USMLE® Step 1 style questions USMLE
A 78-year-old male presents to the emergency department with altered mental status and fatigue. He is found to be critically ill and is admitted to the intensive care unit (ICU) for further evaluation and monitoring. While in the ICU, the patient has a Swan-Ganz catheter placed in the right heart for continuous hemodynamic measurements. An initial set of readings is demonstrated below:
Which of the following is the most likely cause of this patient's clinical presentation?
Content Reviewers:Rishi Desai, MD, MPH
So, when we talk about ischemia, we’re usually talking about this lack of blood flow to a specific area of tissue. For example, with a heart attack, a coronary artery in the heart that supplies the left ventricle with blood gets blocked, so that localized area of heart tissue doesn’t get enough blood and oxygen; that damage is localized to that left ventricle. Shock is like ischemia, but on a global scale. In other words, it’s a circulatory failure of the whole body; blood flow to tissues is dangerously low, which leads to cellular injury, possibly damages multiple organs, and can even lead to multiple organ failure if not treated immediately.
Okay, so with shock, the body’s tissues aren’t getting enough oxygen via the blood, right? Normally, blood perfuses through tissue and delivers oxygen because there’s enough pressure in the circulatory system to push it through; so, blood pressure majorly affects the amount of blood perfusing through tissues.
Now, blood pressure is determined by two components: the resistance to blood flow in the blood vessels, which is affected by things like vessel length, blood viscosity, and vessel diameter; and the cardiac output, which is the volume of blood pumped by the heart through the body per minute. You can break that down into heart rate, or the number of beats per minute, multiplied by stroke volume, or the amount pumped out each beat. The stroke volume is found by taking the total volume of blood left over after contraction, which is called the end-systolic volume, and subtracting it from the total volume in the heart after filling, or the end-diastolic volume.
All right, keeping all this in mind, shock can be caused by many different things, but we can categorize the different types of shock into the three main categories with some subcategories. The first category is called hypovolemic shock. Hypo means “low,” vol refers to “volume,” and emia refers to the blood; thus, hypovolemic shock is shock induced by a low fluid volume of blood. This could be either non-hemorrhagic or haemorrhagic. Non-hemorrhagic means that the loss of fluid volume isn’t from bleeding. For example, if you were stranded in a desert and suffered severe dehydration, eventually your loss of fluid from sweating would reduce blood volume to the point that it wouldn’t be enough to supply your body’s organs, and you’d develop hypovolemic shock. Hecommorrhagic hypovolemic shock, on the other hand, is loss of blood volume through ruptured blood vessels; in other words, it’s loss of blood volume from bleeding. A loss of about 20% of your total blood volume, which is roughly one liter, can be enough to induce hypovolemic shock.
When that liter of blood leaves circulation, the total volume filling into the heart goes down; this means that the end-diastolic volume goes down, which causes stroke volume to go down as well. Therefore, cardiac output goes down, and finally we see blood pressure go down. When cardiac output goes down, catecholamines such as epinephrine and norepinephrine, ADH, and angiotensin II are released. These all cause vasoconstriction of blood vessels, which increases vascular resistance and heart rate, and in turn, this increases cardiac output. These combined effects increase blood pressure.
A super important indicator that tissues are not getting enough oxygen due to hypovolemia is a decreased mixed venous oxygen saturation, or MVO2. MVO2 is the amount of oxygen bound to hemoglobin in the blood coming to the right side of the heart from the tissues. It’s like the amount of oxygen left over, or not extracted and used by the tissues. So, if blood volume is down, it means that oxygen is down, and there’s going to be less left over, right? So MVO2 will be down with hypovolemic shock.
A second main category of shock is cardiogenic shock. Cardiogenic means produced by the heart, right? So, this is when something happens to the heart that prevents it from pumping enough blood to the body’s tissues. The most common cause is acute myocardial infarction, or heart attack. Hold on a second! Didn’t we say at the beginning that a heart attack was more like localized ischemia? Well, the heart attack itself reflects ischemia, but the effects of the initial cardiac damage eventually lead to a state of shock.
When the heart’s muscle cells die, it can’t contract as hard, which means the amount of blood pumped out, or stroke volume, goes down; therefore, cardiac output goes down as well. In the same way as with hypovolemic shock, the body releases vasoconstrictors to increase vascular resistance and help maintain blood pressure.
Also, similar to hypovolemic shock, in cardiogenic shock MVO2 will be down because since there’s less oxygen being pumped out, less will be left over. Sometimes, there might be an obstruction that doesn’t allow the heart to fill properly with blood. For example, we might have the pericardial sac fill up with fluid from an infection or with blood from trauma, like getting stabbed in the chest.
If this sac fills up, it physically constricts and prevents the heart from expanding and contracting normally, reducing the stroke volume. This is sometimes “sub” classified as obstructive shock, but you can see that the cause is still due to the heart’s inability to do its job, right?