In the PALS (Pediatric Advanced Life Support) framework, the cardiopulmonary system has three major functions: oxygenation, ventilation, and circulation. These work together to deliver oxygen to every tissue in the body and remove carbon dioxide. Understanding how each function works in children, and how quickly it can fail, is the foundation of pediatric emergency assessment.
Oxygenation: Getting Oxygen Into the Blood
Oxygenation is the process of moving oxygen from inhaled air into the bloodstream at the level of the lungs’ tiny air sacs, called alveoli. In children, this process has unique vulnerabilities. Neonates and premature infants have far fewer alveoli than adults. These air sacs continue to grow and multiply from birth through adolescence, which means younger children have less lung surface area available for gas exchange.
Children also consume oxygen at a much higher rate. Neonates, infants, and toddlers have a resting oxygen consumption more than double that of adults, driven by their higher metabolic rate. This means any interruption in oxygen supply depletes their reserves faster, and they can deteriorate within minutes rather than the longer window adults typically have.
Newborns face an additional challenge: their blood contains a high proportion of fetal hemoglobin, which grips oxygen more tightly than adult hemoglobin. While this helped pull oxygen from the placenta before birth, it makes it harder to release oxygen to tissues afterward. Over the first weeks and months of life, fetal hemoglobin is gradually replaced, and oxygen delivery to tissues improves.
Ventilation: Moving Air In and Out
Ventilation is mechanically distinct from oxygenation. It refers to the physical movement of air into and out of the lungs, and its primary job is removing carbon dioxide, the waste product of metabolism. When ventilation fails, carbon dioxide builds up in the blood, making it dangerously acidic.
Carbon dioxide and oxygen don’t behave the same way in the lungs. Carbon dioxide dissolves much more easily across lung tissue, so even partially functioning lungs can often clear CO2 reasonably well. Oxygen transfer is less forgiving. This is why a child can have adequate ventilation (normal CO2 levels) but still be poorly oxygenated, or vice versa. PALS treats these as separate problems requiring separate interventions.
Pediatric airways are structurally different from adult airways in ways that make ventilation more fragile. Infants between about 2 and 6 months are preferential nasal breathers, meaning a stuffy nose can significantly impair their ability to move air. The narrowest point of a child’s airway sits below the vocal cords at the cricoid cartilage, unlike adults, where the vocal cords themselves are the bottleneck. The pediatric chest wall is also more compliant, meaning it’s softer and more flexible. This sounds like an advantage, but it actually means the chest can collapse inward during labored breathing rather than generating effective suction to pull air into the lungs.
Premature infants respond poorly to rising CO2 levels. While older children and adults automatically breathe faster and deeper when carbon dioxide accumulates, preterm neonates have a blunted response to this signal, making them vulnerable to dangerous CO2 buildup without obvious warning signs.
Circulation: Delivering Oxygen to Tissues
Circulation is the pump-and-pipeline system that moves oxygenated blood from the lungs to every organ. In PALS, adequate circulation means the heart is generating enough cardiac output (the volume of blood pumped per minute) to meet tissue demand. Cardiac output depends on two variables: heart rate and stroke volume (the amount of blood ejected with each beat).
This is where pediatric physiology diverges sharply from adults. An adult heart can increase output by pumping harder, squeezing out a larger volume per beat. An infant’s heart muscle is less mature and cannot significantly increase stroke volume. Instead, infants and young children are almost entirely dependent on heart rate to maintain cardiac output. In neonates, a drop from a normal rate of 140 to 160 beats per minute down to 120 or below can slash cardiac output by 20 to 30 percent. This is why bradycardia (a slow heart rate) is treated as an emergency in pediatric patients.
Normal Vital Signs by Age
Recognizing abnormal cardiopulmonary function in children requires knowing what’s normal at each age. The ranges shift dramatically from infancy through adolescence.
For heart rate (beats per minute while awake):
- Newborn to 3 months: 85 to 205
- 3 months to 2 years: 100 to 190
- 2 to 10 years: 60 to 140
- Over 10 years: 60 to 100
For respiratory rate (breaths per minute):
- Infant: 30 to 60
- Toddler: 24 to 40
- Preschooler: 22 to 34
- School-age child: 18 to 30
- Adolescent: 12 to 16
Hypotension thresholds also vary. A systolic blood pressure below 60 mm Hg is hypotensive for a term newborn. For children 1 to 10 years, the threshold is 70 plus twice the age in years. For children over 10, the adult threshold of 90 mm Hg applies.
How Children Compensate for Failure
When one or more of these three functions begins to fail, a child’s body activates compensatory mechanisms to buy time. The two primary responses are tachycardia (faster heart rate) and vasoconstriction (narrowing of blood vessels in the arms, legs, and skin to redirect blood toward vital organs like the brain and heart).
During compensated shock, blood pressure stays normal even though perfusion is falling. The signs are subtle but recognizable: a fast heart rate, cool hands and feet, slow capillary refill time, and peripheral pulses that feel weaker than central pulses. This is the critical intervention window in PALS, because the child may look deceptively stable based on blood pressure alone.
When these compensatory mechanisms are overwhelmed, the child enters decompensated shock. Blood pressure drops, mental status deteriorates, urine output falls, and breathing rate climbs as the body tries to blow off the acid building up from oxygen-starved tissues. Central pulses weaken. This progression can be rapid in children precisely because their compensatory reserves, while initially strong, are limited in duration.
Respiratory Distress vs. Respiratory Failure
PALS draws a critical line between respiratory distress and respiratory failure, and the distinction maps directly to the three cardiopulmonary functions. In respiratory distress, the child is working harder to maintain oxygenation and ventilation. You’ll see increased breathing effort: nasal flaring, chest retractions, use of neck and abdominal muscles to breathe. The system is strained but still functioning.
Respiratory failure is the point where those efforts stop working. The signs of impending collapse include exhaustion (visible as listlessness or decreasing breathing effort), oxygen saturation below 92% despite supplemental oxygen, impaired consciousness, recurrent pauses in breathing, and worsening CO2 levels. A particularly dangerous signal is a child who was working hard to breathe and then suddenly becomes quiet and still. That isn’t improvement. It’s the loss of compensatory effort and typically means intervention is needed immediately.
Children with neuromuscular conditions deserve special attention here, because they may not be able to show the visible signs of increased breathing effort even when they are in distress. Their chest wall muscles are too weak to produce the retractions and accessory muscle use that typically signal a struggling respiratory system.
The PALS Systematic Approach
PALS ties all three cardiopulmonary functions together in its initial assessment framework, which evaluates three things at a glance: appearance, work of breathing, and circulation (assessed by skin color). This rapid “across the room” evaluation takes seconds and tells you whether the child’s cardiopulmonary system is meeting demand or falling behind.
Appearance reflects brain perfusion, the end product of all three functions working together. A child who is alert, interactive, and making eye contact has adequate oxygen delivery to the brain. Work of breathing reveals whether ventilation and oxygenation require excessive effort. Skin color, particularly mottling, pallor, or bluish discoloration, signals circulatory compromise. Each observation maps back to one of the three core functions, making the framework both a clinical tool and a way to understand what the cardiopulmonary system is doing in real time.