Introduction and Background
Hemodynamic insufficiency, presenting as hypotension and shock, is a common presentation in both the emergency and critical care setting. The etiology of hypotension may be easily identifiable in such cases as massive hemorrhage, profound dehydration, sepsis, or severe cardiac dysrhythmia. More often, there is ambiguity as to the cause of a patient’s hypotension, especially when no good history is available and the physical examination and laboratory values are unrevealing. In the patient with hypotension and no identifiable cause, appropriate initial management may be delayed. In this situation, the physician may use bedside ultrasonography to rapidly assess the patient for various causes of shock, possibly leading to immediate life-saving interventions.
The usefulness of bedside ultrasound in the hypotensive patient is based on the principles already discussed in previous chapters. A bedside ultrasound is performed rapidly by the physician taking care of the patient, and interpreted immediately while performing the scan. This allows the treating physician to expedite the patient’s diagnosis and definitive care. In addition, a hypotensive patient is usually too hemodynamically unstable to leave the department, and performing an ultrasound at the bedside may help obviate the need for further testing and transport out of the department. Consultant-performed ultrasound is still not always available in every hospital setting and testing may be delayed. This stresses the importance of needing an ultrasound machine available in both the emergency and critical care department and adequately training physicians to use the equipment properly. Along with using ultrasound to search for the cause of a patient’s instability, it is useful for procedures needed to treat the condition, such as central line placement. Finally, a bedside ultrasound can be repeated as necessary to assess response to therapy, evolution of disease, and to search for new problems.
Causes of Shock and the Ultrasound Approach
The causes of shock can be divided into various categories that help to generate a differential diagnosis for a patient’s instability. These categories help to design an ultrasound approach at the bedside tailored to the patient’s presenting complaint and most likely diagnosis.
Etiologies of obstructive shock are either due to intrinsic or extrinsic factors. Common causes include pericardial tamponade, tension pneumothorax, right ventricular (RV) outflow impedance (ie, pulmonary embolism, high ventilator pressures, auto PEEP), and intraabdominal compartment syndrome. Rare causes of obstructive shock include tamponade from massive pleural effusion and intracardiac obstruction (ie, myxoma or massive valvular vegetation).
Ultrasonography permits rapid assessment for obstructive shock. An echo can be performed to evaluate for pericardial effusion and signs of tamponade, including right ventricular (RV) diastolic collapse. The inferior vena cava (IVC) size can also be evaluated, which will appear plethoric with minimal or no respiratory variability.
An echo can be used to look for evidence of RV outflow impedance, showing a dilated chamber size consistent with acute cor pulmonale and pulmonary embolism. A lower extremity ultrasound can also be used to identify deep venous thrombosis (DVT) along with the echo signs of RV strain in order to confirm the diagnosis.
Although, transthoracic echo (TTE) is not the most sensitive modality to identify valvular vegetations or chamber myxomas, if identified at the bedside it may expedite care.
If a lung etiology is suspected, a right and left upper quadrant abdominal view may identify pleural effusions above the diaphragm leading to an obstructive picture. In addition, a dedicated lung ultrasound may diagnose or exclude a pneumothorax.
Hypovolemic shock is due to decreased intravascular volume. This may be related to absolute hypovolemia (blood loss, gastrointestinal fluid loss, third spacing) or relative hypovolemia (loss of vasomotor control function).