The Delta Dispatch
From Pulse to Perfusion: Cardiac Output in Prehospital Medicine
Cardiac output is the foundation of perfusion — the lifeline that keeps every organ functioning. In prehospital care, understanding how heart rate and stroke volume interact helps responders identify shock early, guide treatment, and protect vital organs. This blog breaks down cardiac output in clear, practical terms for EMRs, advanced first aiders, and anyone working in emergency response.
When assessing or treating a patient in the field, one of the most critical yet often overlooked concepts is cardiac output— the amount of blood the heart pumps in one minute. Understanding it helps prehospital providers recognize early signs of shock, guide treatment decisions, and prioritize interventions that can truly make the difference between life and death.
What Is Cardiac Output?
Cardiac Output (CO) = Heart Rate (HR) × Stroke Volume (SV)
Heart Rate (HR): The number of heartbeats per minute.
Stroke Volume (SV): The amount of blood pumped out of the left ventricle with each beat.
For example, if a patient’s heart rate is 80 bpm and stroke volume is 70 mL, their cardiac output is:
80 × 70 = 5,600 mL/min (5.6 L/min)
This means their heart pumps about 5–6 litres of blood per minute, roughly the body’s total blood volume.
Why It Matters in Prehospital Care
Cardiac output is the foundation of perfusion — how well oxygen and nutrients reach tissues. In the field, recognizing how HR and SV interact can help you determine whether a patient is compensating or decompensating.
When cardiac output drops, tissue hypoxia occurs. Cells switch to anaerobic metabolism, producing lactic acid and leading to shock.
Common causes of decreased cardiac output in the field include:
Hypovolemia (bleeding, dehydration)
Pump failure (myocardial infarction, cardiomyopathy)
Obstruction (cardiac tamponade, tension pneumothorax, pulmonary embolism)
Distributive shock (sepsis, anaphylaxis)
Heart Rate and Stroke Volume in Real-Time Assessment
Tachycardia: Often the first sign of compensation. The body increases HR to maintain CO when stroke volume drops (like in hemorrhage).
Bradycardia: Can be catastrophic in shock, as it limits overall output.
Stroke Volume Influencers:
Preload: Volume of blood returning to the heart.
Contractility: Strength of the heart’s contractions.
Afterload: Resistance the heart pumps against.
Understanding how each factor impacts stroke volume helps guide care decisions — for instance, whether a patient needs fluids, vasopressors, or advanced airway management.
In Practice: Assessing Cardiac Output Without a Monitor
Prehospital providers rarely calculate CO directly — instead, you assess it through clinical signs of perfusion:
Level of consciousness – is the brain getting enough oxygen?
Skin signs – pale, cool, diaphoretic skin suggests poor perfusion.
Capillary refill and pulse strength – quick, weak, or absent pulses can all indicate changes in CO.
Blood pressure and pulse pressure trends – narrowing pulse pressure (e.g., 90/80) may indicate a drop in stroke volume.
Your hands, eyes, and ears are your diagnostic tools before monitors confirm what you already suspect.
Improving Cardiac Output in the Field
Depending on the cause:
Fluid resuscitation restores preload in hypovolemia.
Positioning (Trendelenburg or shock position) can temporarily improve venous return.
Oxygen therapy supports oxygen delivery when perfusion is limited.
CPR and defibrillation are lifesaving when the heart fails to generate output altogether.
Remember — every intervention you perform is ultimately aimed at restoring or maintaining cardiac output to sustain vital organ perfusion.
Final Thoughts
For first responders and EMRs, cardiac output isn’t just a number — it’s the lifeline of your patient.
Understanding how heart rate, stroke volume, and perfusion interact allows you to anticipate deterioration before it becomes irreversible.
Every pulse you feel, every set of vitals you take, tells a story about how effectively the heart is doing its job.
Dealing with Shock: A Guide for First Responders
Shock is a serious and potentially life-threatening condition that occurs when there is inadequate blood flow to the body's tissues and organs. There are several different types of shock, including hypovolemic shock (caused by severe blood loss), cardiogenic shock (caused by heart failure), obstructive shock (caused by a physical obstruction), and distributive shock (caused by a loss of blood vessel tone).
Common symptoms of shock include rapid breathing, rapid heart rate, low blood pressure, confusion or disorientation, and pale or cool skin. If left untreated, shock can lead to organ failure and even death.
Effective treatment of shock depends on identifying the underlying cause and quickly providing appropriate interventions. This may include providing high-flow oxygen, addressing the underlying cause (such as stopping severe bleeding), and closely monitoring the patient's vital signs.
Shock is a life-threatening medical condition that can occur in a variety of situations, including trauma, infection, and severe bleeding. It's important for firefighters and other first responders to be able to recognize the signs and symptoms of shock and to know how to treat it effectively.
First responders at a car crash scene attending to a patient in shock.
What is shock
When the body enters a state of shock, it undergoes a process called vasoconstriction. This is where the blood vessels throughout the body narrow and constrict in an attempt to redirect blood flow to the body's core organs, such as the heart, lungs, and brain. This is a protective mechanism that allows the body to prioritize blood flow to the most vital organs during times of stress or injury.
However, this process of vasoconstriction can have negative effects on other parts of the body. Blood flow to the extremities, such as the arms and legs, can be significantly reduced, which can lead to cold, clammy skin and impaired wound healing. The kidneys may also be affected, as decreased blood flow to the kidneys can lead to decreased urine output and impaired kidney function.
If shock is not addressed and blood flow to the body's core organs is not restored, the body's compensatory mechanisms may become overwhelmed and ineffective. This can lead to a state of decompensation, where the body's vital organs begin to fail and serious complications can occur.
What does shock look like?
Shock is characterized by a lack of oxygen and nutrients to the body's tissues and organs. This can cause a range of symptoms, including:
Rapid heart rate and breathing
Low blood pressure
Cold, clammy skin
Confusion or disorientation
Weakness or fatigue
Nausea or vomiting
Different types of shock
There are several different types of shock, each with their own unique causes and characteristics:
Hypovolemic shock occurs when there is a significant loss of blood or other fluids from the body, such as from a severe injury or dehydration.
Cardiogenic shock is caused by a heart that is unable to pump blood effectively due to damage or disease.
Obstructive shock occurs when there is a physical blockage that prevents blood from circulating properly, such as from a blood clot or a collapsed lung.
Distributive shock is caused by a disruption of the body's natural balance of fluids and can be the result of severe infection, an allergic reaction, or neurological damage.
Treatment
Administering high-flow oxygen:
One of the first steps in treating someone in shock is to provide high-flow oxygen. This can help improve oxygen delivery to the body's tissues and organs.
Addressing the underlying cause:
Depending on the type of shock, there may be an underlying cause that needs to be addressed. For example, if the shock is caused by severe bleeding, the bleeding must be stopped in order to effectively treat the shock.
Monitoring vital signs:
It's important to monitor the patient's vital signs, including their heart rate, blood pressure, and oxygen saturation levels, in order to determine the effectiveness of treatment and make any necessary adjustments.
It's important to note that the specific treatment for shock will vary depending on the underlying cause and the patient's individual needs. Therefore, it's crucial for first responders to be properly trained and equipped to identify and treat shock in a timely and effective manner.
Training
At Delta Emergency Support Training, we understand the importance of being prepared for emergency situation. We offer a range of training sessions to help individuals and classes develop the skills and knowledge they need to respond to emergency situations. Our courses include Standard First Aid (SFA), Advanced First Aid (AFA), and Emergency Medical Responder (EMR), and we offer in-person, hybrid, and online options to suit different needs.
If you have any questions about our training sessions or shock, please don't hesitate to reach out to us. You can contact us at info@deltaemergency.com and we'll be happy to assist you.