The systematic approach algorithm is a step-by-step framework taught in Advanced Cardiovascular Life Support (ACLS) and Pediatric Advanced Life Support (PALS) courses. It gives healthcare providers a consistent, repeatable method for evaluating and treating patients in life-threatening emergencies. The algorithm moves through distinct phases: an initial impression, a primary assessment, a secondary assessment, and targeted treatment of reversible causes. Each phase builds on the one before it, ensuring nothing critical gets missed during a high-stress resuscitation.
Initial Impression: The First Few Seconds
The systematic approach begins before anyone touches the patient. In the first few seconds, you form a rapid visual impression using three observations, sometimes remembered as CBC: Consciousness, Breathing, and Color. You’re looking at the patient from a distance to determine whether they appear conscious, whether they’re breathing normally, and whether their skin color suggests adequate blood flow. Pale, gray, or bluish skin can signal poor circulation or oxygen levels even before a monitor is attached.
This quick scan determines which path you take next. If the patient is unconscious, not breathing, or pulseless, you move immediately into a Basic Life Support survey: check for a pulse, begin CPR if needed, and activate the emergency response system. If the patient is conscious and breathing, you proceed directly into the primary assessment at a slightly less urgent pace.
Primary Assessment: The ABCDE Framework
The primary assessment is the core of the algorithm. It follows the ABCDE sequence, and each letter represents a body system you evaluate in order of priority.
- Airway. Is the airway open? Can the patient maintain it on their own, or do they need repositioning, suctioning, or an advanced airway device?
- Breathing. Is the patient breathing adequately? You assess the rate, depth, and effort of breathing. Oxygen saturation monitoring (pulse oximetry) and waveform capnography, which measures exhaled carbon dioxide, help confirm whether ventilation is effective.
- Circulation. Check the pulse, blood pressure, and skin signs like temperature and moisture. An ECG (heart monitor) is attached here to identify any dangerous heart rhythms. If the patient is in cardiac arrest, defibrillation happens during this step.
- Disability. This is a quick neurological check. The AVPU scale categorizes the patient as Alert, responsive to Voice, responsive only to Pain, or Unresponsive. A sudden change in consciousness can point to stroke, low blood sugar, or other time-sensitive conditions.
- Exposure. Remove enough clothing to examine the patient for injuries, rashes, bleeding, or medical devices like insulin pumps that might explain the situation. Keeping the patient warm during this step matters, especially in prolonged resuscitations.
The key principle is that you address each problem as you find it before moving to the next letter. If the airway is blocked, you fix it before assessing breathing. This prevents providers from jumping ahead to a diagnosis while a basic, fixable problem goes untreated.
Secondary Assessment: Building the Full Picture
Once the primary assessment is underway and immediate threats are being managed, the secondary assessment gathers more detailed information. This phase uses the SAMPLE mnemonic to collect a focused patient history:
- Signs and Symptoms. What you can observe (signs) and what the patient reports feeling (symptoms).
- Allergies. Particularly to medications, since drug reactions can mimic or worsen emergencies.
- Medications. What the patient currently takes, including over-the-counter drugs and supplements.
- Past pertinent medical history. Previous heart problems, surgeries, or chronic conditions that could explain the current situation.
- Last oral intake. When the patient last ate or drank, which matters for procedures that may require sedation.
- Events leading up to the emergency. What was the patient doing when symptoms started? Did anything trigger the episode?
The secondary assessment also includes a more thorough physical exam and review of diagnostic data. This is where lab results, 12-lead ECGs, and imaging come into play to narrow down the cause and guide specific treatment.
The H’s and T’s: 12 Reversible Causes
One of the most practical components of the systematic approach is a checklist of 12 reversible conditions that can cause or worsen cardiac arrest. These are grouped by their first letter into “H’s” and “T’s,” and providers are trained to run through this list whenever a patient isn’t responding to standard treatment.
The seven H’s are:
- Hypovolemia (not enough blood or fluid in the body)
- Hypoxia (not enough oxygen reaching the tissues)
- Hydrogen ion excess (the blood has become too acidic)
- Hypoglycemia (dangerously low blood sugar)
- Hypokalemia (too little potassium in the blood)
- Hyperkalemia (too much potassium in the blood)
- Hypothermia (the body’s core temperature has dropped too low)
The five T’s are:
- Tension pneumothorax (air trapped in the chest cavity compressing the lung and heart)
- Tamponade, cardiac (fluid around the heart preventing it from pumping)
- Toxins (poisoning or drug overdose)
- Thrombosis, pulmonary (a blood clot blocking the lung’s blood supply)
- Thrombosis, myocardial (a blood clot blocking blood flow to the heart muscle, or heart attack)
The value of this list is that every item on it has a specific, targeted treatment. Standard CPR and medications alone won’t save a patient whose cardiac arrest is caused by a tension pneumothorax or cardiac tamponade. Identifying and treating the underlying cause is often the difference between survival and death.
How the Algorithm Differs for Children
The Pediatric Advanced Life Support (PALS) version of the systematic approach follows the same general structure but shifts its emphasis. In adults, cardiac arrest most often starts as a heart rhythm problem. In children, the cause is more frequently respiratory failure or shock that eventually leads to cardiac arrest. Because of this, the pediatric algorithm places even heavier weight on the Airway and Breathing steps and on recognizing the early signs of respiratory distress and circulatory shock before the heart stops.
The ABCDE sequence, the SAMPLE history, and the H’s and T’s all still apply. The difference is in what providers are trained to watch for at each step and the thresholds that define “normal” for different age groups.
Why the Sequence Matters
The entire point of the systematic approach is to replace panic with process. Cardiac emergencies are chaotic, and even experienced providers can fixate on a single problem or skip steps under pressure. The algorithm creates a mental checklist that ensures the most lethal, most treatable problems get addressed first.
The American Heart Association’s ACLS flow diagrams are built on this foundation. Every specific protocol for cardiac arrest rhythms, stroke, or acute coronary syndromes assumes you’ve already completed or are simultaneously running the systematic approach. It’s the starting framework that feeds into every other algorithm in the course. The AHA publishes updated guidelines periodically, with the most recent full set released in 2025, but the underlying logic of the systematic approach has remained consistent across revisions: assess in order of lethality, treat what you find, and continually reassess.