The optimization of DO₂ is the underlying premise for shock resuscitation. Let’s review the DO₂ equation:

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Where:

• Hb = hemoglobin concentration (g/dL)

• SaO₂ = arterial oxygen saturation (%)

• CO = cardiac output (L/min)

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Considering this equation, improving hemoglobin or cardiac output (CO) are the two most effective ways to improve DO₂. Manipulating CO requires a solid understanding of heart-lung-vascular physiology. If we look at a very simple model of the circulatory system, we recognize a few key principles:

Principle 1: The circulatory system can be broken down into four principal interfaces. These include the:

1. Left Ventricle → Arterial System (Interface I)

2. Arterioles → Capillaries (Interface II)

3. Capillaries → Venules (Interface III)

4. Right Ventricle → Pulmonary Artery (Interface IV)

Principle 2: The circulatory system is a closed loop system. Therefore, blood return to the heart (venous return) and cardiac output are equal across time.​

Principle 3: The interfaces all follow a unifying hydraulic principle where the pressure gradient across the interface is equivalent to the resistance of the interface multiplied by the cardiac output. (the Hagen-Poiseuille law.)  The Hagen-Poiseuille law states that flow is determined by the pressure gradient across the length of a tube divided by the resistance (R) within the tube.

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Let’s explore these three principles further. We recommend you review them in order.