When the heart stops beating effectively, a person experiences cardiac arrest, leading to the immediate cessation of effective blood circulation. Cardiopulmonary resuscitation (CPR) is instantly necessary to manually circulate oxygenated blood, serving as a temporary, life-sustaining intervention. The goal of chest compressions is to artificially generate blood flow to the body’s most sensitive organs until the heart’s natural rhythm can be restored. This manual process moves blood through the system when the heart muscle is unable to pump.
Normal Cardiac Output: Establishing the Baseline
The measurement of the heart’s efficiency is called cardiac output, representing the volume of blood the heart ejects each minute. This volume is calculated by multiplying the heart rate by the stroke volume (the amount of blood pumped out with each beat). For a healthy, resting adult, cardiac output typically ranges between 4 and 8 liters per minute, with 5 liters per minute often cited as the average value. The stroke volume is usually between 55 and 100 milliliters per heartbeat, which is enough to meet the body’s full metabolic demands.
This continuous flow is necessary to deliver oxygen and nutrients to all tissues while simultaneously removing metabolic waste products. The heart possesses a significant reserve capacity, allowing cardiac output to increase dramatically during periods of physical activity. This baseline output is the standard that artificial circulation must attempt to mimic during a cardiac emergency.
The Mechanism of Artificial Circulation
External chest compressions force blood through the circulatory system by physically altering the pressure within the chest cavity. The most widely accepted explanation is the thoracic pump mechanism, which posits that the entire thorax acts as a pump. When the chest wall is compressed, the resulting increase in intrathoracic pressure squeezes the heart and major blood vessels. This pressure gradient forces blood from the thoracic cavity out into the systemic circulation.
The heart acts primarily as a passive conduit rather than a dedicated pump in this scenario. During the release phase, the negative pressure created allows the great veins to fill the heart with returning blood, preparing it for the next manual squeeze. A secondary theory, the cardiac pump mechanism, suggests the heart is directly compressed between the sternum and the spine, but the thoracic pump is thought to be the dominant mechanism in most adults.
The Quantitative Difference in Blood Flow
Even when executed perfectly, chest compressions generate a blood flow significantly lower than normal physiological output. Optimal cardiopulmonary resuscitation typically achieves only 20% to 30% of a person’s normal cardiac output. If normal output was 5 liters per minute, artificial circulation moves only about 1.0 to 1.5 liters per minute. This minimal circulation is a consequence of the non-physiological pressure gradient created by the compressions.
The low flow rate is insufficient to meet the body’s full metabolic needs, but it is often enough to sustain the most vulnerable organs. This blood flow is directed to minimally perfuse the brain and the heart muscle itself, buying time until definitive medical treatment can be applied. The circulation is limited because the heart chambers do not fill completely between compressions, and the pressure generated is not as efficient as a healthy heart’s contraction.
Parameters for Maximizing Artificial Circulation
Because artificial circulation provides such a small fraction of normal blood flow, adherence to precise technique is necessary to ensure the maximum possible 20-30% is achieved. Current guidelines emphasize specific physical parameters that define high-quality compressions. Failure to meet any of these specific parameters drastically reduces the already limited blood flow, severely impacting the chance of a positive outcome.
Compression Depth and Rate
The correct depth for an adult is to compress the chest by at least 2 inches, but not more than 2.4 inches. The compression rate is also important and must be maintained between 100 and 120 compressions per minute.
Full Chest Recoil
The allowance for full chest recoil between compressions is a primary element of high-quality CPR. Full recoil permits the thoracic cavity to return to its normal shape, which is necessary for effective circulation. This creates the negative pressure necessary for venous blood to return to the heart, ensuring the chambers have enough volume to be pushed out during the next compression.