From the Guidelines
The zone 1 of the lung has the highest PaO2 due to its high ventilation-perfusion ratio, as it receives the greatest ventilation relative to perfusion, allowing for optimal oxygen transfer. This is because, in the upright position, ventilation is highest at the apex of the lung, where zone 1 is located, due to the lung's weight and lower intrapleural pressure, causing it to expand more on inspiration 1. As a result, alveolar pressure exceeds pulmonary arterial pressure in zone 1, which can compress capillaries and reduce blood flow while maintaining good ventilation. This creates an ideal environment for oxygen transfer, resulting in the highest PaO2 levels. Some key points to consider:
- The pulmonary circulation maximizes PaO2 through V/Q matching, ensuring well-ventilated areas receive most of the pulmonary blood flow 1.
- Hypoxic pulmonary vasoconstriction (HPV) plays a crucial role in this process by constricting precapillary pulmonary arterioles in response to low alveolar PO2 (PAO2) in poorly ventilated areas of the lung 1.
- The relationship between PaO2 and CaO2 is not linear, making it essential to maintain optimal ventilation-perfusion ratios to ensure adequate oxygenation 1. In real-life clinical practice, understanding the physiological distribution of ventilation and perfusion in the lungs is crucial for managing patients with respiratory diseases and optimizing oxygen therapy.
From the FDA Drug Label
INOmax appears to increase the partial pressure of arterial oxygen (PaO2) by dilating pulmonary vessels in better ventilated areas of the lung, redistributing pulmonary blood flow away from lung regions with low ventilation/perfusion (V/Q) ratios toward regions with normal ratios. The zone of the lung with the highest PaO2 is the better ventilated areas of the lung, as the drug increases PaO2 by dilating pulmonary vessels in these areas, redistributing blood flow towards them 2.
From the Research
Zone of the Lung with the Highest PaO2
The question of which zone of the lung has the highest PaO2 is related to the effects of different therapies on oxygenation in patients with acute respiratory distress syndrome (ARDS).
- Studies have shown that prone positioning and inhaled nitric oxide (NO) can improve oxygenation in ARDS patients 3, 4, 5, 6, 7.
- The improvement in oxygenation is thought to be due to the recruitment of atelectatic lung regions and the reduction of pulmonary vascular resistance 3, 4.
- However, the studies do not specifically address the question of which zone of the lung has the highest PaO2.
- The lung can be divided into zones based on the relationship between alveolar pressure, arterial pressure, and venous pressure, but the studies do not provide information on the PaO2 in each zone 3, 4, 5, 6, 7.
- Therefore, based on the provided evidence, it is not possible to determine which zone of the lung has the highest PaO2.
Effects of Prone Positioning and Inhaled Nitric Oxide on Oxygenation
- Prone positioning has been shown to improve oxygenation in ARDS patients by increasing the PaO2/FIO2 ratio 3, 4, 5, 6, 7.
- Inhaled NO has also been shown to improve oxygenation in ARDS patients by increasing the PaO2/FIO2 ratio 3, 4, 5, 6, 7.
- The combination of prone positioning and inhaled NO has been shown to have an additive effect on oxygenation 3, 4, 6.