Internal Medicine Correlations and Clinical Scenarios (CCS) USMLE Step 3

CASE 1: Asthma

Setting: emergency department (ED)

CC: “I can’t breathe.”

VS: BP: 124/76 mm Hg; P: 112 beats/minute; T: 97°F; R: 32 breaths/minute

HPI: A 32-year-old man comes to the ED with 1 day of severe shortness of breath after several days of cough. He has a long history of asthma, which is usually mild. He has only been hospitalized once before several years ago. He stopped using his inhalers 2 weeks ago because he ran out of medications.

PMHX:

Asthma

Allergies (ragweed, pollen)

Medications:

Albuterol intermittently

Fluticasone inhaler

What is the best way to tell how severe the patient’s asthma exacerbation is?

a. Pulmonary function test (PFT)

b. Respiratory rate

c. Wheezing on examination

d. Prolonged expiratory phase

e. Oximeter

Answer b. Respiratory rate

If the respiratory rate is low at 10 to 14 breaths/minute, it does not matter how much the patient says he feels short of breath or if there is wheezing. You can have wheezing but not be in severe distress. You can have extremely severe asthma, and wheezing can stop when all air movement stops. An oximeter can show a normal saturation or one that is >90% to 92% with very severe asthma. A person can be markedly hyperventilating to maintain a normal partial pressure of oxygen (PO₂) and oxygen saturation.

Respiratory rate is the fastest way to tell the degree of respiratory illness.

Effects of Hyperventilation

• Lower partial pressure of carbon dioxide (PCO₂) by 4 mm Hg

• Raise PO₂ by 5 mm Hg

For every 4-point DEcrease in PCO₂, the PO₂ should INcrease 5 points.

Acute asthma in the ED is a case when orders should be done even before the physical examination.

Orders:

Oxygen

Oximeter

Arterial blood gas (ABG)

Chest x-ray

Peak expiratory flow

If tests and treatments are ordered at the same time on the computer-based case simulation (CCS), the test is always done first. If you order the oximeter and ABG at the same time as oxygen, the test will not reflect the treatment.

PE:

General: clear respiratory distress, sitting on edge of bed trying to catch his breath

Chest: bilateral wheezing, prolonged expiratory phase

Abdomen: soft, nontender

Cardiovascular: no murmurs, no gallops

What part of pulmonary function testing would wheezing correspond to?

a. Increased total lung capacity (TLC)

b. Increased residual volume (RV)

c. Decreased forced vital capacity (FVC)

d. Decreased forced expiratory volume at 1 second (FEV₁)

e. Decreased expiratory reserve volume

Answer d. Decreased forced expiratory volume at 1 second (FEV₁)

Wheezing is entirely an expiratory problem in which there is an abnormal narrowing of the airway decreasing the FEV₁. Wheezing is an audible decrease in FEV₁.

As soon as wheezing is detected on examination, inhaled or nebulized albuterol should be ordered. There is no contraindication to inhaled albuterol in anyone. In addition, glucocorticoids should be ordered as soon as possible because there is a delay of 4 to 6 hours in seeing a clinical effect of steroids.

Orders:

Albuterol nebulizer

Methylprednisolone intravenously (IV)

Albuterol is a beta-2-agonist.

Reports:

Oximeter: 94% saturation

ABG: pH 7.52; PCO₂ 24 mm Hg; PO₂ 70 mm Hg

Chest x-ray: no infiltrates, no pneumothorax

Peak expiratory flow: 150 L/second

WARNING! Next part is hard! Numbers ahead!

If a healthy person were to hyperventilate himself to a PCO₂ of 24 mm Hg, what should the expected PO₂ be?

a. 70 mm Hg

b. 80 mm Hg

c. 100 mm Hg

d. 120 mm Hg

e. 150 mm Hg

Answer d. 120 mm Hg

Because the PO₂ should go up if the PCO₂ goes down, a 16-point decrease in PCO₂ should result in a 20-point increase in PO₂. This is the “alveolar gas equation.”

1. Atmospheric pressure 760 mm Hg minus 47 mm Hg for water = 713 mm Hg

2. 21% of this 713 is oxygen = 150

3. Inhaled alveolar air: PO₂ 150

4. 150 – PCO₂/0.8 = expected PO₂

5. 24/0.8 = 30

6. 150 – 30 = 120

Hyperventilation should raise the PO₂ in a healthy person.

Which part of the PFTs does peak flow most closely correspond to?

a. FVC

b. FEV₁

c. Residual volume

Answer b. FEV₁

Peak flow is the maximum rate of exhalation. FEV₁ is a volume of air to move in 1 second, but the peak flow is the maximum rate. FEV₁ is similar to the distance you must drive. Peak flow is similar to the maximum speed of the car.

There is no maximum dose of albuterol inhaler.

Move the clock forward 15 minutes.

Order:

Interval History

Vital signs (to recheck respiratory rate)

Albuterol nebulizer

Decreasing PCO₂ raises pH.

PCO₂ down 10 mm Hg = pH up 0.08

Report:

The patient is still very dyspneic and has difficulty speaking in complete sentences.

The patient’s respiratory rate is 30 breaths/minute.

Which of the following will work as an acute rescue medication in asthma exacerbation?

a. Theophylline

b. Cromolyn

c. Ipratropium

d. Omalizumab

e. Montelukast

Answer c. Ipratropium

Ipratropium and tiotropium have some effect in asthma. They are quaternary amine anticholinergic agents. They inhibit acetylcholine effect and act locally in the lungs because they are not absorbed. The other medications are third-line drugs in the chronic maintenance of preventing asthma exacerbation. None of them works in acute circumstances. Theophylline is a phosphodiesterase inhibitor that increases cyclic adenosine monophosphate (cAMP) (Figure 7-1).

Figure 7-1. The pathophysiology of asthma is complex with participation of several interacting inflammatory cells, which result in acute and chronic inflammatory effects on the airway. (Reproduced with permission from Longo DL, et al. Harrison’s Principles of Internal Medicine, 18th ed., Vol. 2. New York: McGraw-Hill; 2012.)

Omalizumab:

• Monoclonal antibody

• Inhibits immunoglobulin E (IgE)

Ipratropium Mechanism:

• Dilates bronchial smooth muscle

• Decreases secretions of bronchial glands

Cromolyn Mechanism:

• Prevents mast cell degranulation

• Prevents release of histamine

Montelukast Mechanism:

• Leukotriene receptor antagonist

• Decreases bronchospasm

• Does not work acutely

Move the clock forward and reexamine the patient.

Orders:

Albuterol nebulizer

Ipratropium nebulizer

Continue steroids

Magnesium IV

Magnesium has a mild relaxing effect on bronchial smooth muscle.

As you move the clock forward, use 15- to 30-minute intervals. This patient has now received multiple doses of albuterol, ipratropium, magnesium, and steroids. There is nothing more in terms of medications that can be done. Epinephrine adds nothing to albuterol. There is no greater efficacy of epinephrine compared to albuterol, but considerably more adverse effects particularly in those with a history of heart disease.

Orders:

Repeat ABG: pH 7.34; PCO₂ 46 mm Hg

Transfer the patient to the intensive care unit (ICU) and consider intubating the patient because of the increased PCO₂ respiratory acidosis. When asthmatics become tired, respiratory failure can happen very suddenly.

Any respiratory acidosis in asthma is life-threatening.

With severe asthma, your patient may or may not be described as needing ICU care. The key issue for you is to know that even a slight respiratory acidosis or slight increase in PCO₂ is an indication for ICU transfer. If there is no respiratory failure, put the patient on the regular hospital ward until the respiratory rate normalizes to less than 20 breaths/minute and dyspnea decreases. If the patient is described as needing ICU and ventilator management, you can expect the stay to be brief, such as 1 to 2 days, and then the patient will go back to the floor.

After moving the clock forward and seeing the symptoms improve, switch steroids to oral prednisone. Once stable on oral medication and inhaled steroids and albuterol, the patient can be safely transferred home.