Postoperative hypotension

A 35-year-old male is in the ICU following emergency surgery for a small bowel obstruction. On arrival to the ICU, he has the following vital signs: HR 115, BP 85/40, SpO2 98. He underwent a 4-hour open lysis of adhesions. He received 2L of crystalloid and made 50 mL of dark urine, and did not require any medication to improve his blood pressure. He remains intubated and sedated.

What is the differential for his hypotension?

Hypovolemia- under-resuscitation relative to the insensible losses from open abdomen and likely preoperative dehydration
Sepsis- bacteremia from gut translocation from small bowel obstruction, pneumonia from aspiration due to obstruction
Tamponade, tension pneumothorax- did he have any intra-vascular devices placed in the OR?
Pulmonary embolism- lengthy surgery, did he have appropriate mechanical prophylaxis?
Cardiomyopathy

The surgical team reports that he has not been tolerating a diet, or even liquids, for the previous 3 days. He received perioperative ertapenem for surgical infection prophylaxis. There was no evidence of aspiration during intubation and his admission CXR was unremarkable. He had a right internal jugular central line placed intra-operatively. He had no issues with oxygenation/ ventilation or high airway pressures intra-operatively.

How can you diagnose shock and differentiate between the different potential etiologies?

Physical exam- evaluation of skin turgor/ color/ temperature and mucous membranes, evaluation of fluid status (open wounds, nasogastric tube output, passive leg raise), examination of urine quality, auscultation of heart/ lungs
Labs- cultures, complete blood count, lactate, liver function tests, BUN/Cr
Ultrasound- gross evaluation of heart function, lung sliding to rule out pneumothorax, volume and collapsibility of the inferior vena cava
Test for fluid responsiveness- based on stroke volume variation (SVV, see below), or response to passive leg raise or a fluid challenge.

On exam, he is tachycardic without murmurs, lungs have equal air movement bilaterally. His nasogastric tube remains on suction with ongoing high output of gastric contents. On ultrasound, he has bilateral lung sliding. His cardiac contractility looks grossly preserved. He has normal oxygenation. His inferior vena cava is collapsible.

He has a known source of infection (positive blood cultures), leukocytosis, elevated lactate, high fluid losses with evidence of fluid responsiveness.

Shock: Undifferentiated Hypotension

Hypotension ≠ shock. So what is shock? Inadequate perfusion to maintain end-organ function

Pathophysiology: effective perfusion requires adequate cardiac output (CO). CO is the volume of blood that the heart pumps each minute, and it depends on stroke volume (SV; the volume of blood ejected with each heartbeat) and heart rate (HR; the number of heartbeats per minute). SV depends on preload (intra-vascular volume returning to the heart), myocardial contractility, and afterload (systemic vascular resistance). Shock is a disruption of preload, contractility, and/ or afterload.

Signs of shock= signs of end-organ hypoperfusion

Altered mental status (brain)
Decreased urine output (kidney)
Change in color/ temperature of extremities (skin)
Abnormal liver function tests (liver)
Ileus (gastrointestinal tract)

Diagnosis of shock + tools for monitoring response to treatment

Elevated lactate (global hypoperfusion)
Ultrasound- evaluate cardiac function, evaluated IVC to assess volume status
Minimally invasive cardiac monitoring (central line or arterial line)- CVP and SVV to assess volume status
Invasive cardiac monitoring (pulmonary artery catheter)- cardiac output, ScVO2 (central venous oxygen saturation)

Four types of shock

Shock is typically categorized as hypovolemic, obstructive, cardiogenic or distributive. However, in order to link the specific category with the associated pathophysiology, I have described each state as it relates to maintaining cardiac output, as described above.

Decreased preload: hypovolemic shock- low circulating blood volume→ decreased blood volume returning to the heart. Etiologies: bleeding, inadequate fluid replacement/ maintenance, high output from nasogastric tube or ostomy, insensible losses that aren't appropriately replaced (burn patients, large open wounds).
Decreased preload: obstructive shock- disease process that impedes venous return to the heart (tamponade, tension pneumothorax, pulmonary embolism).
Decreased contractility: cardiogenic shock- disturbance of the intrinsic function of the heart. Etiologies: heart failure, arrhythmias, valvular insufficiency, or decompensated valvular stenosis.
Decreased afterload: distributive shock- dilated peripheral vasculature, sometimes known as vasoplegia. Etiologies: sepsis, anaphylaxis, neurogenic following spinal cord injury (NOTE- this is NOT the same as spinal shock), burns, trauma, pancreatitis. Neurogenic- hypotension with concurrent bradycardia. Vasoplegia is a term used to describe pathologically low systemic vascular resistance- this can be associated with post-cardiac bypass or any of the other causes mentioned here.

Management of shock

Treat underlying cause (see below).
Restore adequate intravascular volume (aka preload). This is part of the initial treatment of hypovolemic shock, obstructive shock, and distributive shock. Fluids in the management of cardiogenic shock depend on the primary cardiac pathology.
Treat hypotension/ decreased cardiac output that persists despite fluid resuscitation and treatment of the underlying cause.
Septic shock- norepinephrine is the first line vasoactive medication.
Monitor end-points of resuscitation (see above, Diagnosis of shock + tools for monitoring response to treatment)
Supportive care- nutrition, respiratory support, venous thromboembolism, etc.

Specific Treatments Based on Etiology

Hypovolemia from hemorrhage- transfusion, stop the bleeding
Hypovolemia from fluid losses- replace fluid via enteral or intravenous route, as appropriate
Sepsis- antibiotics, control source of infection (appendectomy, drain placement, etc).
Tamponade- drainage of pericardial fluid (pericardiocentesis, pericardial window)
Tension pneumothorax- release of tension physiology (needle decompression or finger thoracostomy)
Cardiogenic- management of primary cardiac pathology, whether that entails treating acutely decompensated heart failure, resolving acute symptomatic arrhythmias, etc.