Acid-Base Diagnosis Flashcards
Acid
- Molecule containing hydrogen atom that can release hydrogen ion when placed in solution
- Strong: Rapid dissociation with release large amount of H+ (HCl)
- Weak: Slow dissociation with release small amount of H+ (H2CO3)
Base
- Ion or molecule that can accept hydrogen ions
- Strong: Reacts strongly and rapidly with H+ and quickly removes larger quantities of H+ from solution (OH-)
- Weak: Reacts slowly forming weak bonds does not remove as much H+ (HCO3-)
Most of the acids & bases in extracellular fluid involved with normal acid-base regulation are
weak acids and weak bases
– H2CO3 and HCO3-
Normal blood [H+] is
40 nEq/liter – 0.00004 mEq/liter
Normal variations of [H+]
3 to 5 nEq/liter
Extreme range variation [H+]
10 nEq/liter to 50 nEq/liter
Hydrogen ion concentration normally expressed on
log scale as pH
pH =
log(1/[H+]) = -log[H+] – (concentration in Eq/liter)
arterial blood pH
7.4
interstitial pH
7.35
VENOUS BLOOD pH
7.35
INTRACELLULAR pH
6-7.4
urine pH
4.5-8
gastric pH
.8
pH Levels at which person can live more than a few hours
6.8-8
Buffer systems
– Bicarbonate system (extracellular) – Phosphate system (extracellular) – Proteins (intracellular)
Lungs control pH by…
– Control of carbon dioxide
Kidneys control pH by
– Control of hydrogen ion concentration – Control of bicarbonate ion concentration
Henderson-Hasselbalch
pH =
pK + log ([base]/[acid]) or pk+ log ([HCO3-]/[CO2dis])
plasma pK @37 degrees
6.1
henderson hasselbach equation applied
pH=pK+log([HC03-]/(.0301 x pCO2))
he bicarbonate/pH chart shows the relationship between the
pH and the bicarbonate ion concentration in the blood. The relationship between the two is defined by the Henderson- Hasselbalch equation.
Two other components must be added to complete the chart.
alkalemia is
pH>7.45
acidemia is
pH< 7.35
hypo and hyperventilation moves
CO2 isobar to the left and right respectively
decreased base/ increased hydrogen and increased base/decreased hydrogen moves
hgb buffer line down and up repectively
metabolic acidosis initially
Buffer Line moves down the pCO2 isobar as [H+] increases and/or [HCO3-] decreases
pH {DN}; [HCO3-] {DN}; pCO2 {NC
metabolic acidosis compensation
Point B to Point C
pCO2 isobar moves to right along Buffer Line as CO2 is blown off pH {UP}; [HCO3-] {DN}; pCO2 {DN}
metabolic acidosis final result
Point A to Point C
pH {DN}; [HCO3-] {DN}; pCO2 {DN}
respiratory acidosis intially
Point A to Point B
pCO2 isobar moves left along Buffer Line as pCO2 increases pH {DN}; [HCO3-] {UP}; pCO
respiratory acidosis compensation
– Point B to Point C
– Buffer Line moves up the pCO2 isobar as the kidney excretes hydrogen ions and retains bicarb
– pH {UP}; [HCO3-] {UP}; pCO2 {NC}
Respiratory Acidosis final
– Point A to Point C – pH {DN}; [HCO3-] {UP}; pCO2 {UP}
Metabolic Alkalosis initial
– Point A to Point B
– Buffer Line moves up the pCO2 isobar as [H+] decreases and/or [HCO3-] increases
– pH {UP}; [HCO3-] {UP}; pCO2 {NC}
Metabolic Alkalosis compensation
– Point B to Point C
– pCO2 isobar moves to left along Buffer Line as CO2 is retained
– pH {DN}; [HCO ] {UP}; pCO {UP} 3- 2
Metabolic Alkalosis final
Point A to Point C – pH {UP}; [HCO3-] {UP}; pCO2 {UP}
Respiratory Alkalosis initial
– Point A to Point B
– pCO2 isobar moves right along Buffer Line as pCO2 decreases
– pH {UP}; [HCO3-] {DN}; pCO2 {DN}
Respiratory Alkalosis compensation
– Point B to Point C
– Buffer Line moves down the pCO2 isobar as the kidney retains hydrogen ions and removes bicarb
– pH {DN}; [HCO3-] {DN}; pCO2 {NC}
Respiratory Alkalosis final
– Point A to Point C – pH {UP}; [HCO3-] {DN}; pCO2 {DN}
Diagnosis of Acid-Base Disorders Step 1 (a)
• Look at pH to identify the most appropriate disorder
– Acidosis (pH < 7.35) – Alkalosis (pH > 7.45)
Diagnosis of Acid-Base Disorders Step 1 (b)
Look at bicarbonate concentration
– If pH indicates acidosis • Metabolic (bicarbonate concentration low) • Respiratory (bicarbonate concentration high)
– If pH indicates alkalosis • Metabolic (bicarbonate concentration high) • Respiratory (bicarbonate concentration low)
Diagnosis of Acid-Base Disorders Step 2
Determine if a second disorder co-exists by looking at compensation for the primary disorder
how to determine second disorder exists
– If primary problem is metabolic acidosis or alkalosis look at the actual pCO2 value
– If primary problem is respiratory acidosis or alkalosis look at [HCO3-]
• If compensation appropriate, there is no secondary problem.
Step 2 Calculations:
Primary Problem Metabolic Acidosis
Calculate predicted value of compensated pCO2. If observed pCO2 is more than 2 mmHg higher than calculated pCO2 there is a good chance of secondary/coexisting respiratory acidosis.
• If observed pCO2 is more than 2 mmHg lower than calculated pCO2 there is a good chance of secondary/coexisting respiratory alkalosis.
step 2 Calculations:
Primary Problem Metabolic Alkalosis
Calculate predicted value of compensated pCO2 pCO2 = 40 + (0.7 x ([HCO3-measured] - ([HCO3-normal]))
Primary Problem Metabolic Alkalosis. If observed pCO2 is more than
5 mmHg higher than calculated pCO2 there is a good chance of secondary/coexisting respiratory acidosis.
• If observed pCO2 is more than 5 mmHg lower than calculated pCO2 there is a good chance of secondary/coexisting respiratory alkalosis.
55 mmHg is the maximum
pCO2 possible when compensating for metabolic alkalosis. pCO2 higher than 55 mmHg means there is a coexisting respiratory acidosis.
Step 2 Calculations:
Primary Problem Respiratory Acidosis
- Calculate predicted value of compensated HCO3-
- It takes the kidneys 24 to 48 hours to compensate. If the onset of the respiratory problem was more than 24 hours ago, use the CHRONIC equation. Otherwise use the ACUTE equation.
Primary Problem Respiratory Acidosis acute/chronic
ACUTE: 1 mEq/L increase in [HCO3-] for every 10 mmHg increase in pCO2 CHRONIC: 3.5 mEq/L increase in [HCO3-] for every 10 mmHg increase in pCO2
• If observed [HCO3-] is more than the calculated [HCO3-] there is a good chance of secondary/coexisting metabolic alkalosis.
• If observed [HCO3-] is less than the calculated [HCO3-] there is a good chance of secondary/coexisting metabolic acidosis.
Step 2 Calculations:
Primary Problem Respiratory Alkalosis
Calculate predicted value of compensated HCO3-
• It takes the kidneys 24 to 48 hours to compensate. If the onset of the respiratory problem was more than 24 hours ago, use the CHRONIC equation. Otherwise use the ACUTE equation.
Primary Problem Respiratory Alkalosis acute/ chronic
ACUTE: 2 mEq/L decrease in [HCO3-] for every 10 mmHg decrease in pCO2 CHRONIC: 5 mEq/L decrease in [HCO3-] for every 10 mmHg decrease in pCO2. • If observed [HCO3-] is more than the calculated [HCO3-] there is a good chance of secondary/coexisting metabolic alkalosis.
• If observed [HCO3-] is less than the calculated [HCO3-] there is a good chance of secondary/coexisting metabolic acidosis.
Diagnosis of Acid-Base Disorders
Step 3
Calculate the anion gap (AG)
AG = [Na+] – ([Cl-] + [HCO3-])
Diagnosis of Acid-Base Disorders
Step 3. AG normal value =
9 to 16 mEq/L
AG > 30 mEq/L
– High anion gap metabolic acidosis
AG > 20 mEq/L
probably high anion gap metabolic acidosis
AG >=16 and <=20 mEq/L
– abnormal but may be due to variety things other than anion gap acidos
Correcting Severe Metabolic Acidosis
- Not all acidosis should be treated with bicarbonate
* If the pH is below 7.10 bicarbonate should be given regardless of cause
Calculate amount of bicarbonate to give:
HCO3-deficit = .5 x Body Weight (kg) x ([HCO3-(desired)] – HCO3-(measured)])
