Pulmonary Embolism: V/Q Mismatch

• Pulmonary embolism (PE) occurs when blood circulation through the pulmonary artery is blocked by a thrombus or other emboli.
• PE is a life-threatening emergency.
• Blood clots causing PE often originate from deep vein thrombosis (DVT).
• A blood clot obstructs blood supply to the lungs, increasing lung blood pressure and causing the heart to work harder, resulting in ventilation-perfusion mismatch.

Ventilation-Perfusion (V/Q) Mismatch

• When alveolar ventilation remains constant but perfusion of the capillary bed decreases, it leads to dead space ventilation and hypoxemia
• Serotonin is released, causing pulmonary vasculature contraction and reduced blood flow, hyperventilation and altered lung surfactant lead to hypocapnia and respiratory alkalosis.
• Increased pulmonary vascular resistance (PVR) raises right ventricular afterload, impairing right ventricular outflow and causing right ventricular dilation

V/Q Mismatch Types

• PE can create a V/Q mismatch due to the embolus obstructing the blood vessel
• V/Q mismatch manifestations: headache, fatigue, dizziness, shortness of breath, and wheezing
• Two classifications of V/Q mismatch: dead space and shunt.

Dead Space V/Q Mismatch

• Ventilation is adequate, but perfusion inadequate; therefore oxygen cannot enter the bloodstream
• Anatomic dead space is the area in the lung with oxygen but no blood flow
• Physiologic mismatch is when the alveoli have oxygen but blood flow is inadequate to move the oxygen
• PE is a common cause of dead space.
• V/Q mismatch is treatable with oxygen.

Shunt

• There is adequate blood flow (perfusion) but inadequate ventilation
• Relative: small amount of ventilation
• Absolute: no ventilation
• Pneumonia and pulmonary edema are conditions associated with shunts.

Saddle Embolism

• A large embolism blocks the bifurcation of the main pulmonary artery.
• Compromised cardiac output results from increased pulmonary vascular resistance increasing right ventricular afterload, impairing right ventricular outflow and causing right ventricular dilation.
• Decreased outflow from the right ventricle leads to reduced filling of the left ventricle and a reduction of cardiac output causing systemic hypotension and hemodynamic instability.
• Right ventricular failure is the main cause of death associated with pulmonary embolism.

Virchow's Triad

• Three main factors increase the risk of pulmonary embolism: hypercoagulability, vessel wall injury, and venous stasis.
• Most emboli arise from the lower extremity proximal veins (iliac, femoral, or popliteal veins).
• If a client has a pulmonary embolism and a DVT, the client is diagnosed with venous thromboembolism (VTE).

Pulmonary Embolism Risk and Diagnosis

• Can be asymptomatic
• Low-risk PE
• Intermediate risk or submassive PE
• High-risk or massive PE (saddle)
• Diagnostic tests include laboratory value assessment and diagnostic imaging.
• Bedside imaging or ultrasound may be used when the client is hemodynamically unstable
• Computed tomographic pulmonary angiogram (CTPA) is the diagnostic test of choice for a suspected PE allowing the pulmonary arteries to be visualized.

Diagnosis via Laboratory Assessment

• ABG analysis: respiratory alkalosis and hypocapnia are revealed.
• Troponin levels elevated in 30-50% of clients with moderate to large PE indicates a possibility of a severe outcome or death.
• Elevated D-dimer levels indicate concurrent activation of thrombotic pathways.
• Electrocardiogram (ECG) abnormalities such as tachycardia.
• Chest x-ray is usually normal but may identify atelectasis, effusion, or other causes for dyspnea.

Treatment

• Initial treatment is supportive.
• Supplemental oxygen is used for oxygen saturation levels below 90%.
• Mechanical ventilation may be considered for unstable clients.
• Hemodynamically unstable clients may require mechanical cardiopulmonary support devices, like extracorporeal membrane oxygenation (ECMO).
• Thrombectomy is used to remove clots if the client is unstable and thrombolysis is ineffective

Pulmonary embolism (PE) is a life-threatening emergency caused by a blocked pulmonary artery. Blood clots often originate from deep vein thrombosis (DVT), obstructing blood supply to the lungs. This leads to ventilation-perfusion mismatch, increased lung blood pressure and strain on the heart.