Distinct test that may not be available - must ensure able to run
Can miss life-threatening conditions: methemoglobinemia, carbon monoxide poisoning
What do you expect PaO2 in CO and MetHb?
NORMAL! (unless patient has lung disease)
What do you expect the SaO2 in CO and MetHb?
Will be artificially high
What does blood gas analysis entail?
Measurements of:
- Blood gases
- Hgb content/binding
SaO2
Percentage of hgb sites chemically combined (saturated) with oxygen
- -Hgb binding site cannot contain more than one gas molecule at a time
- Percentages (%O2Hb, %COHb, %MetHb) = additive –> cannot be greater than 100%
Methemoglobin
Hgb that has iron in its ferric or oxidized state (Fe+++) as opposed to the normal ferrous state (Fe++)
–Hgb w Fe3+ can bind neither oxygen nor carbon monoxide
Basics: how measure electrolytes in the arterial sample? Which electrolytes do we measure?
Incorporating special electrodes into the blood gas machine
–Na, K, Cl, bicarb, +/- Ca, Mg
Important considerations when interpreting ABG
- Patient’s environment –> FiO2, barometric pressure
- Other lab data –> prior BGs, ELs, gluc, BUN, hct, CXR
- Clinical picture –> RR/effort, other vitals, mentation, tissue perfusion
Isolated PaO2
Reveals little useful information without reference to the patient’s mental status and resp effort
Low PaO2 may mean one thing if patient inhaling supplemental air vs room air
What is normal PaO2?
IT DEPENDS!!!
Depends on FiO2, Pb, patient’s age
Air composition
78% nitrogen, 21% oxygen, 1% other inert gases, negligible amount of CO2
T/F: At any altitude the fraction of inspired O2 is 0.21
True
Barometric Pressure
- -Fxn of wght of atms above the point of measurement
- -Sum of all of the pressures of the constituent gases
- -Each gas exerts its own partial pressure, which is the pressure it would exert if no other gases were present
What does it mean that the Pb is 760mmHg at sea level?
Air pressure at sea level will sustain a closed column of mercury 760mmHg high
What is the Pb on Everest?
253 mm Hg
How does altitude affect Pb?
Higher the altitude, lower the weight of the air at that point and lower the Pb
Pressure of a gas in dry air
= percentage of a gas x barometric pressure
Water vapor in ambient air
- -Exerts own partial pressure
- -Must be subtracted from the barometric pressure since dilutes out all of the dry gases
- -Depends on climate: amp of water vapor in ambient air varies from 0 to >50 mm Hg
Water vapor in the upper airway
Becomes fully saturated
Pressure = 47mm Hg
How to maintain acceptable oxygen levels at extreme altitude?
Change the environment 1. Increase FiO2 2. Increase Pb Adapt physiologically 1. Lower PaCO2 by hyperventilating
What is the average airway pressure in the lungs?
= ambient/barometric pressure
(with mechanical ventilation, avg airway pressure will be slightly higher than ambient - difference depends on art of positive pressure being delivered by the ventilator)
What happens when breathe 100% O2?
Extra oxygen displaces nitrogen from the body’s tissues
Amount of N2 displaced depends on the FiO2 and how long supplemental oxygen has been inhaled –> oxygen pressure merely replaces the N2 pressure
What physiological processes are assessed by pH, PaO2, and PaCO2?
Alveolar Ventilation
Oxygenation
Acid-Base Balance
Equation for alveolar ventilation?
PaCo2 = (VCo2 x 0.863)/VA
Alveolar PCo2 = amount of CO2 produced by metabolism and delivered to the lungs x constant divided by alveolar ventilation
Alveolar ventilation
VA = VE - VD –> minute/total ventilation - dead space ventilation
Minute/total ventilation
VE = RR x Tv
Dead space ventilation
VD = RR x Dead space volume
PCO2 Equation
PaCO2 = (VCo2 x 0.863)/VA
Evaluates PaCO2 based on patient’s RR and breathing effort
What is PaCO2 directly proportional to?
VCO2 –> amount of CO2 produced by metabolism and delivered to the lungs
What is PaCO2 inversely proportional to?
Alveolar ventilation
What equations assess oxygenation?
Alveolar gas, oxygen content
What equation(s) assess acid-base balance?
Henderson-Hasselbalch
Short form alveolar gas equation
PAO2 = PIO2 - 1.2PaCO2
Where PIO2 = partial pressure of inspired/tracheal oxygen in mm Hg –> FIO2 (Barometric pressure - water vapor pressure)
Longer form of the alveolar gas equation
PAO2 = FIO2(Pb-PH20)-PACO2 x [FiO2+(1-FiO2)/R)
R = respiratory quotient
PACO2 assumed to equal PaCO2
Oxygen content
Oxygen = gas –> exerts a pressure
Also has a content: CaO2 (mL O2/dL blood)
Oxygen content equation
CaO2 = amt of O2 bound to hgb + amt dissolved in plasma
= (SaO2xHbx1.34) + 0.003PaO2
CaO2
Oxygen content of arterial blood - mL O2/dL arterial blood
SaO2
Percent saturation of arterial hgb with oxygen, expressed as decimal fraction
Hb
Hemoglobin content (g/dL blood)
