Dead Space Calculator

Physiological Dead Space Calculator

Volume of air inhaled in a single breath (mL)

Partial pressure of carbon dioxide in arterial blood (mmHg)

Partial pressure of carbon dioxide in mixed expired air (mmHg)

function calculateDeadSpace() { // Get inputs by ID var vtInput = document.getElementById('ds_tidal_volume'); var paco2Input = document.getElementById('ds_paco2'); var peco2Input = document.getElementById('ds_peco2'); var resultBox = document.getElementById('ds_result_display'); // Parse values var vt = parseFloat(vtInput.value); var paco2 = parseFloat(paco2Input.value); var peco2 = parseFloat(peco2Input.value); // Validation if (isNaN(vt) || isNaN(paco2) || isNaN(peco2)) { alert("Please enter valid numerical values for all fields."); return; } if (vt <= 0 || paco2 = paco2) { alert("Expired PCO2 typically cannot be higher than Arterial PCO2."); return; } // Calculation Logic: Bohr Equation // Vd = Vt * (PaCO2 – PeCO2) / PaCO2 var numerator = paco2 – peco2; var ratio = numerator / paco2; // Vd/Vt ratio var deadSpaceVolume = vt * ratio; // Vd in mL // Update DOM document.getElementById('ds_output_volume').innerText = deadSpaceVolume.toFixed(2); document.getElementById('ds_output_ratio').innerText = (ratio * 100).toFixed(2); // Interpretation Logic var interpretation = ""; if (ratio >= 0.20 && ratio <= 0.35) { interpretation = "Normal Range (20% – 35%)"; } else if (ratio < 0.20) { interpretation = "Low Ratio (Possible error or hyperventilation)"; } else { interpretation = "High Ratio (Suggests wasted ventilation/pathology)"; } document.getElementById('ds_output_interpretation').innerText = interpretation; // Show result box resultBox.style.display = "block"; }

About Physiological Dead Space

Physiological dead space represents the volume of air within the respiratory tract that is inhaled but does not take part in gas exchange. This includes air in the conducting airways (anatomic dead space) and air in alveoli that are ventilated but not perfused (alveolar dead space).

Understanding dead space is crucial in respiratory physiology and critical care medicine, particularly when managing patients on mechanical ventilation. An increase in physiological dead space implies that a larger portion of the tidal volume is not contributing to oxygenating the blood or removing carbon dioxide.

The Bohr Equation Formula

This calculator uses the Bohr Equation to estimate physiological dead space. The formula calculates the ratio of dead space volume (V_D) to tidal volume (V_T) based on carbon dioxide pressures.

V_D / V_T = (P_aCO₂ – P_ECO₂) / P_aCO₂

Where:

• V_D: Physiological Dead Space Volume
• V_T: Tidal Volume (the total amount of air moved in one breath)
• P_aCO₂: Partial pressure of Carbon Dioxide in arterial blood
• P_ECO₂: Partial pressure of Carbon Dioxide in mixed expired air

Interpreting the Results

VD/VT Ratio	Clinical Implication
0.20 – 0.35 (20-35%)	Normal physiological range for spontaneous breathing in healthy adults.
> 0.35 (>35%)	Increased dead space. Common in conditions like Pulmonary Embolism, ARDS, Emphysema, or low cardiac output.
Mechanical Ventilation	Patients on ventilators often have higher ratios (0.40 – 0.60) due to apparatus dead space and changes in ventilation perfusion matching.

Why Calculate Dead Space?

Measuring the dead space fraction helps clinicians determine the efficiency of ventilation. If the dead space is high, the patient must breathe a larger tidal volume or breathe faster (higher respiratory rate) to maintain normal blood gas levels. This increases the work of breathing and can lead to respiratory failure.

Anatomic vs. Alveolar Dead Space

• Anatomic Dead Space: The volume of the conducting airways (nose, trachea, bronchi) where no gas exchange occurs. In a healthy adult, this is roughly 2 mL per kg of body weight (approx 150 mL).
• Alveolar Dead Space: The volume of alveoli that are ventilated but not perfused with blood. In healthy lungs, this is negligible. In disease states (like pulmonary embolism), this increases significantly.

The Bohr equation calculates the Physiological Dead Space, which is the sum of both anatomic and alveolar dead space.