![]() Dead space is the portion of the respiratory system where tidal volume doesn’t participate in gas exchange: it is ventilated but not perfused. One important contributor to ventilation perfusion mismatch is dead space. It will help you understand how you can use these concepts to care for your patient. This article will describe how dead space is different from shunt. Physiologic dead space is ventilation of poor perfused alveoli. The A-a gradient can be calculated by subtracting the alveolar partial pressure of oxygen, which is calculated using the alveolar gas equation, by the arterial partial pressure of oxygen, measured with arterial blood gas (ABG).Shunt is perfusion of poorly ventilated alveoli. All three of these conditions can contribute to an increase in A-a gradient and cause hypoxemia. Diffusion problems can occur when the alveolar walls are fibrosed and oxygen cannot diffuse across the alveolar wall into the blood stream. ![]() High V/Q is also called dead space and low V/Q is called shunting. ![]() V/Q mismatch means a mismatch of ventilation to perfusion (V/Q) ratio and can represent either too little perfusion due to blood flow obstruction (high V/Q) or too little ventilation due to airway obstruction (low V/Q). An increase in A-a gradient can occur in hypoxemia and the causes include V/Q mismatch such as dead space or shunting and diffusion problems. The normal A-a gradient is 10 to 15 mmHg. The A-a gradient represents the difference in partial pressure of oxygen between the alveoli and the arteries.
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