Haemodynamic measurements become interpretable only when related to one another. A low blood pressure may be due to low flow, low vascular tone or both, and the appropriate action differs fundamentally between these cases.
Pressures in the heart and pulmonary circulation
Flow, resistance and oxygen transport
Formulas
- Mean arterial pressure (MAP) = diastolic + (systolic − diastolic)/3
- Cardiac output = stroke volume × heart rate
- SVR = 80 × (MAP − CVP) / CO
- PVR = 80 × (mean PA − PCWP) / CO
- Oxygen delivery (DO₂) = CO × (1.34 × Hb × SaO₂ + 0.003 × PaO₂), normally 950–1150 mL/min
Haemodynamic profiles in shock
| Type | CO | SVR | CVP/PCWP | ScvO₂ |
|---|---|---|---|---|
| Hypovolaemic | ↓ | ↑ | ↓ | ↓ |
| Cardiogenic | ↓ | ↑ | ↑ | ↓ |
| Distributive (septic) | ↑ or normal | ↓↓ | ↓ or normal | ↑ or normal |
| Obstructive (tamponade, PE) | ↓ | ↑ | ↑ | ↓ |
Vital signs and measures of perfusion
| Parameter | Normal | Comment |
|---|---|---|
| Heart rate | 50–100/min | |
| MAP | 70–105 mmHg | Target ≥ 65 mmHg in shock; higher in known hypertension |
| Pulse pressure | 40–60 mmHg | A narrow pulse pressure suggests a low stroke volume |
| Capillary refill | < 3 s | A simple and validated marker of perfusion |
| Lactate | < 1.6 mmol/L | Clearance over time is more informative than a single value |
| Urine output | > 0.5 mL/kg/h |
Normal echocardiographic values
| Measurement | Women | Men |
|---|---|---|
| Left ventricular ejection fraction | 54–74% | 52–72% |
| Left ventricle, diastolic diameter | 3.9–5.3 cm | 4.2–5.8 cm |
| Wall thickness (septum, posterior wall) | ≤ 0.9 cm | ≤ 1.0 cm |
| Left atrial volume index | ≤ 34 mL/m² | ≤ 34 mL/m² |
| TAPSE | > 17 mm | > 17 mm |
| E/e′ (mean) | < 14 | < 14 |
| Aortic valve area | 3.0–4.0 cm² | 3.0–4.0 cm² |
Grading of aortic stenosis: mild, peak velocity 2.6–2.9 m/s; moderate, 3.0–3.9 m/s; severe, ≥ 4.0 m/s or a valve area < 1.0 cm² with a mean gradient ≥ 40 mmHg.
Fluid responsiveness
Static measures such as the CVP predict fluid responsiveness poorly. Dynamic measures are better: a pulse pressure variation > 13% in a ventilated patient in sinus rhythm without spontaneous breathing, passive leg raising with an increase in stroke volume > 10%, or inferior vena cava variability. In the absence of responsiveness, fluid does more harm than good.
