Your doubt is valid.
Let me clear this first:
Pulmonary infarction is NOT common in pulmonary embolism. It is seen in only 10% of patients with pulmonary embolism. The reason is that there is generally intact bronchial arterial and collateral arterial circulation.
In general, the effects of pulmonary embolism depend on:
- Size of embolus
- Size of occluded vessel
- Exent to which blood flow is obstructed
- Availabilty of alternative blood source - bronchial arteries
- Overall cardiovascular function of the person involved
Common effects of pulmonary embolism:
- Increased pulmonary vascular resistance, due to vascular obstruction
- Impaired gas exchange, due to increased alveloar dead space.
- Alveolar hyperventilation, due to reflex stimulation
- Increased airway resistance, due to bronchospasm
- Decreased pulmonary compliance, due to edema, hemorrhage and loss of surfactant.
As you can see, pulmonary infarction does not figure prominently here...
Conditions that favor the formation of pulmonary infarction in pulmonary embolism:
- Pulmonary infection - pneumonia/ consolidation
- Diminished ventilation
- Vascular congestion
These conditions can produce pulmonary infarction even in the ABSENCE of pulmonary embolism!
Shashikiran
CLINICAL SUBJECTS 
CLINICAL SUBJECTS
