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87 Cards in this Set
- Front
- Back
How can you calculate PaCO2? |
PaCO2 = production of CO2/elimination of CO2 by the lung |
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What are the three determinants of PaCO2 and thus adequacy of ventilation? |
1. CO2 production, |
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What are causes of high CO2 production(VCO2)? |
1. fever, |
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What are causes of low CO2 production(VCO2)? |
1. hypothermia, |
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What are causes of decreased minute ventilation(high PaCO2)? |
1. drugs, |
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What are causes of increased minute ventilation(low PaCO2)? |
1. anxiety, |
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What are causes of increased alveolar dead space(increased PaCO2)? |
1. pulmonary vascular disease, |
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What is physiologic dead space? How do you quantify physiologic dead space? |
The sum of anatomic and alveolar dead space. The dead space to tidal volume fraction Vd/Vt. |
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What is the relationship between alveolar ventilation and PaCO2 in a mechanically ventilated patient when displayed graphically? |
curvilinear (rectangular hyperbola) |
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What is the most efficient way to increase oxygen content in the blood? |
increase hemoglobin |
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What is the haldane effect? |
Describes how oxygen concentration determines the affinity of hemoglobin for CO2. Holds that deoxygenation of blood increases its ability to carry CO2, whereas increased oxygenation of blood decreases it's ability to carry CO2. |
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What are the normal values of mixed venous oxygen saturation and tension? |
mixed venous O2 sat: 65-75% |
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What factors determine mixed venous oxygen tension(PvO2)? |
COALS: |
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What is the equation for SvO2 (mixed venous oxygen saturation)? |
SvO2 = SaO2 - (VO2 / (CO x Hb x 13)) |
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What is the normal V/Q ratio? |
0.8 (nml ventilation is 4L/min, nml perfusion is 5L/min |
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How do you calculate shunt fraction? |
Qs/Qt = (CcO2-CaO2)/(CcO2-CvO2); |
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what is the normal shunt fraction? What does it represent? |
Qs/Qt=0.1 |
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What is the normal A-a gradient? |
10-20mmHg |
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What is an approximate A-a gradient based on age? |
1/4 x the patients age |
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Why do patient's under general anesthesia have a widened A-a gradient? |
increased V/Q mismatch due to altered lung and chest wall compliance |
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What are causes of increased A-a gradient? |
1. V/Q mismatch, |
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How do low FiO2 and hypoventilation affect the A-a gradient? |
normal A-a gradient |
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How does hypercarbia affect PaO2 at a particular FiO2? |
limits PaO2 according to the alveolar gas equation |
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What kind of syringes should be used for blood gas and why? |
glass syringes if possible because CO2 and O2 don't dissolve into the wall of the syringe and minimizes the risk of air bubbles in the syringe |
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Why should heparin be used as the anticoagulant in ABG syringes? |
EDTA, citrates and oxalates alter the pH |
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What variables are necessary for proper interpretation of blood gas? |
FiO2, temp, source, ventilator settings |
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What electrode does the pH, PCO2, PO2 require for interpretation of ABG? |
pH-sanz electode |
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What factors influence blood gas values? |
age, sex, barometric pressure |
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How do the kidneys and the lungs compensate for pH abnormalities? |
lungs: hyper or hypoventilation |
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What is the henderson hasselbalch equation? |
pH=6.1+log(base/acid)=6.1+log(bicarb/(0.03 x PaCO2)) |
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How is the pH affected by increases in arterial PaCO2? |
for every 10 mm Hg increase in PaCO2 the pH decreases by 0.08-0.1 |
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What are some causes of respiratory acidosis? |
1. reduced minute ventilation, |
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what are causes of nonanion gap acidosis? |
BADR: |
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How can you determine if the reflex compensation response of the lungs in a metabolic acidosis is appropriate? |
the PCO2 should be equal to the last two digits of the pH |
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What is the major problem with giving NaBicarb for lactic acidosis? |
bicarb reacts with hydrogen ions to form carbonic acid which then dissociates to CO2 and water and the CO2 partial pressure increases which can worsen the acidosis. When CO2 cannot be eliminated, the pH of the system is only minimally changed or in fact worsened. |
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If you are considering giving bicarb to a patient with a combined metabolic and respiratory acidosis what should you do first? |
treat the respiratory acidosis first |
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When should you treat a metabolic acidosis with bicarb? How do you determine the dose of bicarb to give? |
Generally treat if the pH is < 7,2 and a respiratory acidosis does not exist. (if It does tx the respiratory acidosis first) |
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Besides worsening a respiratory acidosis what other problems are associated with the use of bicarb? |
1. intraventricular hemorrhage |
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How is the solubility of O2 and CO2 affected by temperature? |
at lower temperature the solubility of oxygen and CO2 in solution is higher, there are less molecules in the gas phase and the partial pressure of both are decreased |
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What is the alpha stat strategy of interpreting a blood gas? |
uncorrected arterial blood gas values, no attempt is made to correct for partial pressure of O2 and CO2 for changes in temperature |
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What is the pH stat strategy for interpreting a blood gas? |
relies upon temperature corrected values and involves administering CO2 systemically to the patient to correct for lower partial CO2 pressure secondary to its increased solubility in solution |
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what are criteria for intubation? |
Mechanical |
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What are criteria for extubation? |
1. RR<30, |
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What is the normal pH at birth in the umbilical vein, artery, and also what is this value at 60 min? |
umbilical vein: 7.35 |
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What is the normal pCO2 at birth in the umbilical vein, artery, also at 60 min and 24 hrs? |
umbilical vein: 40 |
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What is the normal pO2 at birth in the umbilical vein, artery, also at 60 min and 24 hrs? |
umbilical vein: 30 |
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How are amines affected by the solution becoming more acidic or basic? |
RNH3+ <-> RNH2 + H+ |
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How are carboxylic acids affect by acidic or basic solutions? |
R-COOH <--> RCOO- + H+; |
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what is the pH and pKa of narcotics, local anesthetics, and thiopental? |
thiopental:pH 10.5, pKa 7.6 |
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2 forms oxygen is present in the blood |
1. bound to hgb |
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What is the P50? |
The partial pressure of oxygen at which hemoglobin is 50% saturated. |
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What is the arterial oxygen content equation? |
CaO2 = (1.34 x Hb x O2 sat) + (.003 x PaO2) |
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What does a right shift in oxy-hb dissociation curve signify? |
Results in increased unloading of oxygen at the tissue level. Caused by: |
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What does a left shift in oxy-hb dissociation curve signify? |
Results in decreased unloading at the tissue level. Caused by: |
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How is the Haldane effect clinically significant? |
When deoxygenated blood returns to the lungs and is exposed to high levels of oxygen, hgb loses it's affinity to carbon dioxide and carbon dioxide is more easily released. At the level of the muscle however, where o2 concentration is low, the affinity of carbon dioxide is increased which is what is desirable. |
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What is the Bohr effect? |
Relates oxygen binding to hgb to H+ ion concentration. The more acidic the environment, hgb will bind less avidly I. E. Facilitates oxygen dumping to tissues that need it most |
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Besides arterial oxygen content, what is the second way to assess tissue oxygen delivery? |
By evaluating the mixed venous oxygen level |
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Accurate sampling of true mixed venous oxygen saturation requires sampling from what location? |
Pulmonary artery |
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What happens to ventilation and perfusion as you go up the lung? |
Both decrease but perfusion decreases more, causing V/Q mismatch at the top |
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Explain a V/Q ratio of infinity vs 0. |
- if there is ventilation but no perfusion, ratio is infinity (large) = dead space |
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Alveolar gas equation |
PAO2 = ((Pb - Ph20) x FiO2) - (PaCO2/0.8) |
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What are the 5 causes of hypoxemia and their effect on the A-a gradient? |
1. Hypo ventilation - normal |
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What is the respiratory quotient? |
Typically equal to 0.8 Bc 200 mL of CO2 are produced for every 250 mL of O2 consumed (200/250= 0.8) |
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Symptoms of high altitude sickness. |
1. HA |
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Tx of high altitude sickness. |
1. Descend to lower altitude - pts with high altitude pulmonary edema need to descend slowly with assistance Bc increased blood flow can worsen problem. |
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4 major buffer systems in the body. Which is most important? |
1. H2CO3/HCO3 - carbonic acid/bicarbonate (most important) |
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What are causes of respiratory alkalosis? |
Virtually all cases result from hyperventilation caused by: |
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What is the anion gap? |
The sum of all + charges in the body must be counterbalanced by the sum of all - charges in the body. |
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What does an increased anion gap imply? |
The presence of an unmeasured anion and its conjugate H+ producing a metabolic acidosis. |
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Causes of Increased anion gap metabolic acidosis |
MUDPILES |
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What is the primary abnormality in metabolic alkalosis? |
An increase in bicarb usually from the loss of bicarb poor fluid. The body store of bicarb is therefore contained in a smaller volume and bicarb concentration is increases. |
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Common causes of metabolic alkalosis. |
1. Vomiting |
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Most common ionizing group in biologic systems? 2nd most common. |
-MC = amines (narcotics, local anesthetics) - charged in the protonated form |
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What must one know in assessing lipophilicity? |
The major ionizing group as well as the pKa. |
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Explain how local anesthetics and narcotics are subject to fetal ion trapping. What about thiopental? |
In the more acidic fetal environment, the dissociation is to the left - the charged form which has difficulty crossing the placental barrier and thus traps compounds. |
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Effect of CO2 and bicarb on CSF. |
CO2 diffuses rapidly across the BBB into the CSF. Bc bicarb crosses the BBB much more slowly, a paradoxical CSF acidosis results despite a normal or high blood pH. |
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Describe the effect of fever on ABG values. |
PO2 and PCO2 values will be artificially elevated. The rise in PCO2 will lead to a fall in pH. |
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If an air bubble is introduced into a blood gas sample what changes will occur in the sample? Explain. |
-The PaO2 will increase and the PaCO2 will decrease. |
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The determinants of venous oxygen are identified by rearranging the __ equation. |
Fick equation |
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What factors cause low mixed venous oxygenation? |
1. low CO |
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What are factors that cause a high mixed venous oxygenation? |
1. decreased O2 consumption at the tissue level (ex. SNP poisoning) |
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By far the greatest amount of oxygen in normal arterial blood is bound to.... |
hemoglobin |
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What is the most efficient way to increase oxygen content? |
Increase hemoglobin |
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Right shift of the oxyhemoglobin dissociation curve. |
Results in increases unloading of oxygen at the tissue level. |
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Left shift of the oxyhemoglobin dissociation curve. |
Results in decreased unloading of oxygen at the tissue level. |
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Local anesthetics and narcotics are weak ___. The pKa's of local anesthetics range from ___. What happens as pH decreases? |
Bases |
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Electrolyte abnormalities found in pyloric stenosis. |
Hyponatremia |