Hemodynamic Relationship: Blood Pressure, Cardiac Output, and Systemic Vascular Resistance
Mean arterial pressure (MAP) equals cardiac output (CO) multiplied by systemic vascular resistance (SVR), expressed as: MAP = CO × SVR. This is the fundamental hemodynamic equation that governs the relationship between these three variables 1.
The Core Relationship
The equation demonstrates that blood pressure is determined by two primary factors:
- Cardiac Output (CO): The volume of blood pumped by the heart per unit time (L/min)
- Systemic Vascular Resistance (SVR): The resistance offered by the systemic circulation to blood flow
SVR is calculated as: (MAP - Right Atrial Pressure) ÷ Cardiac Output 1. This inverse calculation allows determination of vascular resistance when pressure and flow are known.
Clinical Implications
Understanding the Relationship
The equation reveals that blood pressure can be maintained through different hemodynamic mechanisms 2, 3:
- High CO with low SVR: Typical in early hypertension or hyperdynamic states
- Normal/low CO with high SVR: Characteristic of established hypertension and most shock states after initial resuscitation
- Low CO with low SVR: Seen in distributive shock (septic shock)
- Low CO with high SVR: Typical of cardiogenic and hypovolemic shock
Critical Clinical Pitfall
Blood pressure is a poor surrogate for cardiac output 4. A study of 100 major abdominal surgery patients demonstrated only a weak correlation (r = 0.34) between MAP and cardiac index, with cardiac index increasing by only 0.014 L·min⁻¹·m⁻² for each 1 mm Hg increase in MAP 4. This means:
- Normal blood pressure does NOT guarantee adequate cardiac output
- Hypotension does NOT always indicate low cardiac output (may reflect low SVR with adequate or high CO)
- Direct measurement of cardiac output is necessary when precise hemodynamic assessment is required 4
Hemodynamic Phenotypes
In hypertension specifically, different phenotypes exist based on the CO/SVR relationship 5:
- 70% of patients: High SVR with normal/low cardiac index (resistance-driven hypertension)
- 15% of patients: High cardiac index with low/normal SVR (flow-driven hypertension)
- 15% of patients: Low/normal values for both parameters
These phenotypes have implications for treatment selection, though age and body mass index only weakly predict which phenotype a patient will have 5.
Shock States and Resuscitation
In shock management, the relationship becomes critical 6:
- Target MAP of 65-70 mm Hg during resuscitation 7, 6
- Achieving this target requires understanding whether the problem is primarily CO (requiring inotropes/fluids) or SVR (requiring vasopressors or vasodilators)
- Multimodal assessment is essential: skin perfusion, lactate, venous oxygen saturation, and direct CO measurement when available 6
Dynamic Nature
The relationship is not static 8:
- During exercise, CO increases 4-6 fold while SVR decreases, maintaining appropriate MAP 8
- Systolic pressure rises with exercise (~10 mm Hg per MET) while diastolic pressure remains stable or decreases slightly 8
- Compensatory mechanisms continuously adjust both CO and SVR to maintain perfusion 9
Mathematical Relationships
From the fundamental equation, several derived relationships exist:
- Cardiac Power Output = (CO × MAP) ÷ 451 (measured in watts) 1
- Pulse Pressure reflects stroke volume and arterial compliance 10
- SVR units: Expressed as dynes·s·cm⁻⁵ or Wood units 1
The equation holds true across the cardiovascular system, with parallel calculations for pulmonary circulation: Pulmonary Artery Pressure = CO × Pulmonary Vascular Resistance 11, 1.