Why Blood Pressure is Low in Septic Shock
Blood pressure drops in septic shock primarily because of massive systemic vasodilation caused by the body's dysregulated inflammatory response to infection, which releases inflammatory mediators—particularly nitric oxide—that relax vascular smooth muscle throughout the body. 1
Pathophysiologic Mechanisms
Primary Mechanism: Inflammatory-Mediated Vasodilation
The fundamental problem is uncontrolled peripheral vasodilation. When bacteria (especially gram-negative organisms) release lipopolysaccharide (LPS) from their cell walls, this triggers toll-like receptors (TLR4) and unleashes a cascade of inflammatory mediators. The most critical consequence is activation of inducible nitric oxide synthase, which produces massive amounts of nitric oxide—a potent vasodilator. This causes the blood vessels to lose their normal tone and dilate excessively 1.
Secondary Mechanism: Vasopressin Deficiency
Patients with septic shock have inappropriately low vasopressin levels (averaging only 3.1 pg/mL) compared to patients with cardiogenic shock (22.7 pg/mL), despite similar or worse hypotension 2. This deficiency occurs because the baroreflex-mediated secretion of vasopressin becomes impaired during sepsis. The lack of this endogenous vasoconstrictor compounds the vasodilation problem 2.
The Clinical Result
The combination of these mechanisms produces:
- Decreased systemic vascular resistance (often dropping to 600-700 dyne·s/cm⁵ or lower)
- Relative hypovolemia from vasodilation and capillary leak
- Persistent hypotension despite often normal or even elevated cardiac output
- Tissue hypoperfusion because blood pools in dilated vessels rather than perfusing vital organs
Clinical Definition and Thresholds
Septic shock is defined as persistent hypotension (systolic BP <90 mmHg or MAP <65 mmHg) after adequate fluid resuscitation (30 mL/kg crystalloid) in a patient with severe sepsis 3. The 2025 ESICM guidelines and 2016 Surviving Sepsis Campaign both recommend targeting an initial MAP of 65-70 mmHg 4, 5.
Important Caveat on Blood Pressure Targets
While 65 mmHg is the standard target, recent evidence shows that hypotension below 85 mmHg is associated with progressively worse outcomes. Every 1-unit decrease in time-weighted average MAP below 65 mmHg increases mortality odds by 11.4%, acute kidney injury by 7.0%, and myocardial injury by 4.5% 6. This suggests the 65 mmHg target may be a minimum threshold rather than an optimal goal.
Why This Matters for Treatment
Understanding the vasodilatory mechanism explains why:
Norepinephrine is first-line therapy 5—it provides both alpha-adrenergic vasoconstriction and some beta-adrenergic cardiac support
Vasopressin (0.03 U/min) can be added 5, 2—it replaces the deficient endogenous vasopressin and provides vasoconstriction through V1 receptors
Fluid resuscitation alone is insufficient—you cannot fill a dilated vascular space adequately; you must restore vascular tone with vasopressors
Early vasopressor initiation is critical 7—patients with initial hypotension on ED arrival have 20% higher 28-day mortality compared to those who develop delayed hypotension
The hypotension in septic shock is fundamentally different from hypovolemic or cardiogenic shock because the primary defect is in the blood vessels themselves, not in volume or pump function.