An end-tidal CO2 (EtCO2) of roughly 35–45 mmHg on capnography is the conventional target range that signifies adequate ventilation during bag-valve mask (BVM) support in a patient with normal baseline physiology. Values persistently below 35 mmHg suggest hyperventilation or reduced pulmonary blood flow, while values above 45 mmHg suggest hypoventilation. These thresholds guide real-time adjustments, not a fixed pass/fail number.
The physiology behind the exhaled CO2 number
Capnography measures the partial pressure of carbon dioxide at the end of exhalation, which approximates alveolar CO2 and, by extension, arterial CO2 (PaCO2). The EtCO2–PaCO2 gradient is normally small, typically 2–5 mmHg in healthy lungs. This relationship holds only when alveolar ventilation, pulmonary perfusion, and cardiac output are reasonably matched. The number on the screen is therefore a perfusion-and-ventilation signal, not just a breathing signal.
Adequate BVM ventilation does three things simultaneously: it moves a sufficient tidal volume, delivers it at an appropriate rate, and allows enough exhalation time. EtCO2 rises when ventilation is insufficient relative to CO2 production. It falls when ventilation exceeds production or when CO2 cannot reach the lungs because blood flow is reduced. That dual dependence is why a falling EtCO2 during resuscitation can mean either overventilation or circulatory collapse.
In a patient with a perfusing rhythm, EtCO2 in the mid-30s to mid-40s mmHg generally reflects matching between minute ventilation and metabolic CO2 production. Clinicians often use 35–45 mmHg as the working target, mirroring the normal PaCO2 range of 35–45 mmHg. The waveform shape matters as much as the number, since a flat or absent capnogram signals a problem the value alone cannot show.
Plain language: the monitor is telling you how well air movement and blood flow are meeting in the lungs, and both must be working for the number to be trustworthy.
Quantified context: rates, volumes, and waveform clues
Ventilation rate is a primary lever on EtCO2. For an adult in cardiac arrest, guidelines emphasize compressions and avoid excessive ventilation, historically warning against rates above roughly 10 breaths per minute during advanced life support; excessive rate lowers EtCO2 and raises intrathoracic pressure. For a spontaneously perfusing patient receiving assisted BVM breaths, targeting a normal respiratory rate of about 12–20 breaths per minute in adults helps keep EtCO2 near the 35–45 mmHg band.
Tidal volume delivered by BVM is hard to measure precisely without adjuncts. A rough adult target is 6–8 mL/kg of predicted body weight, which for a 70 kg adult is roughly 420–560 mL per breath. Overdelivery distends the stomach and raises aspiration risk; underdelivery produces hypoventilation and a rising EtCO2. A visible chest rise is a practical proxy, but it is subjective.
The capnogram waveform provides timing data. A normal rectangular shape with a brisk upstroke and a plateau indicates even alveolar emptying. An upsloping plateau (a "shark fin") can suggest obstructive physiology such as asthma or bronchospasm. A sudden drop toward zero suggests disconnection, obstruction, or esophageal placement.
| EtCO2 reading (mmHg) | Common interpretation | Typical action |
|---|---|---|
| 0–5 | No effective CO2 return | Check airway, circuit, and perfusion immediately |
| 10–20 | Hyperventilation or low perfusion | Reduce rate, reassess circulation |
| 20–34 | Mild hypocapnia | Slow the rate slightly, reassess |
| 35–45 | Adequate ventilation in normal physiology | Maintain current technique |
| 46–55 | Hypoventilation | Increase rate or volume cautiously |
| Above 55 | Significant hypoventilation | Reassess airway, ventilation, and cause |
This table reflects general adult physiology and assumes a perfusing patient. It is not a substitute for waveform review and clinical assessment.
Time course: how EtCO2 behaves during BVM support
EtCO2 responds quickly to changes in ventilation, usually within a few breaths, because alveolar gas composition shifts fast. That responsiveness makes capnography useful for titrating BVM technique in real time. The timeline below describes a typical sequence when ventilation is corrected.
| Time point | What may be seen | Interpretation |
|---|---|---|
| Before BVM | Low or absent waveform | Inadequate ventilation or no perfusion |
| First 30–60 seconds | EtCO2 begins to register | Air movement and CO2 return established |
| 1–2 minutes | Value rises toward target with good technique | Ventilation matching improving |
| 2–5 minutes | Stabilizes in 35–45 mmHg band | Adequate ventilation in normal physiology |
| With overventilation | Value falls below 35 mmHg | Excessive rate or volume |
| With hypoventilation | Value rises above 45 mmHg | Insufficient minute ventilation |
| With circulatory collapse | Value drops despite unchanged ventilation | Reduced pulmonary blood flow |
The last row is the counterintuitive detail worth flagging. A falling EtCO2 during otherwise stable BVM technique can be a perfusion warning rather than a ventilation problem, particularly in shock or cardiac arrest. Chasing the number by increasing ventilation in that setting can worsen the underlying problem.
Where the target breaks down: edge cases
The 35–45 mmHg target assumes normal metabolism and normal perfusion. Several conditions shift the appropriate range.
Chronic hypercapnia: patients with COPD and chronic CO2 retention may have a baseline EtCO2 above 45 mmHg, so "normal" for them differs. Pregnancy: physiologic hyperventilation lowers baseline CO2, so a lower EtCO2 may be expected. Severe metabolic acidosis: compensatory hyperventilation drives EtCO2 below 35 mmHg as a physiologic response, not a ventilation error. Cardiac arrest: EtCO2 values are typically far below 35 mmHg because pulmonary blood flow is minimal; higher values during CPR suggest better perfusion. Hypothermia and sepsis: altered metabolic rate and perfusion change CO2 production and delivery. Shunt physiology and pulmonary embolism: ventilation-perfusion mismatch widens the EtCO2–PaCO2 gradient.
The professional caveat: treat the trend and the waveform, not a single isolated number. A reading of 38 mmHg on one breath means little without the preceding and following values and the shape of the capnogram. Trending across minutes carries more information than any snapshot.
Plain language: one number is a data point; a pattern over time is a diagnosis aid.
Misconceptions worth correcting
A common misconception is that EtCO2 directly equals arterial CO2 in every patient. The gradient widens in shock, embolism, and low cardiac output, so EtCO2 can substantially underestimate PaCO2. An EtCO2 of 25 mmHg in a hypotensive patient may coexist with a normal or elevated PaCO2.
Another misconception is that a "good" EtCO2 proves the airway is correctly placed and ventilation is optimal. Placement is better confirmed by a sustained waveform over several breaths, and optimal ventilation depends on rate, volume, and exhalation time together. A single acceptable value can mask gastric insufflation or inadequate chest rise.
A third misconception is chasing the target aggressively. Overventilating to force EtCO2 into range can cause hypocapnia, cerebral vasoconstriction, and worsened hemodynamics. The goal is adequate gas exchange, not a specific digit.
A fourth is ignoring the waveform. The number without the shape is half the information, since obstruction, leak, and disconnection each alter the shape before the plateau value stabilizes.
Practical actions for titrating BVM ventilation
Use a structured approach rather than reacting to a single reading.
Confirm a sustained waveform over at least 3–5 breaths before trusting the number. Aim for an adult EtCO2 of 35–45 mmHg when physiology is normal, adjusting rate first. Keep adult assisted breaths near 12–20 per minute and avoid hyperventilation during arrest care. Watch for a rising EtCO2 with hypoventilation and a falling EtCO2 with overventilation or shock. Reassess the patient, not just the monitor: chest rise, breath sounds, and perfusion signs. Recheck after any change in rate, volume, or airway position within 1–2 minutes.
Red-flag warnings:
A sudden drop to near zero requires immediate airway and circuit check. A persistently flat waveform despite chest movement suggests esophageal placement or obstruction. Falling EtCO2 with unchanged ventilation may signal circulatory deterioration, not a ventilation error. Very high EtCO2 with poor chest rise suggests severe hypoventilation and possible airway obstruction.
Limitations and what the evidence supports
The 35–45 mmHg target derives from normal PaCO2 physiology and is widely used in emergency and critical care education, but it is a general reference rather than a validated universal threshold for every BVM scenario. Individual targets depend on the patient's baseline, the clinical context, and concurrent perfusion status. Capnography reflects exhaled CO2, which only approximates arterial CO2 when ventilation and perfusion are matched. The EtCO2–PaCO2 gradient can vary widely in shock, arrest, and lung disease.
Capnography also cannot measure tidal volume or airway sealing directly, so BVM adequacy still requires clinical assessment alongside the monitor. Waveform interpretation adds value but depends on operator skill. Where local protocols specify different targets, those protocols and qualified clinical judgment take precedence over general ranges. This article describes mechanisms and general principles rather than individualized targets.
This article is general educational information, not a diagnosis or treatment plan. It does not replace clinical assessment or local protocols. For any patient receiving ventilation support, consult a qualified clinician or emergency service and follow local guidelines.