Obstructive Shock

Positive pressure is the enemy - Induction and mechanical ventilation cut venous return in a circulation that already cannot fill, and positive pressure enlarges a tension pneumothorax. Where the clinical situation allows, decompress first and let the patient keep breathing spontaneously; a chest tube goes in before positive pressure ventilation, not after the pressure drops.

High CVP with low output is the differentiator - Elevated central venous pressure separates obstructive from hypovolemic shock. In trauma the two coexist often enough that finding one should not stop the search for the other.

Point-of-care echo and lung ultrasound answer the question at the bedside - Right ventricular diastolic collapse, right atrial systolic collapse, and a plethoric IVC confirm tamponade. Absent lung sliding with a lung point confirms pneumothorax; ultrasound is about 94% sensitive and 100% specific with a skilled operator.

Read pulsus paradoxus off the arterial line - A greater than 10 mmHg inspiratory fall in systolic pressure; in a patient with a pericardial effusion it is over 80% sensitive for tamponade. Automatic cuffs cannot measure it, and its absence does not exclude tamponade — atrial septal defect, elevated diastolic pressures, pulmonary hypertension, and aortic regurgitation all mask it.

Fluids are judicious, not aggressive - Give crystalloid carefully and start norepinephrine early if shock persists, adding vasopressin if refractory. In massive pulmonary embolism, volume loading can paradoxically worsen the hypotension by dilating the right ventricle and bowing the septum into the left ventricle.

Fix the obstruction, don't treat the number - Tension pneumothorax gets needle decompression at the second intercostal space in the midclavicular line, then a chest tube. Tamponade gets pericardiocentesis — taking off even the first small volume can transform hemodynamics. Massive pulmonary embolism gets thrombolysis. Volume and pressors are temporizing while you set the procedure up.

You can cause this one - Barotrauma from mechanical ventilation caused 69.6% of iatrogenic pneumothoraces in one series and central venous catheter insertion another 13.2%; internal jugular and subclavian access carry a 1% to 13% risk, higher after a failed first pass, with a subclavian approach, and under positive pressure ventilation. Ultrasound guidance lowers it.

Intraoperative tension pneumothorax is a clinical diagnosis - Rising peak airway pressures with hypotension, unilaterally absent breath sounds, hyperresonant percussion, tracheal deviation away from the affected side, distended neck veins, and desaturation. Decompress on that picture; waiting for a film increases mortality.

Give 100% oxygen while you set up - Dropping alveolar nitrogen creates a diffusion gradient that speeds reabsorption of pleural air; without it only about 1.25% of the air resorbs in 24 hours.

Watch for it after cardiac and electrophysiology procedures - Tamponade after cardiac surgery is frequently fatal. Rising filling pressures with a falling output and a narrowing pulse pressure send you to echo, not to more fluid.

Echo discriminates, and these are the findings that do it - Measure the echo-free pericardial space: small is under 9 mm, moderate 10 to 19 mm, large over 20 mm. Left atrial collapse appears in only about 25% of tamponade but is specific for it, and a leftward shift of the interventricular septum is likewise specific - that shift is the mechanism behind the pulsus paradoxus you are reading off the arterial line. Left ventricular collapse is rare because the wall is muscular, so do not wait for it. A dilated, poorly contracting ventricle without chamber collapse points at pump failure instead.

The ECG carries a tamponade signature - Low voltage across all leads, with electrical alternans: beat-to-beat alternation between normal and reduced voltage as the heart swings inside a fluid-filled sac.

Support while the pericardiocentesis is being set up - Oxygen, volume expansion through two large-bore IVs, and legs elevated. These are genuinely holding measures in this shock state - the fluid has to come off.

Most of these never get an anesthetic - Pericardial decompression is done under local anesthesia or mild sedation in almost every case. The safest induction in tamponade is the one you avoid, so if you are being asked to induce, ask first whether the drainage can happen without you.

If you do induce, the agent matters - Ketamine supports cardiac index better than the other induction agents, which makes it the agent of choice in pericardial effusion and tamponade; the induction dose is 1 to 2.5 mg/kg, reduced when catecholamine reserve is already spent, because its direct negative inotropy is masked only by sympathetic tone. Etomidate produces less ventilatory depression than propofol, which is the point when the plan is to keep the patient breathing.

Shock categories convert into each other - A patient compensating for hypovolemia is leaning on venous return, and induction, positive pressure, and PEEP all take it away. That is the mechanism by which a hypovolemic patient becomes an obstructive one on your ventilator, and by which an occult pneumothorax becomes a tension pneumothorax. Re-look after every intervention, because the picture you diagnosed at the start is not necessarily the one you are treating ten minutes later.

Obstructive shock is low cardiac output from a mechanical obstruction outside the heart itself — the pump works and the volume is there, but something will not let the heart fill or eject. Cardiac tamponade and tension pneumothorax compress the chambers and great veins and block diastolic filling; a hemodynamically significant pulmonary embolism or severe pulmonary hypertension obstructs outflow and fails the right ventricle.

It is the least common of the four shock categories and the easiest to miss, because the surface picture — hypotension, tachycardia, cold periphery, rising lactate — is the same as every other shock. The tell is a high central venous pressure with a low output, where hypovolemic shock runs a low CVP. Untreated it moves to PEA arrest quickly, and each cause has a mechanical fix rather than a pharmacologic one.

Obstructive shock sits alongside distributive, hypovolemic, and cardiogenic shock in the four-way classification, and of the four it is the only one whose lesion is mechanical and outside the heart. It shares the low cardiac output and reduced oxygen transport of hypovolemic and cardiogenic shock; distributive shock is the outlier, running a low peripheral vascular resistance with abnormal oxygen extraction.

The high central venous pressure that separates obstructive from hypovolemic shock does not separate it from cardiogenic shock, because pump failure raises filling pressures too. Neither does venous oxygen saturation: mixed venous saturation normally runs 65% to 75% and falls in any low-output state, so a low number tells you output is inadequate and nothing about why. The mechanism has to come from imaging.

Timing shapes the picture as much as volume does. A pericardium filling fast - traumatic hemopericardium is the classic - tamponades at small volumes, while a slowly accumulating effusion stretches the sac and can grow very large before filling is compromised. A big effusion in a chronic patient is not automatically tamponade, and a small one in a stabbed patient is.