Wells Criteria and D-Dimer Interpretation

A pulmonary embolus usually originates as venous thrombus in the deep veins of the leg or pelvis. Once lodged in a pulmonary arterial branch, it creates ventilated but underperfused lung, or dead-space ventilation, and may produce hypoxaemia. The more dangerous consequence is an abrupt rise in pulmonary vascular resistance: the right ventricle must pump against a suddenly increased afterload, may dilate and fail, and cardiac output can fall. Haemodynamic collapse reflects right-ventricular pressure overload and impaired forward flow, not clot size alone. That mechanism determines the first bedside question: Is this patient stable enough for a diagnostic pathway, or is cardiopulmonary failure developing? Hypotension, shock, syncope, worsening hypoxaemia, tachycardia with evidence of right-ventricular strain, altered mentation, oliguria or a rising oxygen requirement should move the patient into an emergency response pathway. A patient whose blood pressure briefly fell and then recovered still requires urgent reassessment; transient instability represents incipient cardiopulmonary failure rather than reassurance. Stabilization, monitoring, urgent imaging when feasible and specialist escalation take priority over waiting for a D-dimer result. Dyspnoea, pleuritic chest pain, haemoptysis and unilateral leg symptoms...