Pocket Medicine: The Massachusetts General Hospital Handbook of Internal Medicine (53 page)

Mixed disorders
(more than one primary disorder at the same time)

• If compensation less or greater than predicted, may be 2 disorders:

P
_a
CO
₂
too low → concomitant 1° resp. alk.

P
_a
CO
₂
too high → concomitant 1° resp. acid.

HCO
₃
too low → concomitant 1° met. acid.

HCO
₃
too high → concomitant 1° met. alk.

• Normal pH
but
…

↑ P
_a
CO
₂
+ ↑ HCO
₃
→ resp. acid. + met. alk.

↓ P
_a
CO
₂
+ ↓ HCO
₃
→ resp. alk. + met. acid.

normal P
_a
CO
₂
& HCO
₃
,
but
↑ AG → AG met. acid. + met. alk.

normal P
_a
CO
₂
, HCO
₃
, & AG → no disturbance
or
non-AG met. acid. + met. alk.

•
Cannot
have resp. acid. (hypoventilation) and resp. alk. (hyperventilation) simultaneously

Figure 4-1 Acid-base nomogram

•
ABG vs. VBG
: concordant for pH (~0.04), HCO
₃
(~2 mEq) but
not
P
_a
CO
₂
(~8±17 mmHg) VBG can be used to
screen
for hypercarbia w/ P
_a
CO
₂
cutoff ≥45 mmHg (100% Se),
but
does not accurately assess
degree
of hypercarbia (
Am J Emerg Med
2012;30:896)

METABOLIC ACIDOSIS

Initial workup
(
Nat Rev Nephol
2010;6:274)

• ✓
anion gap
(AG) = Na+ – (Cl
^-
+ HCO
₃
^-
) = unmeasured anions - unmeasured cations

if ↑ glc, use measured
not
corrected Na

expected AG is [albumin] × 2.5 (ie, 10 if albumin is 4 g/dL, 7.5 if albumin is 3 g/dL)

↑ AG → ↑ unmeasured anions such as organic acids, phosphates, sulfates

↓ AG → ↓ alb or ↑ unmeasured cations (Ca, Mg, K, Li, bromine, immunoglobulin)

• If ↑ AG, ✓
delta-delta
(ΔΔ = DAG/DHCO
₃
) to assess if there is an additional metabolic acid-base disturbance; DAG = (calculated AG – expected AG), DHCO
₃
= (24 – HCO
₃
)

ΔΔ = 1–2 → pure AG metabolic acidosis

ΔΔ < 1 → AG metabolic acidosis
and
simultaneous non-AG acidosis

ΔΔ > 2 → AG metabolic acidosis
and
simultaneous metabolic alkalosis

Workup for AG metabolic acidosis

• ✓ for
ketonuria
(dipstick acetoacetate) or plasma b-hydroxybutyrate (bOHB) nb, urine acetoacetate often not present in early ketoacidosis due to shunting to bOHB; ∴ acetoacetate may later turn
, but does not signify worsening disease

• If
ketones, ✓
renal function
,
lactate
,
toxin screen
, and
osmolal gap

•
Osmolal gap
(OG) = measured osmoles – calculated osmoles

calculated osmoles = (2 × Na) + (glucose
18) + (BUN
2.8) (can + [EtOH/4.6] if have EtOH level and want to test if other ingestions)

OG >10 → suggests ingestion (see below)

for methanol/ethylene glycol: early on, OG precedes AG; later OG may be nl with
AG

Workup for non-AG metabolic acidosis
(
CJASN
2012;7:671)

• Evaluate history for causes (see above)

• ✓
urine anion gap
(UAG) = (U
_Na
+ U
_K
) – U
_Cl

UAG = unmeasured anions – unmeasured cations; as NH
₄
+ is primary unmeasured cation, UAG is indirect assay for renal NH
₄
+ excretion (
NEJM
1988;318:594)

•
UAG → ↑ renal NH
₄
+ excretion → appropriate renal response to acidemia Ddx: GI causes, proximal RTA, ingestions or dilutional