Haemodynamic instability (HI) describes a state where the circulatory system fails to maintain adequate blood flow, or perfusion, to the body’s tissues and vital organs. This instability signifies a breakdown in the system’s ability to regulate the forces of blood circulation. This failure results in the lack of oxygen and nutrients reaching cells, a condition known as shock, which is a life-threatening medical emergency requiring immediate intervention.
Understanding the Circulatory Breakdown
The circulatory system relies on the interaction of three main components to ensure stable haemodynamics and adequate tissue perfusion. These are the heart’s function, which determines cardiac output (the volume of blood ejected per minute); the total circulating blood volume, which dictates preload (the blood returning to the heart); and the vascular tone, which determines systemic vascular resistance (SVR).
Instability begins when one or more of these components fails, leading to a severe imbalance between oxygen supply and the body’s metabolic demand. Reduced cardiac output or a drastic drop in SVR results in a fall in mean arterial pressure (MAP). When this pressure drops below a critical threshold, blood flow to the microvasculature is insufficient to deliver necessary oxygen. This inadequate perfusion forces cells to switch to anaerobic metabolism, producing lactic acid. Elevated blood lactate levels indicate that tissue oxygenation is failing and the patient is in circulatory shock.
Recognizing the Clinical Signs
The body attempts to compensate for failing perfusion by activating the sympathetic nervous system, leading to a cascade of observable effects. A rapid heart rate (tachycardia) is a common sign as the heart attempts to increase cardiac output by beating faster to compensate for a reduced stroke volume. Low blood pressure (hypotension) is a later and more ominous sign, indicating that the compensatory mechanisms are being overwhelmed.
The body prioritizes blood flow to vital organs, such as the brain and heart, by constricting blood vessels in less essential areas like the skin, muscles, and kidneys. This peripheral vasoconstriction is why a person experiencing haemodynamic instability often presents with cool, pale, and clammy skin. Simultaneously, reduced blood flow to the brain can cause an altered mental status, resulting in confusion or decreased responsiveness.
Poor perfusion also manifests as significantly decreased urine output (oliguria), as the kidneys attempt to conserve the remaining fluid volume in the circulation.
Primary Causes of Instability
Haemodynamic instability is the manifestation of an underlying condition categorized as a form of circulatory shock, with three main physiological types based on the component of the circulatory system that fails first.
Hypovolemic Shock
This occurs due to a significant and sudden loss of circulating blood volume, caused by severe hemorrhage or non-hemorrhagic causes like severe dehydration or burns. The primary problem is critically low preload, meaning insufficient blood returns to the heart to be pumped out effectively.
Cardiogenic Shock
This is defined by a primary failure of the heart pump, where the myocardium cannot contract with enough force to generate adequate cardiac output. This is commonly caused by a large myocardial infarction (heart attack). Although the blood volume and vascular tone may initially be normal, the insufficient pumping action leads to congestion and poor forward flow.
Distributive Shock
This is a failure of vascular tone, characterized by pathological widespread vasodilation and a severe drop in systemic vascular resistance. The total volume of blood remains the same, but the vascular system becomes too large, resulting in relative hypovolemia. The most common example is septic shock, where a systemic infection triggers an overwhelming inflammatory response. Anaphylaxis also causes distributive shock through the massive release of inflammatory mediators.
Immediate Stabilization and Treatment Strategy
The immediate treatment for haemodynamic instability aims to rapidly restore adequate tissue perfusion and address the underlying cause. Initial management often involves volume resuscitation, particularly for hypovolemic and distributive shock, using intravenous crystalloid fluids like normal saline or lactated Ringer’s solution to quickly increase the circulating volume and preload. In cases of hemorrhagic shock, blood products are administered to replace lost red blood cells and clotting factors.
If fluid administration alone is insufficient to stabilize blood pressure, pharmacological support is initiated using vasoactive medications tailored to the specific type of shock. These drugs fall into two main categories: vasopressors and inotropes. Vasopressors, such as norepinephrine, act primarily to constrict blood vessels, increasing systemic vascular resistance and raising the mean arterial pressure. This is particularly effective in distributive shock, where low vascular tone is the main defect.
Inotropes, such as dobutamine, are used when the primary issue is pump failure, as they work to increase the force of the heart muscle’s contraction, thereby boosting cardiac output. The choice of agents depends on the specific physiological deficit identified through continuous monitoring. The final step is source control, which involves identifying and treating the root cause, such as administering broad-spectrum antibiotics for septic shock or surgically stopping a source of internal bleeding.