V/Q Mismatch 

Definition: V/Q mismatch occurs when ventilation (V) and perfusion (Q) of the lungs are not properly matched, leading to impaired gas exchange and hypoxemia. 

Normal V/Q ratio: ~0.8 

Types of V/Q Mismatch: 


 
Patients at Risk: 

Low V/Q (shunt-like): – Chronic obstructive pulmonary disease (COPD) – Asthma exacerbation – Pneumonia – Pulmonary edema – Atelectasis (post-op or immobilized patients) 

High V/Q (dead space): – Pulmonary embolism – Emphysema 

Mixed/Complex: – Acute Respiratory Distress Syndrome (ARDS) – Severe heart failure with pulmonary congestion 

Other risk factors: – Bedridden or postoperative patients (atelectasis) – Smoking history (chronic lung disease) – History of DVT/PE (pulmonary embolism) – ICU patients (risk of ARDS) 
 
Clinical Clues to the presence of a shunt: 

– Hypoxemia not fully corrected by oxygen → consider low V/Q or shunt  
– Sudden dyspnoea and pleuritic chest pain → suspect pulmonary embolism  
– Chronic cough, wheezing → chronic airway disease (COPD, asthma)  
– Imaging: infiltrates (pneumonia), emboli (CT pulmonary angiography), atelectasis 
 
Key Points:  

– V/Q mismatch is the most common cause of hypoxemia.  
– Low V/Q → airway problem, poor oxygenation. 
– High V/Q → vascular problem, dead space.  
– Remember common at-risk patients: COPD, asthma, pneumonia, PE, ARDS, postoperative patients. 
– Patient positioning in key to improve VQ mismatch and recruitment of alveoli in the lungs