Peter Stewart
Solving Acid-base problems Stewart Approach Roop Kishen
Interactive Cases Joint tutorial with Dr Patrick Honoré 8th Critical Care Symposium, Manchester
[email protected]
Reminders! • Decrease in HCO3 is not the cause of acidosis! • 3 independent variables determine pH • pCO2 • SID • Total weak acid (ATOT)
• HCO3 & H+ are NOT independent variable! • Best treatment – treat the basic problem • Never forget to look at the patient
Solving metabolic acidosis problems by the bedside • Base excess (SBE) from blood gases • Calculate SID effect (ignore K+, Mg, +Ca+) • {[Na] – [Cl]} – 38 = SBENaCl • Calculate weak acid effect • 0.25 x [42 – measured albumin] = SBEAlb • SBECalc = SBENaCl + SBEAlb • Calculate unmeasured anion effect or BEG • True SBE (or SBEG) = SBE - SBECalc
Case 1: increasing acidosis! • , 59, post-operative patient • Emergency laparotomy for acute abdomen • Perforated colonic diverticulum; contained • • • •
abscess, no peritoneal soiling Haemodynamically stable Uneventful anaesthesia and surgery Increasing acidosis!!! Admitted to ICU
Case 1 Time
Pre-operative
Intra-operative
pH
7.23
7.11
pCO2
4.2 kPa
5.1 kPa
pO2
14.6 kPa (on O2)
22.6 (ventilated, FiO2 - .5)
Bicarb
20 mmol/l
15 mmol/l
BE
-6.3 mmol/l
-12.6 mmol/l
Lactate
1.5 mmol/l
1.2 mmol/l
Case 1 • Anaesthetist (and surgeon) worried • Something is not right! • There is increasing acidosis • Patient, warm, stable, good UO • What is going on???? • Intra-operatively – given 5000 ml fluid • 1000 Hartmann’s, rest 0.9% saline and gelofusin
Case 1 Time pH pCO2 pO2 Bicarb BE Lactate Na Cl Albumin
Pre-operative 7.23 4.2 kPa 14.6 kPa (on O2) 20 mmol/l -6.3 mmol/l 1.5 mmol/l 134 mmol/l 101 mmol/l 36 g/l
Intra-operative
Case 1 – pre-op status • SID effect: [134-101] – 38 = 33-38 = -5 • ATOT effect: 0.25 x [42-36] = 0.25 x 6 = +1.5 • Total effect on BE = -5 + 1.5 = -3.5
• True BE = -6.3 – [-3.5] = -6.3 + 3.5 = -2.8
Case 1 Time Pre-operative pH pCO2 pO2 Bicarb BE Lactate Na Cl Albumin
Intra-operative 7.11 5.1 kPa 22.6 (ventilated, FiO2 - .5) 15 mmol/l -12.6 mmol/l 1.2 mmol/l 138 mmol/l 115 mmol/l 22 g/l
Case 1 – intra-operative • SID effect: [138-115] – 38 = 23 -38 = -15 • ATOT effect: 0.25 x [42-22] = 0.25 x 20 = +5.0 • Total effect on BE = -15 + 5.0 = -10.0
• True BE = -12.6 – [-10] = -12.6 + 10.0 = -2.6
Case 1 – intra-operative
• Metabolic acidosis entirely HCMA due to infusion of large quantities of Cl containing fluid!
Case 2: Shouldn’t she be acidotic!
• , mid sixties, known COPD • Infective exacerbation of COPD; ?LVF • Antibiotics, bronchodilators, steroids and • • • •
diuretics Acute abdomen on the ward ?Sepsis, ?septic shock Hypotensive, tachycardia, hypoxia, Low UO Blood gases – Type 2 Respiratory Failure
Case 2: Ward blood gases Patient on 24% O2
pH 7.48 pCO2 8.2 kPa pO2 8.9 kPa Bicarb 33.5 mmol/l SBE
+7.6
Case 2: • ‘Type 2’ respiratory failure???? • Septic shock but not acidotic • Why???? • There is no lactate measurement • Electrolytes? • ICU admission and resuscitation begun
Case 2: Original Results (ward)
New results
pH
7.48
7.45
pCO2
8.2 kPa
8.6kPa
pO2
8.9 kPa
19..7 kPa (on high flow O2)
Bicarb
33.5 mmol/l
32.1 mmol/l
SBE
+7.6
+7.2
Lactate
?
1.6 mmol/l
Na
?
132 mmol/l
Cl
?
78 mmol/l
Albumin ?
18 g/l
PO4
0.89 mmol/l
?
Case 1 – doing the sums! • SID effect: [132-78] – 38 = 54 - 38 = +16 • ATOT effect: 0.25 x [42- 18] = 0.25 x 24 = +6.0 • Total effect on BE = 16 + 6.0 = 22.0
• True BE (SBEG) = 7.2– 22 = -14.4!!!
Case 2: So?
• Large quantity of unmeasured anion • •
present despite a ‘near normal’ pH! Acidosis missed by traditional approach Labelled as ‘Type 2 respiratory failure’
• Acidosis masked due to widened SID (diuretics) and reduced ATOT
Case 3: Oh my God, what’s happening?
• 35, , known case of intestinal failure • High enteric fistula (just below duodenum) • Presented after a short & minor illness • Stopped her codeine, loperamide (usual •
•
fistula losses <1000 ml/day) Breathless, tachycardic, looked ill on presentation Admitted surgical HDU, ABG’s
Case 3: Arterial blood gases! Blood gases on SHDU pH
7.1
pCO2
1.8 kPa
pO2
21.6 (on face mask high flow O2)
Bicarb
4.8 mmol/l
BE
-21.6
Lactate
0.75 mmol/l
All astounded! What is going on?
Case 3: Next steps • • • •
• •
??Sepsis, ???PE Various other unknown conditions suspected She had:
• • •
Abdominal CT with contrast (Normal) Pulmonary angiogram (Normal) Various other investigations – all within normal limits
ICU resident called – prescribed infusion of 1.4% NaHCO3! Next morning, still acidotic, ‘breathless’, now oliguric Asked to see
Case 3: My doings! • Go and see the patient • Take a detailed history • She had been feeling a bit rough (had flu!) • Although ↑ RR did not look much in distress! • Afebrile (WWC, Hb etc normal) • Stopped loperamide; fistula out put ↑ 6l/day! • Was dehydrated and tachycardic • Now oliguric
Case 3: My doings • Asked for fistula loss Na (106 mmol/l) • Losing Na >600 mmol/day • Na - 121, Cl – 107 (had 0.9% saline as part of • • •
initial resuscitation); Albumin – 31 Routine fluids prescribed Stopped bicarbonate infusion Prescribed Hartmann’s to replace fistula losses
Case 3 – Mystery resolved! • SID effect: [121 - 107] – 38 = 14 - 38 = -24 • ATOT effect: 0.25 x [42- 31] = 0.25 x 11 = +2.75 • Total effect on BE = -24 + 2.75 = -21.25
• True BE (SBEG) = -21.6 – [-21.25] = -0.35!!!
Case 3: Conclusion • The patient settled. • Her subsequent BE settled around -4.5 (as she •
was constantly hyponatraemic) This patient demonstrates that you should never interpret blood gases without looking at the patients and trying to relate investigation to the patient’s conditions
• NEVER TREAT THE NUMBER
Case 4: She needs ventilating!
ABG results! pH = 7.52 PCO2 = 13.6 (102) PO2 = 7.6 (57) Bic = 38.4 BE = +14.7
Case 4: • Another lady, Grade I SAH • 24 hours wait before admitted to our NHDU • Arterial line put in • A ‘blood gas’ • Came to ICU and told me she needs ventilating!
Case 4 • Why does she need ventilating? • Patient conscious • Just finished her breakfast! • Complaining of head ache • A bit confused but otherwise OK • Received a lot of ‘f’ drug in referring hospital!
Case 4: She needs ventilating!
ABG results! pH = 7.52 PCO2 = 13.6 (102) PO2 = 7.6 (57) Bic = 38.4 BE = +14.7 Na = 132 Cl = 63
Case 4: She needs ventilating!
ABG results! pH = 7.52 PCO2 = 13.6 (102) PO2 = 7.6 (57) Bic = 38.4 BE = +14.7 Na = 132 Cl = 63
ABG later that day pH = 7.48 PCO2 = 8.3 (62.2) PO2 = 15.8 (118.5) Bic = 27.9 BE = +4.3 Na = Cl =
137 91
What have we learned? • Acid-base disorders can be complex • Traditional approaches may not explain all • •
things!! Stewart approach can explain a lot Stewart approach can also point to aetiology
• Never interpret blood-gases without reference to history, examination and the patient!
