Respiratory Calculations Vocabulary Flashcards

Vocabulary flashcards covering key gas exchange formulas, oxygen system calculations, unit conversions, and mechanical ventilation parameters.

5-Step Calculation Habit

A systematic problem-solving routine: 1. Circle what the question is asking for. 2. Write the matching formula before inserting numbers. 3. Convert units first. 4. Calculate one step at a time and round only at the end. 5. Ask: Does the answer make clinical/math sense?

Alveolar Gas Equation (PAO₂)

The equation used to calculate alveolar oxygen tension: $\text{PAO}_2 = \text{FiO}_2 \times (\text{PB} - \text{PH}_2\text{O}) - (\text{PaCO}_2 \div \text{R})$. Standard values are $\text{PB} = 760\,\text{mmHg}$, $\text{PH}_2\text{O} = 47\,\text{mmHg}$, and $\text{R} = 0.8$.

A–a Gradient

The difference between alveolar and arterial oxygen partial pressures: $\text{A-a gradient} = \text{PAO}_2 - \text{PaO}_2$. A larger gap indicates that more oxygen is being lost between the alveoli and arterial blood.

Arterial Oxygen Content (CaO₂)

The total volume of oxygen carried in arterial blood, combining hemoglobin-bound oxygen and dissolved oxygen: $\text{CaO}_2 = (1.34 \times \text{Hb} \times \text{SaO}_2) + (0.003 \times \text{PaO}_2)$.

Oxygen Tank Duration

The total remaining operational time of an oxygen cylinder in minutes: $\text{Duration (minutes)} = (\text{PSI} \times \text{Tank Factor}) \div \text{Flow (L/min)}$.

Air:O₂ Ratio

The proportion of air entrained per unit of oxygen, calculated as: $\text{Air:O}_2\text{ ratio} = (100 - \text{FiO}_2\%) : (\text{FiO}_2\% - 21)$, using a baseline room-air oxygen concentration of $21\%$.

Male Ideal Body Weight (IBW)

The standard predicted baseline weight formula for adult males: $\text{IBW} = 50\,\text{kg} + 2\,\text{kg}$ for each inch over $5\,\text{ft}$.

Female Ideal Body Weight (IBW)

The standard predicted baseline weight formula for adult females: $\text{IBW} = 45.5\,\text{kg} + 2\,\text{kg}$ for each inch over $5\,\text{ft}$.

Tidal Volume (VT)

The target volume of air delivered per breath during mechanical ventilation, calculated using ideal body weight: $\text{VT} = \text{IBW} \times \text{prescribed mL/kg}$.

Minute Ventilation (VE)

The total volume of gas exhaled from the lungs per minute: $\text{VE} = \text{Respiratory Rate (RR)} \times \text{Tidal Volume (VT)}$. To convert $\text{mL/min}$ to $\text{L/min}$, divide by $1,000$.

Total Breath Time

The duration in seconds of a single complete respiratory cycle: $\text{Total breath time (seconds)} = 60 \div \text{RR}$.

Inspiratory Time (Ti) and Expiratory Time (Te)

The division of total breath time into inspiration and expiration based on the I:E ratio: $\text{Ti} = \text{total breath time} \div \text{total parts} \times \text{inspiratory parts}$, and $\text{Te} = \text{total breath time} \div \text{total parts} \times \text{expiratory parts}$.

Inspiratory Flow

The rate at which gas is delivered to the patient during inspiration: $\text{Inspiratory flow (L/min)} = \text{VT (L)} \div \text{inspiratory time (sec)} \times 60$.

Mechanical Ventilation Chain

The sequential order used to solve full ventilation problems: 1. Find IBW. 2. Use IBW to find VT. 3. Use RR × VT to find VE. 4. Use 60 ÷ RR to find total breath time. 5. Use the I:E ratio to calculate Ti and Te. 6. Use VT ÷ Ti × 60 to find inspiratory flow.