four types of shock

Shock is defined as inadequate tissue perfusion leading to organ system compromise or failure. In clinical practice, shock is often considered synonymous with hypotension (although that isn't always the case). There are four types of shock by mechanism: hypovolemic, cardiogenic, distributive, and obstructive. A patient may have more than one mechanism at a time, of course--someone with sepsis (distributive shock) might also have significant systolic dysfunction (cardiogenic shock). 

 

Hypovolemic Shock is due to low circulating blood volume. This can be due to acute hemorrhage (trauma, GI bleeding) or loss of plasma volume (vomiting, diarrhea, third spacing, high urinary losses). The treatment is replacement of blood volume with isotonic crystalloid, like Lactated Ringer's or 0.9% saline, or blood products.

 

  • ​Lactated Ringer's is preferred in most cases to reduce the incidence of hyperchloremic acidosis and acute kidney injury

  • For active, significant hemorrhage, transfusing PRBC and FFP in a 1:1 ratio is superior to transfusing separately based on the hemoglobin and INR.

Cardiogenic Shock is due to poor function of the heart, leading to hypoperfusion. This can be mechanical (poor pump  function, valvular disease) or electrical (dysrhythmias). The most common reason for cardiogenic shock is systolic failure due to acute myocardial infarction. The treatment should address two things--supporting the circulation and correcting the underlying cause.

 

  • Inotropes and vasopressors are used to support the circulation. Dobutamine and milrinone are inotropes and chronotropes. Dopamine has these properties as well, and causes peripheral vasoconstriction; this is helpful when the systolic blood pressure is less than 90. Norepinephrine is primarily a peripheral vasoconstrictor but it does have some beta-1 adrenergy, and can be used when tachycardia is a concern. For refractory shock, intra-aortic balloon counterpulsation (IABP) should be considered.

  • Correction of the underlying cause is vital in cardiogenic shock. Reperfusion of myocardium and treatment of unstable dysrhythmias should be done as quickly as possible.

Distributive Shock is due to pathologic vasodilation—the blood volume is normal, but the space it has to fill is now greater. Cardiac function is often normal or hyperdynamic, and the extremities are warm and well-perfused despite hypotension. This is also known as “warm shock.” The most common reasons for distributive shock are sepsis, anaphylaxis, adrenal insufficiency, and loss of vasomotor tone (as is often seen with spinal cord injury). The treatment is expansion of the blood volume with isotonic crystalloid, followed by vasoconstricting agents. Norepinephrine is preferred because many patients will benefit from the additional inotropy and chronotropy that it provides. Phenylephrine is a pure vasoconstrictor and can also be used. 
 

Obstructive Shock occurs when something obstructs blood return to the left ventricle. The most common causes of obstructive shock are tension pneumothorax, pericardial tamponade, dynamic hyperinflation, and massive pulmonary embolism.

 

  • Tension pneumothorax causes shock by obstruction of the great veins. As the pneumothorax enlarges, the increased intrathoracic pressure causes the mediastinum to shift to the opposite side. This kinks the SVC and IVC, leading to hypotension. Signs include absent breath sounds on the side of the pneumothorax, distended neck veins, subcutaneous air (not always present), and shift of the trachea to the opposite side. The treatment is emergent evacuation of the pneumothorax, either with needle decompression or a chest tube. Don’t wait on the X ray to make the diagnosis!

  • Pericardial tamponade, either from bleeding or pericardial fluid, constricts the chambers of the heart. This leads to an “equalization of pressures,” where the right atrial, pulmonary artery diastolic, and left atrial pressures are the same. If the pressures are the same, blood doesn’t flow very well, and this leads to an underfilled left ventricle. Signs include distended neck veins, hypotension, and muffled heart tones (Beck’s Triad). These may not be seen, however, and bedside ultrasound can be very helpful to make the diagnosis. The treatment is emergency pericardiocentesis or a subxiphoid pericardial window.

  • Dynamic hyperinflation happens when the patient can’t exhale completely. This is most often seen in those with COPD or status asthmaticus—it’s easy to get the air in, but not out. When it’s severe, it can reduce venous return to the heart and cause hypotension. Signs include diffuse wheezing, forced exhalation, and distended neck veins. The ventilator waveform will show expiratory flow not going all the way to baseline. The treatment is to give the air time to escape. Shorten the inspiratory time, increase inspiratory flow, and decrease the ventilator rate.

  • Massive pulmonary embolism is a form of obstructive shock because the large clot burden prevents blood from getting from the right side of the heart to the left. Signs include distended neck veins, hypoxemia, and hypercapnia. If you have a pulmonary artery catheter in place, the CVP will be higher than the PAOP. Treatment involves reducing the clot burden with thrombolytic therapy or surgical embolectomy, and supporting the circulation with norepinephrine. Giving large amounts of isotonic fluids may be counterproductive—as the right ventricle gets more full, it will bow into the left ventricle and reduce left ventricular stroke volume.