A 56-year-old male with advanced idiopathic pulmonary fibrosis presents to the ICU in respiratory distress. He is put on high flow nasal cannula with 50 L flow, 80% FiO2 . ABG obtained has:
Given his underlying disease, what is the primary physiological aberration leading to the patient’s hypoxemia?
Correct Answer: D
The patient may have several physiologic derangements, but given his diagnosis of fibrosis, diffusion limitation may be the primary physiologic aberration leading to hypoxemia. There are several different mechanisms of hypoxemia: hypoventilation, V/Q mismatch, right to left shunt, diffusion limitation, and reduced inspired oxygen tension. Hypoventilation and reduced inspired oxygen tension will lead to reduced PAO2 , which results in hypoxemia. In the case of hypoventilation, a concurrent rise in PaCO2 will also be noticed. Increasing ventilation and FiO2 will improve oxygenation in these cases.
V/Q mismatch refers to an imbalance between alveolar ventilation and alveolar perfusion. A certain degree of V/Q mismatch exists in normal lungs with V/Q being higher in the apex of the lungs compared to the bases. In pulmonary disease states, the degree of V/Q mismatch will worsen resulting in hypoxemia. In cases of V/Q mismatch, worsening of Aa gradient will be noticed. Right to left shunt results from poorly oxygenated blood passing from the right to left side of the heart without being oxygenated, leading to hypoxemia. Two kinds of right to left shunt exist: anatomic and physiologic. Anatomic shunt refers to when the alveoli are bypassed as in cases of intracardiac shunt or AV malformations. Physiologic shunt refers to when nonventilated alveoli are perfused as in cases of atelectasis.
Diffusion limitation exists when there is a destruction of lung parenchymal tissue, which impairs the movement of oxygen from alveoli to pulmonary capillary. Often times in cases of lung parenchymal disease, V/Q mismatch and diffusion limitation coexists and the exact cause of hypoxemia can be difficult to distinguish.
References:
A 72-year-old male with past medical history significant for pulmonary hypertension and congestive heart failure presents to the ICU in cardiogenic shock with hypotension. Despite initiation of vasopressor therapy, the patient remains hemodynamically unstable and develops worsening hypoxemia. Bedside transthoracic echocardiography shows bowing of the intra-atrial septum toward the left atrium with positive Doppler color flow across the intraatrial septum and a hypokinetic right ventricle.
What is the most likely cause of the patient’s hypoxemia?
Correct Answer: B
In this scenario, based on the patient’s echocardiography results, the patient is showing signs of right-sided heart failure with increase in right atrial pressure leading to right to left shunt through an atrial septal defect. In patients with pulmonary hypertension, it is crucial to avoid hypoxemia, hypercarbia, and acidosis as this can increase pulmonary vascular resistance and if severe enough lead to right heart failure. When the right-sided heart fails to eject blood against the high pulmonary vascular resistance, significant tricuspid regurgitation can develop and rise in right atrial pressure higher than the left atrial pressure can result. Bowing of the intra-atrial septum toward the left atrium indicates increased right atrial pressure, which in presence of an atrial septal defect (seen as color Doppler flow across the septum) causes flow of unoxygenated blood from the right side of the heart to mix with oxygenated blood in the left atrium causing systemic hypoxemia. The resulting hypoxemia can further worsen the patient’s pulmonary hypertension and lead to cardiopulmonary decompensation. Measures to decrease pulmonary hypertension may reduce the right to left shunting.
Reference:
A 38-year-old, 155 cm, 50 kg, previously health female presents to the ICU with altered mental status, tachycardia, hypotension, and high fever. She is intubated for airway protection. Her laboratory test results reveal a markedly elevated T3 and T4 level with a decrease in thyroid-stimulating hormone level consistent with thyroid storm. Her arterial blood gas shows:
Her ventilator settings are volume control, tidal volume of 350 mL, respiratory rate of 14, positive end expiratory pressure (PEEP) of 5, FiO2 of 50%.
Which of the following is the next appropriate step in management?
Increasing the patient’s respiratory rate to increase minute ventilation is the next appropriate step in management of this patient. The patient’s ABG is shows primary respiratory acidosis with elevation in PaCO2 levels. PaCO2 is a measure of the patient’s ventilation, whereas PaO2 is a measure of oxygenation. Elevation in PaCO2 indicates inadequate ventilation, which can be improved by increasing minute ventilation: either by increasing tidal volume or respiratory rate. Increasing the patient’s tidal volumes to 500 mL would not be recommended as this is inconsistent with lung protective ventilation strategy. So, the appropriate course of action here would be to increase respiratory rate, which in turn will decrease PaCO2 levels.
PaO2 measures the patient’s oxygenation and can be adjusted by altering the level of PEEP or fraction of inspired oxygen. Because the PaO2 is within normal range, a change in this parameter is not warranted. Similarly, a change in mode of ventilation is also not indicated.
A 63-year-old male presents to the hospital in acute respiratory distress, with high fevers and cough with purulent sputum. The patient has a history of adenocarcinoma of the lung and has underwent a right upper lobectomy 10 days ago. His vitals are:
Chest X-ray reveals a 3 cm pneumothorax in the right chest.
What is the next appropriate treatment option for this patient?
Correct Answer: C
The first step in managing a bronchopleural fistula (BPF) is to insert a chest tube to allow drainage of air and fluid from the pleural space. BPF is a known complication after lung resection surgery and carries a high mortality and morbidity. BPF should be suspected in patients with a recent history of lung surgery presenting with dyspnea, chest pain, and hemodynamic instability. Once BPF is suspected, imaging (chest X-ray or CT) should be obtained to look for evidence of pneumothorax, or pneumomediastinum. Although supportive therapy such as high flow nasal cannula, initiation of broad-spectrum antibiotics, and diagnostic bronchoscopy are all warranted, the first step in managing BPF is chest tube insertion to drain air and fluid. This prevents worsening of the patient’s pneumothorax. Initiation of NIPPV should be deferred until a chest tube is inserted as it could also potentially worsen the pneumothorax.
A cardiothoracic surgery consult should then be obtained to determine if the patient is a candidate for surgical closure of the BPF. For those who are not a surgical candidate, placement of stents, angiographic coils, or other occlusive material can be considered as a treatment option.
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:
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?
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.
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