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what will arterial blood gases help you determine?
- - how you treat pts
- - from radial artery (painful) or femoral
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what is an acid?
- - a substance that releases H+
- - low pH levels
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What is a base?
- - substance that absorbs H+
- - pH- alkaline
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pH
- - more H+ ions- lowers pH
- - less H+ ions- higher pH
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What is acidemia/ alkalaemia?
- a decrease or increase in arterial blood pH
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What is acidosis/ alkalosis?
- a process which causes acidemia/ alkalaemia
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What is base excess?
- the amount of a strong acid that would need to be added to 1L of blood to return the pH to normal
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What is compensation?
- normal body process to return arterial pH normal
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What is a buffer?
- a solution containing substances which have the ability to minimise changes in pH when an acid or base is adde to it
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Acid base balance refers to?
- bicarb
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Look at notes pg 109 to talk about the buffer
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How does the body get rid of CO2?
- - through alveolar ventilation
- - the body uses this to control H+ in the blood
- - PaCO2 is a direct reflection of the level of alveolar ventilation.
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How is the bicarb controlled?
the renal system
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With more CO2 what is the result in the blood?
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less CO2 in the blood means?
- less H+ ions and a higher pH
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If there is more bicarb in the blood?
- - less H+ ions
- - higher pH
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less bicarb in the blood?
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How can you determine if the problem with pH is a result from respiratory or metabolic systems?
- Acidosis Alk
- CO2 (resp) increase decrease
- HCO3 (met) decrease increase
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What affects does increasing alveolar ventilation have on the respiratory system?
- - decrease PaCO2 = decrease H+ = increase pH
- - respiratory alkalosis
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What affects does decreasing alveolar ventilation have on the respiratory system?
- - respiratory acidosis
- - increase in PaCO2 = increase H+ = decrease pH
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Metabolic system affects on pH?
- - decrease in HCO3 = increase H+ = decrease pH = metabolic acidosis
- - increase HCO3 = decrease H+ = increase pH = metabolic alkalosis
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what can cause respiratory acidosis?
- reduced alveolar ventilation- alveolar hypoventilation
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what can cause respiratory alkalsosis?
- - increased alveoar ventilation
- eg anxiety, fever, cerebral disease, mechanical ventilation
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what causes metabolic acidosis?
- - excess H+ production eg lactic acidosis from anaerobic metabolism
- - renal failure- failure to excrete H+
- - bicarbonate loss eg diarrhoea
- - uncontrolled diabetes (increased acids)
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What causes metabolic alkalosis?
- -vomiting- loss of acid from stomach
- - ingesting bicarbonate
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what does it mean by compensation?
- - if there is a change in acid-base status of the blood- attempts to return the pH back to normal
- - primary disturbances (CO2 levels) are compensated by the metabolic systems and if the primary disturbance is bicarb, then they are compensated by respiratory system
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How is respiratory acidosis compensated? (caused by alveolar ventilation)
- - compensation by bicarb retention by the metabolic system by the metabolic (renal) system
- - 6-12 hrs
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How is respiratory alkalosis be compensated? (caused by increased alveolar ventilation)
- - compensation by increased excretion of bicarb by the metabolic (renal) system.
- - slow compensation - up to 2 weeks
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How is metabolic acidosis compensated? (caused by less bicarb- renal failure)
- - rapid
- - compensation by the respiratory system, increased ventilation to reduced CO2 = reduced H+
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How is metabolic alkalosis compensated? (caused by loss of hydrogen, more bicarb, eg vomiting)
- - respiratory compensation
- - reduction in alveolar ventilation in order to increase CO2
- - doesnt often occur due to the number of other factors affecting respiratory drive
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What are the normal pH values?
7.38- 7.42
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What are the normal PaO2 value?
80-100mmHg
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what is the normal PaCO2 levels?
38-42 mmHg
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What is the normal levels of HCO3?
22-26
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What is the normal level of BE?
-3 to +3
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what is the normal SaO2 levels?
95-100%
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What are the steps to work out what the cause is for a change in the pH of a pt?
- Step 1: look at the H, if it is increased- alkalosis, if it is decreased- acidosis
- Step 2: now work out what caused the change- respiratory or metabolic. Look at CO2- if it is increased or decreased- respiratory. Bicarb and base excess (metabolic)
- Step 3: determine if compensation has occurred. Compensating (not complete)- change in other system but pH is back to normal. Compensated- change in other system and pH back to normal
- - if CO2 and bicarb are both in the diff directions means their is a combined condition
- - what end of the pH scale will tell you what compensated
- LOOK AT PG 112 for an example
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Alveolar- arterial oxygen difference
- PA(alveoli)O2 - Pa(artery)02 = 10
- PAO2 = FiO2 (always a fraction) x 713 - PaCo2/ 0.8
- pg 113
- A-a > 10 could be due to a V/Q mismatch
- PaO2 is gained from the ABGs
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How can you tell if an A-a gradient >10 is a mismatch or hypoventilation?
- V/Q mismatch- purely this CO2 will be normal
- hypoventilation and V/Q mismatch- CO2 will be high
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Carbon dioxide mesures
- CO2 measurement of arterial or alveolar PCO2 is the only reliable indicator of the adequacy of overall alveolar ventilation
- - level of ventilation can aslo affect PaO2, so both should be examined
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interpreting PaO2 and PaCO2
- - good at determining adequacy of total alveolar ventilation
- - determine if there is a problem of gas exchange eg V/Q mismatch
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Adequacy of alveolar ventilation
- - hyperventilation PaCO2 <35mmHg
- - hypoventilation PaCo2 > 45mmHg
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Gas exchange problem?
- - determine is PaO2 is within expected limits (consdier age, height, FiO2)
- - if unexpected PaO2 then determine if it is due to the level of alveolar ventilation ie consider PaCO2. If it is high then low PaO2 may be due to alveolar hypoventilation as well as a V/Q mismatch
- - if PaCO2 is normal then the problem must be with gas exchange ie V/Q
- - if the PaCO2 is high but the PaO2 is lower than expected from the amount of hypoventilation present then the problem must be hypoventilation and low V/Q
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