Week 9 - Blood Gas Analysis Flashcards
Why do we care about acid-base balance in anesthesia?
A physiologic acid-base balance optimizes enzyme function, myocardial contractility, and saturation of hemoglobin with oxygen
*increase in carbon dioxide forces pH to drop, which causes the affinity of Hbg to oxygen to decrease
What is the carbonic acid-bicarbonate buffer equation?
Equation
What role does the kidneys play in acid-base balance?
Proton secretion (H+) and HCO3 filtration
- if PCO2 rises, proton secretion becomes dominant and the kidney excretes acid, raising blood pH
- if HCO3 rises, HCO3 filtration increases and the kidney excretes alkali, reducing blood pH
*adjusts H+ ion secretion and HCO3 filtration in response to elevated CO2 or HCO3
What are the normal arterial blood gas values for the following on room air?
- pH
- PCO2
- HCO3
- PO2
pH 7.35-7.4
PCO2 35-45 mmHg
HCO3 22-28 mEq/L
PO2 80-100 mmHg
What are the normal venous blood gas values for the following on room air?
- pH
- PCO2
- HCO3
- PO2
pH 7.30 - 7.40
PCO2 42-48 mmHg
HCO3 24-30 mEq/L
PO2 35-45 mmHg
Fill in the chart
How does the body compensate for acidemia and alkalemia?
- Metabolic Acidosis –> Respiratory compensation (CO2 <40)
- Respiratory Acidosis –> Renal compensation (HCO3 >24)
- Metabolic Alkalosis –> Respiratory compensation (PCO2 >40)
- Respiratory Alkalosis –> Renal compensation (HCO3 <24)
How long does it take for acid-base compensation to occur?
Respiratory compensation is almost immediate
Metabolic compensation takes 6-12 hrs to appear, several days to maximize
What is acute respiratory acidosis? What can cause this in anesthesia?
Increase in CO2
- Hypoventilation (pressure control ventilation)
- Increased CO2 production (MH, sepsis, shivering, prolonged seizure, thyroid storm)
- Rebreathing (exhausted or missing CO2 absorber, incompetent one way valves)
- Laparoscopic insufflation
How do you treat respiratory acidosis?
- Most cases, increase ventilation (bronchodilation, reversal of narcosis or sedation)
- Decrease CO2 production (dantrolene, tx shivering)
- In chronic respiratory acidosis (return to baseline PCO2, respiratory drive may be dependent upon low PaO2)
What is chronic respiratory acidosis?
Elevated PCO2 with near normal pH
Shift to right of respiratory response curve
What is the cause of respiratory alkalosis?
- Usually due to over-ventilation and usually easily remedied by decreasing total minute ventilation
- If spontaneous, maybe be caused by increased breathing due to pain, anxiety, drugs, CNS disease, fever etc…
- Decreases stimulus to breathe
What is metabolic acidosis? What is typical compensation?
- Accumulation of any acids in the body
- Normal physiologic compensation is to increase respiratory rate – if mechanically ventilated we need to compensate
How is metabolic acidosis diagnosed?
Differential Diagnosis = Anion Gap
-provides info as to whether the acidosis is due to increased acid accumulation or bicarbonate loss
What is the anion gap?
Difference between major measured cations (Na+) and anions (Cl- + HCO3-)
-there are always more unmeasured anions (albumin…) than cations (K+, Ca++..)
Normal Gap is 7-14
Anion Gap = [Na] - [Cl + HCO3]
*any process that increases unmeasured anions or decreases unmeasured cations will increase the gap (and vice versa)
What is metabolic acidosis with an increased gap characterized by?
An increase in relatively strong nonvolatile acids
- these acids dissociate into H+ and their respective anions
- the H+ consumes HCO3 to produce CO2, whereas their anions (conjugate bases) accumulate and take the place of HCO3 in extracellular fluid
What is typical causes of high anion gap acidosis?
Lactic acidosis
Ketoacidosis
*acid present that dissociates with H+ which consumes bicarb
What are causes of intra-op high anion gap lactic acidosis?
Tissue Hypoperfusion
Hypoxemia
What are the causes of intra-op high anion gap non-lactic acidosis?
Ketoacidosis – diabetes, ethanol, starvation
Uremia Salicylates Ethylene glycol Methanol Paraldehyde Isoniazid
What is intraop development of metabolic acidosis frequently attributed to?
Hypovolemia
Tissue Hyoperfusion
Lactic Acidosis
How do you treat lactic acidosis?
- Increase oxygenation
- Controlled respiration, keep PaCO2 in low 30s
- Fluid resuscitation
- Circulatory support, maintain BP and CO!
- Bicarb (only for severe acidosis <7.1) – doesn’t treat problem, only temp measure and may cause intracellular acidosis, must increase RR to compensate for CO2
What are causes of normal anion (hyperchloremic) gap acidosis?
- GI loss of bicarb (diarrhea – pancreatic, small bowel, biliary fluids are all rich in bicarb)
- Renal wasting of bicarb (renal tubular acidosis)
- Dilutional hyperchloremic acidosis (rapid volume expansion with 0.9% NS results in excessive chloride and impaired bicarb reabsorption)
What is the equation to determine the bicarb deficit in mEq?
Deficit in mEq = Base Deficit x 0.3 x bodyweight (kg)
Define base excess
The amount of strong acid or base that has to be added to a sample of blood to produce a pH of 7.4
-reflects the non respiratory contribution to acid-base balance (the metabolic component)
What are the base deficit categories?
Normal: -2 to 2
Mild: -5 to -3
Moderate: -9 to -6
Severe: < -10
What is metabolic alkalosis and its causes?
Gain of bicarbonate or loss of hydrogen
- loss of H+ from stomach (vomiting) or kidney (diuretic therapy)
- Alkali administration (citrate in blood products, TPN, giving bicarb)
- hypovolemia leading to decreased chloride which increased bicarb reabsorption with sodium
*may be compensatory from over-ventilation
What is the 1 for 10 rule for ACUTE Respiratory Acidosis?
HCO3 will increase by 1 mmol/L for every 10 mmHg elevation in pCO2 above 40 mmHg
Expected HCO3 = 24 + {(Actual pCO2 - 40) / 10}
- ex. pt with acute respiratory acidosis (pCO2 60mmHg) has an actual HCO3 of 31 mmol/L.. the expected HCO3 for this acute elevation of pCO2 is 24+2 = 26 mmol/L
- the actual measured value is higher than this indicating that a metabolic alkalosis must also be present
What is the 4 for 10 rule for CHRONIC respiratory acidosis?
HCO3 will increase by 4 mmol/L for every 10 mmHg elevation in pCO2 above 40 mmHg
Expected HCO3 = 24 + 4 {(Actual pCO2 - 40) / 10}
- pt with chronic respiratory acidosis (pCO2 60) has an actual HCO3 of 31.. the expected HCO3 for this chronic elevation is 24 + 8 = 32
- the actual measured value is extremely close to this so renal compensation is maximal and there is no evidence indicating a second acid-base disorder
What is the compensation formula for metabolic acidosis?
pCO2 = 1.5 [HCO3] + 8
What is the compensation formula for metabolic alkalosis?
pCO2 = 0.9 [HCO3] +16
What is the compensation formula for respiratory acidosis?
For every 10 change in pCO2, pH decreases by:
- 08 in acute resp acidosis
- 03 in chronic resp acidosis
What is the compensation formula for respiratory alkalosis?
For every 10 change in pCO2, pH increases by:
- 08 in acute respiratory alkalosis
- 03 in chronic respiratory alkalosis
What is the step by step approach to interpreting blood gas?
- Look at pH
- What is the process? look at pCO2 and HCO3 to distinguish the initial change from the compensatory response
- Calculate the anion gap (Na - (Cl + HCO3))
- Is compensation adequate? calculate estimated pCO2 (1.5 x HCO3 + 8 +/- 2)
