Acid/Base Physiology Flashcards
What is the henderson hasselbalch equation?
pH = pKa + log ( [A-] / [HA] )
Are the end products of metabolism generally acids or bases?
Acids
What is pKa?
This is the pH at which the concentration of acid and its conjugate base are equal in a buffering system.
pH = pKa + log ( [A-] / [HA] )
When [A-] = [Ha], pH = pKa
What makes an optimal buffer system?
A buffer system in which the pKa is as close as possible as your target pH
Name 3 intracellular buffer systems
- Organic phosphates
- Proteins
- Hemoglobin
Name 4 extracellular buffer systems
- Proteins
- Albumin
- Phosphate
- Bicarbonate
What is the bicarbonate buffering system and what enzyme participates?
This is the bicarbonate buffering system. Notice that ventilation can get rid of CO2 and can drive the reaction towards the left.
Why is it okay to substitute CO2 as the conjugate acid for bicarbonate (HCO3-)?
H2CO3 is rapidly converted by carbonic anhydrase so CO2 can be substituted as bicarbonate’s conjugate acid in equations.
What is the Henderson–Hasselbalch equation for the bicarbonate buffering system?
pH = 6.1 + log ( [HCO3-] / 0.03PaCO2)
What is a typical [HCO3-]?
24 meq/L
What is a typical PaCO2?
40 torr
Plug in typical values to the bicarbonate Henderson–Hasselbalch equation. What do you get for the pH?
pH = 6.1 + log (24/ 0.03(40))
= 7.4
What is the pH of a typical arterial blood sample?
7.4
What is normal arterial blood pH range? how about for venous blood?
Arterial: 7.38-7.43
Venous: 7.34-7.37
Is blood pH typically higher or lower than normal in Denver?
A little higher because at higher altitudes you tend to blow down more CO2.
Is venous pH higher or lower than arterial pH? Why?
Venous pH is more acidic because metabolic end products are acidic.
What is the buffer that keeps venous pH only slightly lower and venous PCO2 only slightly higher despite the amount of CO2 being carried?
Deoxyhemoglobin
What is acidemia?
More acid in the blood than normal, resulting in a lower pH (less than 7.40)
What is alkalemia?
More base (or less acid) in the blood than normal resulting in a higher pH (greater than 7.40)
What are two ways that the body can try to compensate for disturbances in blood pH?
- Lungs can regulate CO2 levels (takes minutes)
- Kidneys can regulate bicarbonate (takes hours to days)
True or False: compensation will never completely correct to normal pH, nor will it over-compensate
True
What is respiratory acidosis?
Acidosis (Low pH from too much CO2) caused by ineffective ventilation.
What is acute respiratory acidosis vs. chronic respiratory acidosis?
Acute respiratory acidosis is before the kidneys can compensate
What are the compensation rules for respiratory acidosis in both acute and chronic situations?
Acute respiratory acidosis: for every 10 torr increase in CO2, pH decreases by 0.08
Chronic respiratory acidosis: for every 1 torr increase in CO2, HCO3- increases about 0.4 meq/L
Respiratory acidosis is caused by ineffective ventilation. What are the 2 main things that cause acute respiratory acidosis?
- CNS depressants (opiates, benzodiazepines, alcohol, etc)
- Respiratory muscle fatigue (increased work of breathing)
Respiratory acidosis is caused by ineffective ventilation. What are the 4 main things that cause chronic respiratory acidosis?
- central hypoventilation (e.g. obesity hypoventilation syndrome)
- Neuromuscular disease (e.g. ALS)
- Chronic lung diseases (e.g. emphysema, bronchiectasis, etc)
- Hypothyroidism
What is respiratory alkalosis?
Increased ventilation that causes a decrease in CO2 resulting in higher pH.
What are the compensation rules for acute and chronic respiratory alkalosis?
Acute respiratory alkalosis: for every 10 torr decrease in CO2, pH increases by 0.08
Chronic respiratory alkalosis: for every 1 torr decrease in CO2, HCO3- decreases about 0.4 meq/L
What are 4 acute causes for respiratory alkalosis?
- Pain
- Anxiety/Panic attack
- Fever
- Mechanical Ventilation
What are 4 chronic causes for respiratory alkalosis?
- Living at altitude
- Brain injury
- Chronic salicylate toxicity
- Pregnancy
What is metabolic acidosis?
When the body produces excessive quantities of acid or when the kidneys are not removing enough acid from the body, resulting in decreased HCO3- and lower pH
How does respiratory compensation work with metabolic acidosis?
Increased ventilation (respiratory rate) results in decreased PCO2
What are the compensation rules for metabolic acidosis?
Winter’s formula
Expected PCO2 = 1.5[HCO3-] + 8 +/- 2
This formula is to see if respiratory compensation for metabolic acidosis is adequate
What is winter’s formula?
Expected PCO2 = 1.5 [HCO3-] + 8 +/- 2
What are the 2 categories of metabolic acidosis?
- Anion gap
- Non-anion gap
What is a typical anion gap value?
12-14
How do you calculate anion gap?
[Na+] - ( [Cl-] + [HCO3-] )
You are subtracting the major anions from the major cation
What does an elevated anion gap indicate?
An elevated anion gap indicates that there is presence of additional acid in the blood which is being buffered by bicarbonate and thereby increases the amount of unmeasured anions
Anion Gap = [Na+] - ( [Cl-] + [HCO3-] )
You can see that if there is additional acid in the blood being buffered by HCO3- , you will end up with less HCO3- so you’re subracting less anions from cations and resulting in a larger value for the anion gap.
What causes non-anion gap metabolic acidosis? (name 3)
Loss of bicarbonate by 3 main ways:
- GI losses (e.g. diarrhea)
- Renal losses
- Too much IV saline (increases Cl- with loss of bicarbonate)
What causes anion gap metabolic acidosis? (name 8 things)
Mudpiles:
Methanol
Uremia
DKA (and other ketoacids, namely EtOH and starvation)
Propylene Glycol (agent in some sedatives)
INH (Isoniazid - used to treat tuberculosis)
Lactate
Ethylene Glycol (antifreeze)
Salicylates
What is metabolic alkalosis?
Metabolic alkalosis is a metabolic condition in which the pH of tissue is elevated beyond the normal range (7.35-7.45) due to too much HCO3-.
What is the respiratory compensation for metabolic alkalosis?
Respiratory compensation happens as decreased ventilation resulting in increased PCO2.
*will not hypoventilate to the point of hypoxemia, so this means that the compensation is generally inadequate
What are the compensation rules for metabolic alkalosis?
Increase [HCO3-] of 1 meq/L increases PaCO2 by 0.7 Torr
What are 5 causes of metabolic alkalosis?
- vomiting or NG tube suction (loss of gastric acid)
- Ingestion of NaHCO3
- Ingestion of other alkali (milk-alkali syndrome)
- Hypovolemia, so-called contraction alkalosis
- Diuretics
A patient presents to the ED with a broken arm and the following blood gas:
pH = 7.52, PaCO2 = 25 torr; PaO2 = 85 torr; [HCO3-] = 21 meq/L
What is the condition?
Acute Respiratory Aklalosis
pH = 7.52, PaCO2 = 25 torr; PaO2 = 85 torr; [HCO3-] = 21 meq/L
pH is high = Alkalosis
PaCO2 is low = Respiratory
Note: The rise in pH fits with the compensation rule. There was a 15 torr drop in the PaCO2 which resulted in a 0.12 increase in pH (0.08 increase per 10 torr drop)
A patient presents to the ED with some nausea/vomiting in past few days with the following blood gas:
pH = 7.53; PaCO2 = 42 Torr; PaO2 = 110 Torr; [HCO3-] = 36 meq/L
What is the condition? Is the compensation appropriate?
Metabolic Aklalosis
pH = 7.53; PaCO2 = 42 Torr; PaO2 = 110 Torr; [HCO3-] = 36 meq/L
pH is high = Aklalosis
PaCO2 and [HCO3-] are high = Metabolic
Is the compensation appropriate?
Increase [HCO3-] of 1 meq/L increases PaCO2 by 0.7 Torr
Expected PaCO2 compensation = (36-24)(0.7) = 8.4
42 is less than 48.4 so the compensation is incomplete.
Note that in order to increase PaCO2 by 8.4 points, you would have to hypoventilate to the point of hypoxemia, which the respiratory centers of the brain generally will not allow.
A patient was brought into the ED by a friend because he is stuporous. Pulse oximetry shows SaO2 of 85%. He has an arterial blood gas drawn and it shows:
pH = 7.31; PaCO2 = 48 torr; PaO2 = 55 torr; [HCO3-] = 23 meq/L
What is the condition?
Acute Respiratory Acidosis
pH = 7.31; PaCO2 = 48 torr; PaO2 = 55 torr; [HCO3-] = 23 meq/L
pH is low = Acidosis
PaCO2 is high = respiratory
This is acute because bicarbonate is normal (kidney has not had time to compensate yet).
The drop in pH follows the compensation rules as we expect a decrease of about 0.08 pH per 10 torr rise in PaCO2
Patient presents to the ED with some nausea/vomiting in the past few days with the following blood gas:
pH = 7.07; PaCO2 = 18 torr; PaO2 = 78 torr; [HCO3-] = 5 meq/L
What is the condition?
Metabolic Acidosis
pH = 7.07; PaCO2 = 18 torr; PaO2 = 78 torr; [HCO3-] = 5 meq/L
pH is low = acidosis
Bicarbonate and PaCO2 are low = metabolic
Is this an anion gap or non-anion gap metabolic acidosis?
[Na+] = 132 meq/L, [Cl-] = 94 meq/L, glucose = 560mg/dl
132 - 94 - 5 = 33
33 is greater than the typical gap of 12-14 so this is an anion gap metabolic acidosis which means that there is a presence of additional acid in the system.
Is this compensated or not?
Expected PaCO2 = 1.5(5) + 8 +/- 2
= 15.5 +/- 2
This patient is adequately compensated as the patients PaCO2 of 18 torr is very close to the expected PaCO2. Even though the patient’s pH is very low, the patient’s lungs have adequately compensated for this acidosis.
