advanced acid base on the pediatric intensive care unit
DESCRIPTION
acid base, fencl, figge, stewart, anion gap, corrected anion gap, outcomes, mortality, ICU length of stay, education, medicalTRANSCRIPT
Advanced Acid/Base on the PICU David Schmidt SUMMA / Clinician Scientists Training Program
Stewart 101
Definitions
• Ions, Kations, Anions • Strong electrolytes and weak electrolytes • Acid • pH
Governing principles • Law of Electrical Neutrality • Law of Mass Conservation • Law of Mass Action
Acid Base Chemistry
H2O! "# H + +OH $
Kd = [H+][OH $ ]H2O
Kd = the dissocia-on constant Dependent on temp, molecular structure
Strong vs weak electrolytes (e.g. acids)
KA =[H + ][A! ][HA]
HA! "# H + + A$
HA = an acid H+ = a hydrogen ion that lost an electron A-‐ = a deprotonated acid Ka = the dissocia-on constant for the acid (a)
NaCl(s) H 2O! "!! Na+ +Cl#(aq)S = solid Aq = aqueous
Proteins = acids and bases
Alberts, NCBI Bookshelf
Albumin (weak acid) IgA (weak acid) IgG (weak ka-on)
Stewart’s Message: [H+] and [HCO3
-] are dependent
Strong Ion Difference (SID)
• Completely dissociated ions at body pH 1 organic acids with a pKa <4 (’strong acids’) 2 strong electrolytes
• Q: Law of electrical neutrality?
• Unmeasured anions: e.g. lactate, ketones, SO42-
• Unmeasured kations: e.g. lithium, Mg, Ca
pKa = acid dissocia-on constant
SID = [strongkations]![stronganions]"[Na+ ]+[K + ]![Cl! ]
Weak non-volatile acids
HA! "# H + + A$
ATOT = HA+ A!
Meet the Stewart players
• Strong ion difference • Non-volatile weak acids • pCO2
These determine • H+ • HCO3- by • Law of Electrical Neutrality • Law of Mass Conservation • Law of Mass Action
The formulas that govern acid base status
HA! "# H + + A$
SID+[H + ]![HCO3! ]![CO3
2! ]![A! ]![OH ! ]= 0
For each acid (including water)
CO32-‐ = carbonate
A-‐ = total anionic weak non-‐vola-le acids
CO2 +H2OCA! "# H2CO3! "# H + +HCO3
$
SID & pH
PCO2 is held constant at 40 mm Hg. ATOT 20 mEq/L
Morgan TJ. Clin Biochem Rev. 2009 May 13;30(2):41–54.
SID & pH
PCO2 is held constant at 40 mm Hg. SID = 42mEq/L
Morgan TJ. Clin Biochem Rev. 2009 May 13;30(2):41–54.
Summary
• Strong ion difference • Non-volatile weak acids • pCO2
These determine • H+ • HCO3-
Stewart Acid Base Physiology
Stewart Acid Base Status
www.acidbase.org Paul Elbers, MD ©
Stewart Acid Base Status
www.acidbase.org Paul Elbers, MD ©
Stewart Acid Base Status
www.acidbase.org Paul Elbers, MD ©
Hyperchloremic SID acidosis
• 0.9% saline • Rapid infusion
– reduces SID (metabolic acidosis) – reduces ATOT (metabolic alkalosis) SID change predominates over ATOT
• SID changes may also be induced by low Cl- fluids
– 0.45% saline – mannitol – 5% dextrose
Morgan. Clin Biochem Rev 2009
Fluids & Stewart Acid Base
• The balanced crystalloid SID lower than plasma (acidotic) to counteract ATOT dilutional alkalosis
• Experimentally: SID 24 mEq/L
• E.g. Ringer’s lactate (SID=28), Hartmann’s (SID=27)
Morgan TJ. Crit Care. 2005 Apr;9(2):204–11.
Calculating the Strong ion gap
• A- = albumin * constant + phosphate * constant • SID = Na + K + Mg + Ca – Cl – A- • SIG = SID – lactate – bicarb
• Anion gap = Na + K – Cl – bicarb – lactate • AGc = AG / (2.5x reference albumin – pt albumin)
5 Minutes of ICU Acid Base
Acidosis • Sympathoadrenal activation • Right-shift Hb dissociation
curve (+Haldane effect) • Hyperkalemia (shift) • SMC Cathecolamine
responsiveness decreases
Alkalosis • Left-shift Hb dissociation
curve • Hypocalcemia (plasma
protein binding) - cardiac - neuromuscular
• Reduced cerebral blood flow
• Increased peripheral vascular resistance
• Coronary vasospasm • Bronchoconstriction
The physiological effects of pH changes
Morgan & Mikhail’s Clinical Anesthesiology 5e (AccessMedicine)
Prognostic value
• Metabolic acidosis predicts mortality, lactate > other causes
• Increased anion gap acidosis predicts morbidity & mortality
• Metabolic acidosis = 2x mortality risk [adults] • Hyperchloremic acidosis associated with ICU stay, renal
dysfunction, mortality [adults]
Lucking SE, Maffei FA, Tamburro RF. Pediatric Cri-cal Care Study Guide. Springer; 2012. Gunnerson KJ, Saul M, He S, Kellum JA. Crit Care. 2006 Feb;10(1):R22. McCluskey SA, Karou- K, Wijeysundera D, Minkovich L, Tait G, Beadle WS. Anesthesia & Analgesia. 2013 Nov 6.
Balasubramanyan N, Havens PL, Hoffman GM. Unmeasured anions identified by the Fencl-Stewart method predict mortality better than base excess, anion gap, and lactate in patients in the pediatric intensive care unit. Critical Care Medicine. 1999 Aug 1;27(8):1577. - n=255, retrospective cohort - inclusion: PICU + acid base status measured - ATOT determination superior to AG/BE/lactate for predicting
mortality - Discussion: Possibly same with AGc ? Multiple
measurements, error
AGc = albumin corrected anion gap
Dubin A, Menises MAM, Masevicius FD, Moseinco MC, Kutscherauer DO, Ventrice E, et al. Comparison of three different methods of evaluation of metabolic acid-base disorders*. Critical Care Medicine. 2007 May;35(5):1264–70 - n=953, prospective observational cohort - inclusion: ICU - Stewart detected metabolic alterations in 14% of patients
with normal HCO3- / BE
- SIG and AGc are correlated R2=.97 - Stewart and AGc perfomed as good in detecting metabolic
acidosis, and were superior to the traditional approach
SIG = strong ion gap AGc = albumin corrected anion gap
No Consequence for Clinical Practice ?
• Traditional approach is more intuitive and well known, supported by robust experience and evidence
• Provision of clear epidemiological evidence for Steward approached Dx / Tx is not given yet (sample size of research…)
Rastegar A. Clinical Journal of the American Society of Nephrology. 2009 Jul;4(7):1267–74.
5 Minutes of Periop Cardio Acid/Base
Murray DM, Olhsson V, Fraser JI. Defining acidosis in postoperative cardiac patients using Stewart’s method of strong ion difference*. Pediatric Critical Care Medicine. 2004 May;5(3):240–5. - n=44, prospective - inclusion: PICU post-cardiac Sx - Daily acid-base status - Metabolic acidosis: lactate, UA, SID - CPB results in more SID acidosis - Stewart detection of acids is superior: 13% of normal BE
samples had UA. However, AGc almost as good by ROC AUC
UA = unmeasured acids, part of ATOT AGc = albumin corrected anion gap ROC AUC = receiver operated characteris-cs area under the curve
Hatherill M. Hyperchloraemic metabolic acidosis following open cardiac surgery. Archives of Disease in Childhood. 2005 Dec 1;90(12):1288–92. - n=97, prospective - inclusion: PICU post-cardiac Sx - Metabolic acidosis: lactate, UA, SID (less SID than Murray) - No association with CPB time, ventilation, but complexity of
surgery - No association with PICU length of stay - Chloride from CPB priming / renal hypoperfusion?
Durward A, Tibby SM, Skellett S, Austin C, Anderson D, Murdoch IA. The strong ion gap predicts mortality in children following cardiopulmonary bypass surgery*. Pediatric Critical Care Medicine. 2005 May;6(3):281–5. - n=85, prospective - inclusion: PICU post-cardiac Sx - 41% (admission) and 52% (24h) raised strong ion gap - SIG and lactate increased with surgical complexity, but not
length of CPB or aortic cross-clamping
- 5 deaths, 4 of which persistent SIG, 2 lactaemia
Mann C, Held U, Herzog S, Baenziger O. Impact of normal saline infusion on postoperative metabolic acidosis. Pediatric Anesthesia. 2009 Nov;19(11):1070–7. - n=119, prospective - inclusion: PICU post-cardiac Sx - Intervals of Saline infusion / no saline infusion - Saline infusion post-op is associated with metabolic acidosis - This can be calculated by chloride effect of SID
WKZ PICU: 2 patient analyses
• pH, bicarbonate, PCO2, PO2, lactate, urea, ketones, Hb/Ht, Na, K, phosphate, albumin, glucose, osmolality
• Pt A, pH 7.27 – mixed acidosis: low SID (high Cl), increased ATOT (explained by lactate and ketones), phosphate effect (high P). Alongside hypoalbuminemia (metabolic alkalosis).
• Pt B, pH 7.23 – low SID acidosis (high Cl), phosphate effect (high P)
Discussion Translation WKZ Practice
David Schmidt [email protected]
Figge-Fencl Stewart Modification
• Albumin charge is approximately linear over pH 6.9-7.9 • Can thus be calculated
Stewart modification
• Story DA. Strong ions, weak acids and base excess: a simplified Fencl-Stewart approach to clinical acid-base disorders. British Journal of Anaesthesia. 2004 Jan 1;92(1):54–60.
The Scheingraber Gynecology Trial
• Two groups of 12 patients undergoing major intraabdominal gynecologic surgery, saline or lactated Ringer at 30ml / kg BW/h
• Saline caused metabolic acidosis with hyperchloremia and SID decrease
• Infusion of both fluids results in hypoproteinemia and decreased anion gap
• Authors consider condition benign, but argue for treatment
• Complication of respiratory acidosis by opiate analgesics ?
Scheingraber et al. Anesthesiology. 1999 May;90(5):1265–70. Prough DS. Anesthesiology. 1999 May;90(5):1247–9.
Further Repots Saline & Metabolic Acidosis
• Stephens RCM, Mythen MG. Saline-Based Fluids Can Cause a Significant Acidosis That May Be Clinically Relevant. Critical Care Medicine. 2000 Sep 1;28(9):3375.
• Prough DS, Bidani A. Hyperchloremic metabolic acidosis is a predictable consequence of intraoperative infusion of 0.9% saline. Anesthesiology. 1999 May;90(5):1247–9.
• Dorje P, Adhikary G, Tempe DK. Avoiding latrogenic hyperchloremic acidosis--call for a new crystalloid fluid. Anesthesiology. 2000 Feb;92(2):625–6.
• Constable PD. Hyperchloremic Acidosis: The Classic Example of Strong Ion Acidosis. Anesthesia & Analgesia. 2003 Apr;:919–22.
• Eisenhut M. Causes and effects of hyperchloremic acidosis. Crit Care. 2006;10(3):413; authorreply413.