Cardiovascular Physiology

Oxygen Delivery

     Adequate cardiovascular function is vital to maintaining perfusion to the organs and tissues in the body. Perfusion drives oxygen delivery (O2) and removal of byproducts of cell metabolism (CO2). The amount of oxygen that is delivered (DO2) is a function of cardiac output (CO; the volume of blood ejected from the heart every minute) and the arterial oxygen content (amount of oxygen in the blood).

     Cardiac output is determined by the volume of blood the heart pumps out into the body with each heartbeat (stroke volume, SV) and the frequency of the heartbeat (heart rate, HR). Stroke volume depends on preload (blood volume returned to the heart), contractility (effectiveness of cardiac muscle activity), and afterload (pressure in the peripheral vasculature that the heart has to overcome to eject blood).

     Arterial oxygen content (CaO2) is the amount of O2 in the blood that is ejected from the heart. This is determined by dissolved O2 + O2 bound to hemoglobin. Hemoglobin carries O2, and the percentage of Hgb molecules that are saturated (bound) with O2 is determined by arterial blood gas (SaO2, arterial oxygen concentration) or pulse oximetry (SpO2, peripheral arterial oxygen concentration). Pulse oximetry is non-invasive and is a reliable surrogate (as long as SaO2 >90%). The O2 carrying capacity of one gram of hemoglobin is 1.38 (this is a constant in the equation). So this is the first part of the equation: the number of hemoglobin molecules x the % of those molecules that are saturated with O2 x how much O2 saturated hemoglobin can carry. 

     The second part of the equation is the dissolved oxygen (partial pressure of arterial oxygen, PaO2, reported as mmHg). This value is multiplied by the constant 0.003, which is the mL of O2 dissolved per mmHg plasma. This number is infinitesimally small relative to the other half of the equation and it is typically ignored when determining oxygen concentration. This means that the significant modifiable factor in CaO2 is Hgb. Oxygen has to have something to bind to (Hgb) because dissolved oxygen has minimal oxygen-carrying capacity. 

• Oxygen delivery (DO2)= CO x CaO2

• Cardiac Output (CO)= heart rate (HR) x SV

• Stroke volume (SV)= the volume of blood ejected from the heart each heartbeat. 

• Arterial oxygen concentration (CaO2)= [1.38 x Hgb x SaO2] + [PaO2 x 0.003]  

How can oxygen delivery be increased? One of the components of the equation has to be adjusted.

1. Increase cardiac output.

   • Increase SV- use of an inotropic agent (*medication that increases the strength of the heart contraction), ensure adequate preload (volume resuscitation).

   • Increase HR- use of a chronotropic agent (*medication that increases heart rate).

2. Increase arterial oxygen content

   • Increase blood hemoglobin concentration 

*See pharmacology below

Oxygen Consumption

     Oxygen consumption (VO2) is determined by how much oxygen the peripheral tissues extract and use. It is the difference between oxygen delivery (DO2) and oxygen return(ed) (SvO2). 

• Oxygen consumption (VO2)= DO2 - SvO2. Oxygen consumption is calculated by subtracting SvO2 or ScVO2 from the amount of oxygen delivered.

• Venous oxygen saturation (SvO2 or ScVO2)- concentration of oxygen in the blood returning to the heart. Measured with a central venous catheter. *See below under CV monitoring for more details.

Cardiovascular Monitoring

     There are several techniques for monitoring cardiovascular parameters, ranging from non-invasive to maximally invasive. Non-invasive methods include telemetry, pulse oximetry, and blood pressure monitoring. The benefit of these devices is their simplicity of use and interpretation. But these are error-prone, and regarding blood pressure, it doesn't provide continuous monitoring. For more info, see lecture entitled "Hemodynamics".

     Arterial lines can be placed to provide continuous cardiac monitoring. The arterial waveform can indicate specific pathology (see Edwards Quick Guide to Cardiovascular Care). In addition, an arterial line can report stroke volume variation. Stroke volume variation (SVV) is a surrogate of arterial pressure changes with inspiration/ expiration. If the change in pressure with respiratory cycles is >10-15%, it suggests the patient is fluid responsive, meaning they are likely to improve their preload (and cardiac output and blood pressure) with IV fluid administration.

     Central venous catheters can be placed to deliver intravenous medication as well as provide cardiac monitoring. A central venous catheter can measure the pressure of the blood returned to the right atrium (central venous pressure, CVP), which is a crude measurement of preload and right heart function. In addition, the oxygenation of the blood returning to the right heart (from the head and upper body) is reported as Central venous oxygenation saturation (ScVO2). ScVO2 reflects the balance between oxygen delivery and consumption. Arterial lines and central venous catheters are considered "minimally invasive".

     A pulmonary artery (PA) catheter is the most invasive device for cardiac monitoring. Similar to a central venous catheter, a PA catheter can determine the oxygenation of the blood returning to the right heart, which is the mixed venous oxygen saturation (SvO2). However, in contrast to the central venous catheter which is located in the superior vena cava (proximal to the right atria), this device is measuring blood oxygenation in the pulmonary artery (from the right ventricle), so it accounts for the blood from the entire body (unlike the ScVO2).

Cardiac Pharmacology

     Vasoactive medications are frequently used in the ICU for the management of shock, heart failure, and other acute pathology. There are several key receptors, and understanding the function of each receptor is the key to using these different agents correctly.

Receptors

* α (alpha) 1- vasoconstriction

* α2- inhibit norepinephrine release from presynaptic neurons

* β (beta) 1- chronotrope (↑HR), inotrope (↑Ca in cardiac myocytes ↑contractility), dromotrope (↑cardiac impulse conduction velocity)

* β2- vasodilation

* Dopa 1- vasodilation

* Dopa 2- neurotransmitter release

Pharmacologic Agent Classification

Each medication has a specific physiologic effect based on its particular mechanism of action. Agents may stimulate or inhibit receptors (see above) or alter the concentration of a key substance (cAMP, calcium, potassium, nitric oxide (NO)).