Intraabdominal Pressure During Deep Breathing
During deep breathing in healthy adults, intraabdominal pressure increases during inspiration as the diaphragm contracts and descends, compressing abdominal contents, then decreases during expiration as the diaphragm relaxes and ascends. 1
Physiological Mechanism
The changes in intraabdominal pressure (IAP) during deep breathing follow a predictable biomechanical pattern:
- During inspiration, the diaphragm contracts and moves caudally (downward), directly compressing the abdominal cavity and increasing IAP 1
- During expiration, the diaphragm relaxes and ascends cranially (upward), reducing compression on abdominal contents and decreasing IAP 1
- This cyclical pressure change is fundamental to deep diaphragmatic breathing (DDB), where slow, full contraction of the diaphragm causes belly expansion during inhalation, followed by abdominal muscle contraction and belly reduction during exhalation 1
Measurement Standards and Context
Understanding normal IAP values provides important context for interpreting these respiratory variations:
- Baseline IAP in healthy adults ranges from approximately 5-7 mmHg in the supine position 2
- IAP should be measured at end-expiration in the supine position with abdominal muscle contractions absent, using the transducer zeroed at the midaxillary line 2
- Normal healthy adults demonstrate mean IAP of 16.7 mmHg when sitting and 20 mmHg when standing, indicating that body position significantly affects baseline pressure 3
Magnitude of Pressure Changes
The respiratory-related IAP fluctuations are relatively modest compared to other physiological activities:
- Routine activities like bending at the knees or lifting 10-pound weights generate average pressures around 25.5 mmHg 3
- Forceful respiratory maneuvers produce substantially higher pressures: coughing generates mean IAP of 107.6 mmHg, while a Valsalva maneuver creates similar elevations 4, 3
- During normal deep breathing, the pressure changes are considerably smaller than these forceful maneuvers, representing gentle oscillations around baseline values 1
Clinical Implications for Respiratory Mechanics
The diaphragm-abdomen interaction during breathing has important effects on thoracic mechanics:
- Increased IAP elevates the diaphragm cranially, reducing lung volume and potentially causing atelectasis formation 5
- Approximately 50% of measured bladder pressure (in cm H₂O) is reflected in airway plateau pressure during mechanical ventilation, regardless of PEEP level 6
- The caudal-cranial displacement of abdominal contents and the diaphragm during breathing is the major determinant of changes in lung mechanics and function 5
Important Caveats
Several factors can modify the normal IAP response to breathing:
- Body mass index correlates significantly with IAP: higher BMI patients generate increased IAP during various activities, though sex does not affect pressure generation 3
- Abdominal muscle activity must be absent when measuring IAP to obtain accurate readings, as expiratory muscle contraction can confound measurements 2
- Pathological conditions such as intra-abdominal hypertension (IAH ≥ 12 mmHg sustained) or abdominal compartment syndrome (ACS ≥ 20 mmHg with organ dysfunction) fundamentally alter normal respiratory-IAP relationships 2, 7