Pocket Medicine: The Massachusetts General Hospital Handbook of Internal Medicine (34 page)

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Authors: Marc Sabatine

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•
Cyanosis:
seen when >4 g/dL of reduced Hb in blood vessels of skin/mucous membranes central: ↓ S
a
O
2
(pulm disease, shunt); abnl Hb [metHb, sulfHb, COHb (not true cyanosis)] peripheral: ↓ blood flow → ↑ O
2
extraction (eg, ↓ CO, cold, arterial or venous obstruction)
CO binds to Hb more avidly than does O
2
. Pulse oximeter (Ox) misreads COHb as HbO
2
→ falsely nl sat. Oxidizing drugs Δ Hb (ferrous) to MetHb (ferric), which cannot carry O
2
. Pulse ox misreads MetHb as HbO
2
. Cyanide inhibits mitochondrial O
2
use → cellular hypoxia but pink skin and ↑
venous
O
2
sat.
MECHANICAL VENTILATION

Indications

• Improve gas exchange
↑ oxygenation
↑ alveolar ventilation and/or reverse acute respiratory acidosis
• Relieve respiratory distress
↓ work of breathing (can account for up to 50% of total oxygen consumption)
↓ respiratory muscle fatigue
• Apnea, airway protection, pulmonary toilet

Choosing settings
(
NEJM
2001;344:1986)

1. Choose method (including potentially noninvasive ventilation, see later)

2. Pick ventilator mode, and (if appropriate) volume targeted or pressure targeted

3. Set or ✓ remaining variables (eg, F
i
O
2
, PEEP, I:E time, flow, airway pressures)

Tailoring the ventilator settings

• To improve oxygenation: options include ↑ F
i
O
2
, ↑ PEEP
First, ↑ F
i
O
2
. If >0.6 and oxygenation remains suboptimal, then try ↑ PEEP:
If ↑ P
a
O
2
/F
i
O
2
and
P
plat
stable, suggests recruitable lung (ie, atelectasis). Continue to ↑ PEEP until either can ↓ F
i
O
2
to <0.6 or P
plat
≥30 cm H
2
O. If PEEP 20 & F
i
O
2
1.0 and oxygenation remains suboptimal, consider rescue/expt strategies (see “ARDS”).
If ↑ PEEP yields no Δ
or
↓ P
a
O
2
/F
i
O
2
or
↑ P
a
CO
2
, suggests additional lung
not
recruitable and instead overdistending lung → ↑ shunt & dead space; ∴ ↓ PEEP
• To improve ventilation: ↑ V
T
or inspiratory pressure, ↑ RR (may need to ↓ I time). Nb, tolerate ↑ P
a
CO
2
(permissive hypercapnia) in ALI/ARDS (qv) as long as pH >7.15.

Acute ventilatory deterioration (usually ↑ PIP)

• Response to ↑ PIP: disconnect Pt from vent., bag, auscultate, suction, ✓ CXR & ABG
Figure 2-7 Approach to acute ventilatory deterioration

Weaning from the ventilator
(
NEJM
2012;367:2233)

• Perform daily assessment of readiness for spontaneous breathing trial (SBT)
• Clinical screening criteria: VS stable, minimal secretions, adequate cough, cause of respiratory failure or previously failed SBT reversed
• Vent parameters: P
a
O
2
/F
i
O
2
>200, PEEP ≤5, f/V
T
<105, V
E
<12 L/min, VC >10 mL/kg rapid shallow breathing index (f/V
T
) >105 predicts failure; NPV 0.95 (
NEJM
1991;324:1445)

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