Respiratory Failure

Question 1

Calculation of A-a gradient

Answer 1

PAO2 = [(Pbar - PH2O) x FiO2] - (PaCO2/R)

Normal A-a gradient is 12 +/- 2

Question 2

Parameters of oxygenation

Answer 2

FiO2 - between 21% (room air) and 100% (pure O2)

PEEP - prevents end-expiratory atelectasis, maintaining alveolar recruitment

Question 3

Parameters of ventilation

Answer 3

Respiratory Rate

Tidal Volume

Question 4

Diagnosis of ARDS

Answer 4

Occurs within 1 week of known clinical insult or worsening of respiratory symptoms

1. Diffuse bilateral radiographic infiltrates
2. PaO2:FiO2 ratio < 300 (ALI) or < 200 (ARDS)
3. No evidence of cardiogenic etiology

Question 5

Common causes of ARDS

Answer 5

Sepsis
Pancreatitis
Trauma
Aspiration
Transfusion
Amniotic fluid / fat emboli

Question 6

Characteristics of ARDS

Answer 6

Intense alveolar inflammation

Increased pulmonary capillary permeability

Alveolar flooding with proteinaceous edema (hyaline membrane deposition)

Hemorrhage

Question 7

Outcomes of ARDS

Answer 7

Mortality rate 30-40%

Surviving patients may experience near-complete resolution or development of fibroproliferative disease and chronic interstitial lung disease

Question 8

Hypoxemic respiratory failure

Answer 8

Respiratory failure due to inadequate oxygenation (SpO2 < 87% or PaO2 < 55)

Caused by:

Impaired gas diffusion
V/Q mismatch
Alveolar hypoventilation
High altitude

Question 9

Hypercapneic respiratory failure

Answer 9

Respiratory failure due to inadequate CO2 removal; caused by any process that impairs ventilation

Obstructive lung disease
Restrictive lung disease
Central causes of hypoventilation

Question 10

Acute ABG response to hypercapneic respiratory failure

Answer 10

HCO3- increases 1mEq/L for every 10 mmHg increase in PaCO2

Change in pH / change in pCO2 = 0.008

Question 11

Chronic ABG response to hypercapneic respiratory failure

Answer 11

HCO3- increases 4mEq/L for every 10 mmHg increase in PaCO2

Change in pH/Change in PCO2 = 0.003

Question 12

What is the ONLY management strategy that improves survival in ARDS?

Answer 12

Ventilation with low tidal volumes (6cc/kg)

RR can be increased as far as possible; acidosis may be corrected with sodium bicarbonate (“permissive hypercapnea”)

Ventilating patients in the prone position may confer an additional advantage