Hypoxemia
Hypoxemia and hypoxia are commonly used interchangeably as a way to describe a state of low oxygen levels. However, hypoxemia literally means low oxygen levels in a patient’s arterial blood (PaO2) and hypoxia refers to low oxygen levels in tissue. You can commonly see both if hypoxemia persists long enough, and you can also see them individually (ie. tourniquet applied to a distal extremity creating a lack of oxygen perfusion/flow). Monitoring oxygenation is one of our 4 major vital signs that we follow in the operating room. We can accomplish this three ways: a pulse oximeter, an FiO2 analyze, and monitoring the patients skin and mucous membranes for cyanosis.
There are 4 binding site for oxygen per hemoglobin (red blood cell). Hemoglobin will pick up oxygen molecules from the alveoli of the lungs and travel through arteries to peripheral capillaries. The capillaries is typically where we measure a pulse ox (ie. finger tips, toes, ears). After the hemoglobin deposits oxygen molecules in the capillaries of peripheral tissue, the deoxygenated hemoglobin will return back to the lungs to pick up more oxygen molecules. The pulse oximeter uses plethysmography and absorption of red and blue light waves to determine the percentage of hemoglobin that are fully bound by oxygen. Our FiO2 analyze will alarm if we are delivery a hypoxic mixture of gas (FiO2 < 21%). And if observing for physical signs of hypoxia in our patient, cyanosis will begin to present around 70-80% SpO2 (a late and severe finding). As hypoxia progresses, you can start to see mental status decline and ultimately cardiovascular collapse.
In the operating room, hypoxemia can happen quickly and it is important to have an organized approach to finding the underlying problem and correcting it. The differential diagnosis of hypoxemia can be divided into 5 categories:
1) Low FiO2
2) Hypoventilation
3) Diffusion Abnormalities
4) V/Q Mismatch
5) Right to Left Shunt
Another way to assess hypoxemia is to look at the A-a gradient. This refers to the partial pressure of oxygen in the alveoli (PAO2) minus the partial pressure of oxygen in the arteries (PaO2). The first two categories above are examples with a normal A-a gradient (5-10 mmHg) and are considered extrathoracic causes. The last three groups are examples intrathoracic causes of hypoxemia with high A-a gradients ( >10 mmHg).
Starting with low FiO2, in the general population we think about this cause for patients who live or are traveling to areas of high elevation (ie. Denver, CO). As you increase in altitude, the partial pressure of oxygen decreases, decreasing the FiO2 that patients breath. This can also happen during diving explorations when a diver runs out of oxygen in their tank. In the operating room, this can also happen a couple different ways. The most common way if for part of your ventilator to get disconnected from the patients breathing tube. If you suspect this, start at the ventilator and work your way towards the patient to assess for any disconnection in the multiple joints of the breathing circuit. Secondly, although rare there can be an oxygen failure in the main gas supply for the operating rooms. If you suspect this, you must disconnect the O2 tubing from the wall mount which will allow your ventilator to use oxygen from the E-cylinder located behind the ventilator. Also, when transporting a patient from the OR to an ICU, make sure you have enough oxygen in your portable tank.
The most common reason for hypoventilation in the OR is medication induced from the anesthetics that we use. Pretty much all of our medications that we use to keep patients asleep will cause some degree of respiratory depression. If you give too much, the patient will go apneic. If you aren’t prepared to take over the patient’s airway, airway obstruction will lead to hypoventilation which will lead to hypercarbia and ultimately hypoxemia. Oxygen is unable to get to the alveoli and CO2 builds up. The alveoli are depleted of oxygen molecules from passing blood in the pulmonary capillaries. Other causes of hypoventilation in the operating room include a kinked circuit or endotracheal tube and a pneumothorax.
Interstitial lung disease creates a pathological state that thickens the interface between the alveoli and the pulmonary capillaries. This makes it more difficult for oxygen molecules to diffuse across and enter the blood stream. Diseases that fall into this category include pulmonary fibrosis and emphysema. They are diagnosed with a pulmonary function test called DLCO - diffusion of lung capacity for carbon monoxide in one inhalation after 10 seconds. These patients can look short of breath at rest and are often on home oxygen. Be prepared to delivery a higher FiO2 than their baseline needs once they are under anesthesia.
Right to left shunt refers to any pathological state when blood enters the right side of the heart and bypasses the lungs to enter the left side of the heart. In other words, unoxygenated blood is entering into systemic circulation. Diseases that fit this description are intracardiac shunts like atrial/ventricular septal defects, pulmonary arteriovenous malformation, and hepatopulmonary syndrome. This is the one cause that DOES NOT improve with oxygen. No matter how much oxygen you put in the alveoli, the blood flow will not see it due to the underlying disease. For these cases, you do your best to manage hemodynamics that favor a low shunt fraction. But ultimately the patient will likely need some intervention to correct the abnormal blood flow.
And finally, let’s talk about V/Q mismatch. This is probably the most common cause for hypoxemia in the general hospitalized population. “V” stands for ventilation, and “Q” stands for perfusion. There can be states of high V/Q mismatch and low V/Q mismatch - both will end in hypoxemia. A state of high V/Q describes normal ventilation with decreased blood flow. The most common example of this is a pulmonary embolism, blocking blood flow to segments of a lung up to potentially a full lung if large enough. A state of low V/Q describes low ventilation with normal blood flow. In other words, you are perfusing a diseased lung or lungs that are unable to be ventilated. Examples of this would be a pneumonia, pulmonary edema, and atelectasis.
