Formulas / Calculations / Labs Flashcards
Winters Formula
Expected CO2 in metabolic acidosis
(1.5 x HCO3) + 8
Compensation in respiratory metabolic disturbance
In acute disturbances the pH changes 0.08 for every 10 mmHg PaCO2 varies from 40.
1-2-HCO3-4-5 rule for change in HCO3 for every 10 mmPaCO2 varies from 40:
- Acute respiratory acidosis HCO3 increases by 1
- Chronic respiratory acidosis HCO3 increases by 4
- Acute respiratory alkalosis HCO3 decreases by 2
- Chronic respiratory alkalosis HCO3 decreases by 5
A-a Gradient
A-a gradient is calculated as PAO2 – PaO2
PAO2 is the ‘ideal’ compartment alveolar
PAO2 = FiO2 x (760-47) – PaCO2/0.8
- normal A–a gradient for a young adult non-smoker breathing air, is between 5–10 mmHg.
- increases with age (see limitations)
CLASSIFICATION OF HYPOXIA BASED ON A-a GRADIENT
Normal A-a gradient
- Alveolar hypoventilation (elevated PACO2)
- Low PiO2 (FiO2 < 0.21 or barometric pressure < 760 mmHg)
Raised A-a gradient
- Diffusion defect (rare)
- V/Q mismatch
- Right-to-Left shunt (intrapulmonary or cardiac)
- Increased O2 extraction (CaO2-CvO2)
LIMITATIONS
- Gradient varies with age and FiO2:
- FiO2 0.21 – 7 mmHg in young, 14 mmHg in elderly
- FiO2 1.0 – 31 mmHg in young, 56 mmHg in elderly
Anion Gap
primarily used for evaluation of metabolic acidosis to determine the presence of unmeasured ions
(Na+ (+ K+)) - (Cl- + HCO3-)
Normal gap (4-12mmol/L)
- Normal anion gap depends on serum phosphate and serum albumin.
- Albumin is the major unmeasured anion and contributes almost the whole of the value of the anion gap
- every 1g/L decrease in albumin will decrease anion gap by 0.25 moles
Causes of HAGMA
results from accumulation of organic acids or impaired H+ excretion
Causes (CATMUDPILES)
- CO, CN
- Alcoholic ketoacidosis and starvation ketoacidosis
- Toluene
- Metformin, Methanol
- Uremia
- DKA
- Paracetamol, Phenytoin, paraldehyde
- Iron, Isoniazid
- Lactic acidosis
- Ethylene glycol
- Salicylates
Causes of NAGMA
Results from loss of HCO3- from ECF
Causes (CAGE)
- Chloride excess
- Acetazolamide / Addisons
- GI causes - V&D, fibulae (pancreatic, ureteric, biliary, small bowel, ileostomy)
Extra - Renal tubular acidosis
Corrected Sodium
Na + ((glucose - 5)/3)
calculated Osmolarity
2 Na + Ur + gluc
Compensation in Metabolic alkalosis
In metabolic alkalosis the response is hypoventilation, therefore the change is not linear. It will not increase greater than 50-55 to compensate for a metabolic alkalosis. Also a patient will be alkalaemic if PaCO2 is elevated to compensate for a metabolic alkalosis. If acidemic, then an additional respiratory acidosis is present.
- Expected pCO2 = 0.7[HCO3] + 20 (range +/- 5)
- Expected pCO2 = 0.9[HCO3] + 9
Wells PE score
3 - Clinical signs and symptoms of DVT
3 - PE is #1 diagnosis OR equally likely
1.5 - Heart rate > 100
1.5 - Immobilization at least 3 days OR surgery in the previous 4 weeks
1.5 - Previous, objectively diagnosed PE or DVT
1 - Hemoptysis
1 - Malignancy w/ treatment within 6 months or palliative
Wells Score - Probability
Three-Tier Model
0 - 1 Low Risk
2 - 6 Moderate Risk
>6 High Risk
Two Tier Model
≤4 PE Unlikely (with d-dimer)
≥5 PE Likely (with CTA)
The largest study demonstrated risk stratification with:
Low score of 0-1 having a 1.3% prevalence.
Moderate score of 2-6 having a 16.2% prevalence.
High score of >6 having a 37.5% prevalence.
The Christopher study divided the Wells scoring system into 2 categories:
A score of 4 or less was defined as “PE unlikely” and tested with a d-dimer.
A score of 5 or more was defined as “PE likely” and went straight to CTA
PERC rule
1 - Age >50
1 - HR > 100
1 - Sats of RA <95%
1 - Unilateral leg swelling
1 - Haemoptysis
1 - Recent surgery or trauma
Surgery or trauma ≤4 weeks ago requiring treatment with general anesthesia
1 - Prior PE or DVT
1 - Hormone use
Oral contraceptives, hormone replacement or estrogenic hormones use in males or female patients
