Pocket Medicine: The Massachusetts General Hospital Handbook of Internal Medicine (9 page)

•
Fick method
: O
₂
consumption (
O
₂
) (L/min) = CO (L/min) × ∆ arteriovenous O
₂
content

∴ 
CO
=
O2 / C(a-v)O2

O
₂
ideally measured (esp. if ↑ metab demands), but freq estimated (125 mL/min/m
²
)

C(a-v)O
₂
= [10×1.36 mL O
₂
/g of Hb × Hb g/dL × (S
_a
O
₂
–S
_v
O
₂
)].
S
_v
O
₂
is key variable that
Δ
s
.

If S
_V
O
₂
>80%, consider if the PAC is “wedged” (ie, pulm vein sat), L→R shunt, impaired O
₂
utilization (severe sepsis, cyanide, carbon monoxide), ↑↑ FiO
₂
.

Tailored therapy in cardiogenic shock (
Circ
2009;119:e391)

•
Goals
: optimize both MAP and CO while ↓ risk of pulmonary edema

MAP = CO × SVR; CO = HR × SV (which depends on preload, afterload and contractility)

pulmonary edema when PCWP >20–25 (↑ levels may be tolerated in chronic HF)

•
Optimize preload
= LVEDV
LVEDP
LAP
PCWP (
NEJM
1973;289:1263)

goal
PCWP
~
14–18 in acute MI
, ≤
14 in acute decompensated HF

optimize in individual Pt by measuring SV w/ different PCWP to create Starling curve

↑ by giving NS (albumin w/o clinical benefit over NS; PRBC if significant anemia)

↓ by diuresis (qv), ultrafiltration or dialysis if refractory to diuretics

•
Optimize afterload
wall stress during LV ejection = [(~SBP × radius) / (2 × wall thick.)] and ∴ ∝ MAP and ∝ SVR = (MAP – CVP / CO); goals:
MAP
>
60
,
SVR 800–1200

MAP >60 & SVR ↑: vasodilators (eg, nitroprusside, NTG,  ACEI, hydral.) or wean pressors

MAP <60 & SVR ↑ (& ∴ CO ↓): temporize w/ pressors until can ↑ CO (see below)

MAP <60 & SVR low/nl (& ∴ inappropriate vasoplegia): vasopressors (eg, norepineph-rine [a, b], dopamine [D, a, b], phenylephrine [a] or vasopressin [V
₁
] if refractory)

•
Optimize contractility
∝ CO for given preload & afterload;
goal CI
=
(CO / BSA)
>
2.2

if too low despite optimal preload & vasodilators (as MAP permits):

inotropes
: eg, dobutamine (mod inotrope & mild vasodilator) or milrinone (strong inotrope & vasodilator, incl pulm), both proarrhythmic, or epi (strong inotrope & pressor)

mechanical support devices
: eg, IABP, percutaneous or surgical VAD (left-sided, right-sided or both) or ECMO (
Circ
2011;123:533)

HEART FAILURE

Definitions (
Braunwald’s Heart Disease
, 9th ed., 2012)