Critical Care Medicine-Neurologic Disorders>>>>>Hypoxemia and Oxygen Delivery
Question 5#

A 68-year-old male with past medical history significant for COPD, HTN, and heart failure with preserved ejection fraction with newly discovered left upper lobe speculated mass presents from a nursing home facility with complaints of acute onset shortness of breath and chest pain. The patient had undergone an uneventful left upper lobe wedge resection 10 days ago. Chest X-ray obtained in unremarkable except for bibasilar atelectasis. The patient’s vital signs are:

• heart rate of 101 bpm
• blood pressure of 98/72 mm Hg
• oxygen saturation of 90% on 50% FiO₂ , on 50 L flow through high flow nasal cannula

Bedside point of care transthoracic ultrasound is showing normal function in the left ventricle, moderate tricuspid regurgitation, flattening of the intraventricular septum, and moderately depressed right ventricular function.

Which physiologic factor is not involved in this patient’s respiratory distress?

A. Increase in shunt fraction
B. Increase in dead space
C. Limitation in diffusion
D. Decreased cardiac output
E. Ventilation/perfusion mismatch

Correct Answer is C

Comment:

Correct Answer: C

Based on the patient’s clinical presentation (and presence of right heart strain), he is most likely suffering from an acute pulmonary embolism. There are four main mechanisms behind hypoxemia: V/Q mismatch, right to left shunt, impaired diffusion, and hypoventilation. Shunt refers to areas of lung in which there is a complete cessation of ventilation but continued perfusion. Lung consolidation as seen in pneumonia, pleural effusions, or atelectasis represents shunt. In cases of shunt, increasing FiO2 in the alveoli will have minimal improvement on hypoxemia. Technically, pulmonary emboli, a clot in the circulation, should only cause an increase in deadspace. But it also causes shunt and V/Q mismatch by causing edema around the clot. This patient had atelectasis on his chest X-ray, which increases shunt and added to his hypoxemia. Hypoventilation is usually accompanied by hypercapnia along with hypoxemia and can be treated by increasing minute ventilation in most cases (although in cases of severe hypermetabolism, hypoventilation may still exist despite having high minute ventilation).

Right- and left-sided heart failure can lead to a decrease in forward blood flow, leading to more desaturated blood returning to the pulmonary circulation and thus making it even more difficult to have the lungs oxygenate the blood in a normal fashion. Impaired diffusion is likely caused by destruction of lung parenchymal tissue as seen in interstitial lung disease, and hypoxemia is caused by inefficient exchange of gases in this case. This is not seen in pulmonary embolism.

Reference:

1. Rodríguez-Roisin R, Roca J. Mechanisms of hypoxemia. Intensive Care Med. 2005;31:1017.