USMLE® Step 1 style questions USMLE
A 57-year-old man comes to the emergency department for evaluation of acute-onset shortness of breath and chest pain that began three hours ago. The patient reports poorly-localized chest pain worse with inspiration. He is a business executive and recently returned from an overseas trip. Past medical history includes hypertension and hyperlipidemia. His temperature is 37.3°C (99.1°F), pulse is 91/min, blood pressure is 142/71 mmHg, and respirations are 21/min. Breath sounds are clear to auscultation bilaterally. Examination of the lower extremities reveals swelling and erythema in the right calf. Dorsalis pedis and posterior tibial pulses are 3+ bilaterally. A chest CT is ordered and reveals the following:
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An arterial blood gas is pending. Which of the following findings will most likely be present on laboratory analysis?
Content Reviewers:Rishi Desai, MD, MPH
With respiratory alkalosis, “alkalosis” refers to a process that raises blood pH above 7.45, and “respiratory” refers to the fact that it’s a failure of the respiratory system carrying out its normal pH- balancing job.
Normally, during an inhalation, the diaphragm and chest wall muscles contract to pull open the chest and that sucks in air like a vacuum cleaner. Then, during an exhalation, the muscles relax, allowing the elastin in the lungs to recoil, pulling the lungs back to their normal size and pushing that air out.
CO2 binds to water H2O in the blood and forms H2CO3 carbonic acid, which then dissociates into hydrogen H+ and bicarbonate ions HCO3-. So, in order to prevent pH fluctuations, the CO2 concentration, or the partial pressure of CO2, called PCO2, needs to be kept within a fairly narrow range.For this reason, lungs maintain the ventilation rate they need to get rid of CO2 at the same rate that it’s created by the tissues.
If PCO2 levels start to fall and pH starts to rise, peripheral chemoreceptors that are located in the walls of the carotid arteries and in the wall of the aortic arch start to fire less, and that notifies the respiratory centers in the brainstem that they need to decrease the respiratory rate and depth of breathing.
As the respiratory rate decreases and breaths become more shallow, the minute ventilation decreases - that’s the volume of air that moves in and out of the lungs in a minute.
In respiratory alkalosis, the normal mechanism of ventilation gets disturbed, and the minute ventilation goes higher than what’s needed to balance the pH.
For ventilation to increase, the respiratory centers have to start firing more than usual.
This increased firing may be a normal compensatory response, or an abnormal response to a situation that doesn’t really call for increased ventilation.
Increased ventilation is a normal response to things like hypoxia, a low oxygen level, which can happen with diseases like pneumonia or a pulmonary embolism, or even when a person climbs a high mountain like Mount Everest.
Rarely, brainstem disorders can irritate the respiratory centers and make them fire more.
Sometimes, increased minute ventilation is iatrogenic, meaning that it’s a result of a medical intervention. For example, a person may be intubated and on a ventilator. If the ventilator settings aren’t correct, it can cause a respiratory alkalosis.
In all of these situations, the result is that lungs get rid of too much CO2.