Dead Space Calculator - Bohr Equation and Ratio

The dead space calculator turns the alveolar CO2 partial pressure, the mixed expired CO2 partial pressure, and the tidal volume from a ventilation assessment into a physiological dead space in mL, a wasted-ventilation percent, and a comparison against the 2 mL per kg reference range. The Bohr equation is the standard mass-balance formula used to estimate it.

• • Reading a capnography trace: Enter arterial and end-tidal CO2 plus the measured tidal volume to get a physiological dead space in mL and a Vd/Vt percent.
• • Reviewing a ventilated patient: Run the Bohr equation on a snapshot of PaCO2, PeCO2, and tidal volume from a ventilator screen to check whether dead space is climbing.
• • Comparing values to body weight: Add a body weight to see the 2 mL per kg reference range and whether the calculated dead space is normal or elevated.
• • Teaching the Bohr equation: Use a worked example with normal adult values to show how alveolar and expired CO2 combine into a wasted-ventilation fraction.

Most readers meet the Bohr equation in a respiratory physiology class or a ventilator review, so the calculator is built around the three values a real report shows: alveolar CO2, mixed expired CO2, and tidal volume.

Body weight is optional but useful, because the textbook reference rule for physiological dead space is about 2 mL per kilogram, which gives a quick reasonableness check on the calculated value.

When the focus shifts from alveolar gas exchange to sleep apnea severity, AHI Calculator turns apnea and hypopnea events plus sleep time into an apnea-hypopnea index score.

The dead space calculator applies the Bohr equation in three steps: subtract the mixed expired CO2 from the alveolar CO2, divide by the alveolar CO2 to get a dimensionless wasted-ventilation fraction, and multiply by the tidal volume to return the physiological dead space in mL.

• paCO2: Alveolar or arterial CO2 partial pressure in mmHg. The Enghoff modification uses the arterial value as a practical substitute.
• peCO2: Mixed expired CO2 partial pressure in mmHg from a capnography trace or Douglas bag.
• tidalVolume: Tidal volume in mL from a ventilator readout or a measured breath.
• bodyWeight: Optional body weight in kilograms used to compute the 2 mL per kg reference range.

The result panel shows the dead space in mL, the same value in liters, the Vd/Vt percent, and the body-weight reference comparison together, so a reader can quote all four values from a single pass.

When mixed expired CO2 climbs close to alveolar CO2, the wasted-ventilation fraction shrinks and the result panel will show a low Vd/Vt and a status of Normal against the body-weight reference.

According to Wikipedia, Bohr equation, the Bohr equation named after Christian Bohr gives the ratio of physiological dead space to tidal volume as the difference between alveolar and mixed expired CO2 partial pressures divided by alveolar CO2, and the Enghoff modification replaces the alveolar value with the arterial PaCO2

For the companion gas exchange view on the arterial side of the alveolar-capillary barrier, Aa Gradient Calculator returns PAO2 and the A-a oxygen gradient from age, FiO2, and blood gas values.

Four concepts keep the Bohr equation from being read as a single fraction, and they explain why the same dead space value can mean different things across patients.

The Bohr equation is a fraction rather than a direct measurement, so the same dead space value can be reached through different combinations of tidal volume and CO2 partial pressure.

In a healthy person, alveolar dead space is small, and physiological dead space is close to the anatomical value of the conducting airways, but lung disease can shift the ratio in either direction.

When readers want to know the body weight value that 2 mL per kg is built around, Ideal Body Weight Calculator returns the ideal body weight used in clinical reference rules.

The form follows the same order as a capnography report, so the top of the form down mirrors the order of a real ventilation note.

1. 1 Find the alveolar or arterial CO2 value: Open the arterial blood gas report or capnography trace and locate the PaCO2 line, usually shown in mmHg.
2. 2 Find the mixed expired CO2 value: Use the PeCO2 reading from a capnography trace or Douglas bag. Both are partial pressures in mmHg.
3. 3 Enter the CO2 partial pressures: Type the PaCO2 into the Alveolar CO2 field and PeCO2 into Mixed Expired CO2, then double-check the units are mmHg.
4. 4 Enter the tidal volume: Locate the tidal volume from the ventilator readout or a measured breath, then enter it in mL. A resting adult sits near 500 mL.
5. 5 Add body weight if you have it: Enter body weight in kilograms to surface the 2 mL per kg reference range and a Normal or Elevated status label.
6. 6 Read the result panel as a set: Use the dead space in mL, the Vd/Vt percent, and the status label together rather than the raw mL value alone.

A 70 kg adult with arterial PaCO2 40 mmHg, mixed expired PeCO2 30 mmHg, and tidal volume 500 mL enters 40, 30, 500, and 70. The result reads dead space 125 mL, Vd/Vt 25 percent, expected 140 mL, status Normal.

For ventilation charts that reference body size alongside weight, Body Surface Area Calculator returns body surface area from height and weight.

Doing the math on paper takes a moment of arithmetic, and the calculator adds the units, percent, and reference comparison that a hand calculation usually leaves out.

• • Bohr equation arithmetic in one pass: The form returns the dead space in mL, the same value in liters, and the Vd/Vt percent from a single set of inputs, so the reader does not have to switch between unit conversions.
• • Body-weight reference built in: The calculator applies the 2 mL per kg rule when body weight is entered, so the result is paired with a Normal or Elevated status label rather than a raw number.
• • Mixed unit support through documentation: The help text and worked example cover kPa and L conversions, so a reader who reads capnography in kPa can convert to mmHg and still use the same form.
• • Validation for the wasted-ventilation fraction: The form flags the case where mixed expired CO2 is greater than alveolar CO2, which would produce a negative dead space and is not physiologically valid.
• • Rounding that fits a chart note: Dead space is rounded to one decimal in mL and to three decimals in liters, which mirrors the precision used in ventilation chart notes.
• • Reusable for snapshots and trends: The same form works for a single capnography snapshot, a series of ventilator readings, and a classroom Bohr equation example.

The 2 mL per kg comparison is a quick reasonableness check, especially for ventilated patients.

The form keeps mmHg and mL as the visible inputs and trusts the reader to convert other units before entering them, because the Bohr equation is unit-agnostic once both CO2 inputs share a unit.

Five factors can move the calculated dead space by enough to cross the 2 mL per kg reference, so knowing them helps a reader interpret a value that surprises them.

• • The Bohr equation assumes mixed expired CO2 is a single value, but the expired CO2 waveform is shaped by the distribution of ventilation and perfusion, so the Enghoff modification is an approximation.
• • A Vd/Vt percent is a ratio rather than an absolute volume, so a 50 percent Vd/Vt at 300 mL describes a different situation from 50 percent at 600 mL, and the form's reference comparison uses body weight to flag the difference.

The reference comparison is a quick reasonableness check, not a diagnosis, and a Vd/Vt above 35 percent can have many causes from low cardiac output to underperfused lung regions.

The rest of the interpretation belongs in a clinical review alongside the capnography waveform, the PaCO2 to EtCO2 gradient, and the patient's ventilator settings.

According to Wikipedia, Dead space (physiology), normal adult physiological dead space is about 150 mL, roughly 2 mL per kilogram of body weight, and the Bohr equation is the standard mass-balance method used to calculate it

When the discussion turns to body composition and how it changes wasted ventilation, BMI Calculator turns height and weight into a body mass index value.