Transcription
Hypotension in adults is generally defined as a systolic blood pressure below 90 millimeters of mercury or a diastolic pressure below 60 millimeters of mercury. When the blood pressure is low enough to cause deprivation of blood flow to organs, this is termed shock.
Blood pressure is determined by the cardiac output, the blood viscosity (meaning the thickness of the blood), and total peripheral resistance. The causes of hypertension we will cover affect these three factors.
Signs and symptoms can include lightheadedness and dizziness, lethargy, and possibly syncope, while other features of the underlying cause may also be present. For example, chest pain, shortness of breath, fever, signs of infection, or anaphylaxis.
The causes of a low blood pressure can be divided into four main types. These are hypovolemic, cardiogenic, obstructive, or distributive, or an impaired peripheral vasoconstriction.
First, we have hypovolemia, which causes a low blood pressure due to inadequate intravascular volume to maintain the blood pressure. Hypotension due to hypovolemia may be due to hemorrhage, which can be overt or obvious cases such as gastrointestinal bleeding or trauma, or maybe more hidden bleeds such as a rupture of an aortic aneurysm or rupture of an ectopic pregnancy.
Non-hemorrhagic causes of hypovolemia can be caused by dehydration or they can be divided by system. For example, fluid depletion from gastrointestinal causes include diarrhea and vomiting. Renal system losses coming from excess diuretics or other causes of polyuria. Respiratory losses through pleural effusion and skin losses through significant burns, as well as potentially third space sequestration such as ascites.
Cardiogenic causes result mostly from pump dysfunction generating a lower cardiac output. Cardiac output is the volume of blood pumped in one minute and is given by the stroke volume multiplied by the heart rate. The stroke volume is the volume of blood pumped into the aorta by the left ventricle in one beat. This is usually around 70 milliliters. Therefore, at a heart rate of around 70 beats per minute, the cardiac output is around 5 liters per minute.
As we mentioned earlier, cardiac output directly affects the blood pressure. That is, a lower cardiac output causes a lower blood pressure. Causes can include acute coronary syndrome leading to ventricular dysfunction and therefore a reduction in stroke volume. Another is valvular pathology such as aortic stenosis, which also reduces the stroke volume. Then we have arrhythmias. Bradyarrhythmias in particular can cause a lower blood pressure because the lower heart rate produces a lower cardiac output. But also tachyarrhythmias in severe cases can cause hypotension because the diastolic time is shortened, meaning less time for blood to fill the ventricles, meaning a lower stroke volume.
Next, we have obstructive causes, which refer to causes where the pumping function of the heart is preserved, but something else is interrupting the flow of blood. This may be obstruction to cardiac filling, therefore a decreased venous return to the heart. Examples include tension pneumothorax or a cardiac tamponade. On the other hand, we may have an obstruction to blood flow between the heart and the lungs, and the primary example is a pulmonary embolism.
The fourth category is distributive or impaired peripheral vasoconstrictive causes. Blood pressure is usually regulated by circulating catecholamines and the peripheral alpha-adrenergic receptors. It occurs when there is a dysfunction with the nervous system or with the vessels. Sepsis and anaphylaxis are both examples where there is an impairment in peripheral vasoconstriction, but there is also an increase in the permeability of the vessel wall, leading to a loss of fluid into the extravascular space. This can also occur due to autonomic dysfunction, which can be seen acutely after a stroke or in chronic illnesses like diabetes, Addison's disease, or amyloidosis.