Chapter 9. Critical Care

Respiratory failure is commonly classified as one or both of the following:

Hypoxic (type 1): Characterized by failure of gas exchange resulting in PaO2 <50 mm Hg breathing a gas mixture of at least 50% oxygen (Figure 9-1)
Hypercapneic (type II): Characterized as failure of ventilatory pump or chronic structural changes (BPD, cystic fibrosis) (Figure 9-2)

eFigure 9-1

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Workup of hypoxemia.†Alveolar to arterial gradient (A–a) gradient = PaO2 estimated - PaO2 measured. PaO2 estimated = FiO2 × (PB– 47) – PaCO2/R (atmospheric pressure or PB at sea level is 760 mm Hg; respiratory quotient R is a unitless number representing basal metabolic rate; 0.7 is typically used in our ICU.) A normal A–a gradient is ∼10 mm Hg. (Adapted from Marino PL: The ICU Book, 2nd ed. Baltimore: Williams & Wilkins; 1990:349.)

eFigure 9-2

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Mechanisms of hypercapnia.

Clinical Predictors of Impending Respiratory Failure

Early: Use of accessory muscles of respiration, markedly diminished or absent breath sounds, diaphoresis, inability to speak, AMS, irritability, cyanosis, hypercapnia
Late: Lethargy, apnea, gasping or agonal respiration, bradycardia, hypotension

Management of Respiratory Failure

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Modality

Indication

Dose and Administration

Advantages

Risks and Adverse Effects

O2 via nasal cannula (provides roughly 25%–40% oxygen)*

(Use humidification system)

For non–life-threatening conditions, hypoxemia (asthma, pneumonia, bronchiolitis)

100% oxygen†, typically no more than 4 L/min flow

Well tolerated, easy to use, readily available, no associated toxicity

Skin irritation from cannula, drying of nasal passages, nose bleeds

O2 via simple face mask (provides roughly 35%–50% oxygen)*

For non–life-threatening conditions, hypoxemia (asthma, pneumonia, bronchiolitis)

100% oxygen†at no less than 5 L/min, typically 5–15 L/min

Well tolerated, easy to use, readily available, no associated toxicity, provides more oxygen than nasal cannula

If flow is <5 L/min, “rebreathing” CO2 may occur

O2 via Venturi™ mask (provides 24%–50% oxygen incrementally)

Weaning oxygen from high flow rates

The flow of 100% oxygen through the Venturi mask draws in a controlled, adjustable amount of room air (21% oxygen)

Well tolerated, easy to use, no associated toxicity, fixed and accurate concentration of oxygen

Not readily available

O2 via partial rebreathing face mask (provides roughly 50%–100% oxygen)*

For non–life-threatening conditions, severe hypoxemia (asthma, pneumonia, bronchiolitis)

100% oxygen, 5–15 L/min

Maintain reservoir at least half full on inspiration

High-flow system readily available, provides more oxygen than simple face mask

Nitrogen washout may lead to atelectasis

O2 via nonrebreather facemask (provides ∼80%–100% oxygen)*

Severe hypoxemia, hemodynamically stable pneumothorax (nitrogen washout)

100% oxygen 10–15 L/min

Maintain reservoir at least 2/3 full on ...

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