What is the Pressure Time Product (PTP) in a ventilator?

Pressure Time Product (PTP) in Mechanical Ventilation

Pressure Time Product (PTP) is the integration of respiratory pressure over time (∫P·dt), representing the energy expenditure and metabolic work of the respiratory muscles during breathing, typically expressed in units of cm H₂O·minutes over a 1-minute interval. 1

Definition and Calculation

PTP quantifies respiratory muscle activity by measuring the area under the pressure-time curve, providing a more comprehensive assessment of respiratory effort than simple pressure measurements alone. 1

• The calculation involves integrating respiratory pressure over time, similar to how flow is integrated to obtain minute ventilation 1
• PTP can be expressed as mean pressure (P̄) over an entire breath cycle using the formula: Mean pressure = PTP / sampling period 1
• For a single breath, the sampling period equals the total breath period (Ttot) 1

Measurement Sites and Clinical Applications

PTP can be measured at multiple anatomical locations depending on which respiratory muscle groups you want to assess: 1

• Mouth/airway opening (Pmo): Estimates average pressure generated by all respiratory muscles working against external loads 1
• Transpulmonary pressure (PL): Measures chest wall and muscle activity against the lung and airways 1
• Transdiaphragmatic pressure (Pdi): Isolates diaphragm activity alone 1
• Total respiratory muscle pressure (Pmus): Assesses synergic respiratory muscles against lung and rib cage 1

Advantages in Ventilated Patients

PTP is superior to work of breathing (WOB) measurements in mechanically ventilated patients because it captures isometric muscle contractions and ineffective triggering efforts that WOB misses. 1

• PTP quantifies "wasted" inspiratory efforts that fail to trigger the ventilator, which are invisible to WOB measurements 1
• Under conditions of relatively constant ventilation, PTP correlates with respiratory muscle endurance, blood flow, and oxygen consumption 1
• PTP remains predictive of muscle activity even when ventilation levels vary, unlike WOB which becomes less reliable at high ventilation rates 1

Measurement Technique in Ventilated Patients

Integration can be performed by medical amplifiers, digital computers, or signal averaging circuits with time constants of approximately 20 seconds. 1

• Critical requirement: Exclude expiratory pressures during the expiratory phase and inspiratory pressures from chest wall elastic recoil from the inspiratory PTP analysis 1
• Signal averaging circuits designed for mean vascular pressure measurement can measure mean pressure directly online 1
• The measurement requires knowledge of chest wall compliance, which can be assumed at approximately 4% of predicted vital capacity per cm H₂O 2

Pressure-Time Index (PTI)

When PTP is normalized to maximum inspiratory pressure capacity, it becomes the Pressure-Time Index (PTI), which indicates the fraction of respiratory muscle reserve being utilized. 1

• Formula: PTI = P̄ / PI,max (where PI,max is maximum inspiratory pressure) 1
• Critical threshold values: Most normal subjects can sustain PTIdi up to approximately 0.18 for the diaphragm and PTI up to 0.3 for chest wall muscles 1
• Values exceeding these thresholds predict impending respiratory muscle fatigue 1

Clinical Significance in Mechanical Ventilation

PTP values guide ventilator weaning decisions and optimization of support levels: 2

• Higher PTP values indicate increased patient effort and potentially inadequate ventilatory support 2
• CPAP-5 decreases PTP by 40% compared to T-piece during weaning 3
• Pressure support ventilation (PS-5) reduces PTP by 34% compared to T-piece 3
• The contribution of reducing intrinsic PEEP (PEEPi) to PTP reduction amounts to approximately 36% 3

Comparison Between Ventilatory Modes

Biphasic CPAP produces significantly higher PTP values than pressure support ventilation at comparable support levels, making it more exhausting for patients. 4

• This discrepancy occurs because nearly half the breaths during biphasic CPAP are not assisted by the ventilator 4
• Demand flow systems during high-frequency oscillatory ventilation can reduce total PTP from 66 to 51 cm H₂O·sec·min 5
• Pressure-limited variable-flow breaths are more responsive to vigorous patient effort than fixed-flow approaches in reducing PTP 6

Common Pitfalls

Expiratory muscle activity increases PTP variability and generates unreliable values—ensure measurements exclude active expiration. 1

• Failure to account for intrinsic PEEP underestimates true respiratory effort 2
• Patient-ventilator asynchrony during measurement produces inaccurate readings 2
• At high ventilation levels with varying inspiratory flows, PTP becomes less predictive and mechanical work rate becomes more relevant 1