Define the following terms:
- Intra-abdominal Hypertension (IAH)
- Abdominal Compartment Syndrome (ACS)
- Abdominal Perfusion Pressure (APP)
- List the steps required to measure the intra-abdominal pressure (IAP) via a catheter inserted in the bladder.
- List the adverse cardiorespiratory effects of an increase in IAP in a mechanically ventilated patient and outline the physiological mechanisms that account for these effects.
- IAH is defined as a sustained IAP ≥12 mmHg
- ACS is defined as a sustained IAP >20 mmHg (with or without APP <60 mmHg) that is associated with new organ dysfunction OR as IAH-induced new organ dysfunction without a strict IAP threshold.
(iii) IPP = MAP – IAP
- Measuring IAP
- Patient is supine and no active abdominal muscle contractions
- Clamp the urinary catheter, after ensuring it is freely flowing and not obstructed.
- 25 ml of sterile saline is instilled into the bladder via a port in the urinary catheter catheter and the catheter filled with fluid
- A pressure transducer is connected to the urinary catheter, between the clamp and the bladder
- Allow 30-60 seconds after instillation of the saline so as to allow for bladder detrusor muscle relaxation
- Zero transducer at the mid-axillary line and at the level of the iliac crest
- Measure pressure at end-expiration
- Physiological mechanisms
Decreased cardiac output –
- Reduced venous return - due to intrabdominal venous compression and raised intrathoracic pressure.
- Increased systemic afterload – due to increased compression of intrabdominal arterial vessels, and PVR due to raised intrathoracic pressure.
- Decreased Right Ventricular output from raised intrathoracic pressure, raised PVR Increased CVP and LVEDP - Reduced compliance - due to elevation of diaphragm displacing the heart and increased afterload (see above).
Hypotension – decreased cardiac output.
- Deteriorating O2 A-a gradients due to increased – raised diaphragm and atelectasis, increased intrapulmonary shunt / V/Q mismatch
- Hypercapnia – decreased chest wall and lung compliance.
- Increased airway pressure – altered respiratory compliance.
A slightly less formal discussion of abdominal compartment syndrome takes place in Question 21from the second paper of 2006: "Outline the causes, consequences and the management of abdominal compartment syndrome. "Brief notes on the pathophysiology of abdominal compartment pressure and its measurement are available, with references for the time-rich exam candidate.
First, the definitions. These are derived from a 2011 consensus statement, from which much of the below information is derived.
Specifically, the definitions were copied verbatim from Table 1, "Consensus definitions list"; one might notice that they are identical to the college answer.
Intra-abdominal Hypertension (IAH)
A sustained or repeated pathological elevation in IAP ≥ 12 mmHg
Abdominal Compartment Syndrome (ACS)
A sustained IAP> 20 mmHg (with or without an APP < 60 mmHg) that is associated with new organ dysfunction/failure
Abdominal Perfusion Pressure (APP)
APP = MAP − IAP.
Measurement of intra-abdominal pressure
- Empty the bladder
- Clamp IDC
- Attach noncompressible tubing and transducer
- Inject 20ml of fluid into the bladder
- Wait for the detrusor to relax (60 seconds)
- Zero the transducer to atmosphere, at mid-axillary line and at the level of the iliac crest
- Measure the pressure at end-expiration
- The measurement is valid under the following conditions:
- Patient is supine
- Muscle contraction is eliminated (eg. NMJ blockers)
List the adverse cardiorespiratory effects of an increase in IAP in a mechanically ventilated patient and outline the physiological mechanisms that account for these effects.
The college demands we focus on the cardiorespiratory consequences.
- Decreased respiratory compliance, increased peak airway pressures- due to the pressure of the abdominal contents on the diaphragm
- Risk of pneumonia
- Increased intrathoracic pressure due to the compensatory reliance on higher PEEP
- Decreased preload, thus hypotension
- Increased afterload, thus increased LV workload
Oh's Intensive Care manual: Chapter 45 (pp. 520) Abdominal surgical catastrophes by Stephen J Streat
Malbrain, Manu LNG, et al. "Results from the international conference of experts on intra-abdominal hypertension and abdominal compartment syndrome. I. Definitions." Intensive care medicine 32.11 (2006): 1722-1732.
Cheatham, Michael L., et al. "Abdominal perfusion pressure: a superior parameter in the assessment of intra-abdominal hypertension." Journal of Trauma-Injury, Infection, and Critical Care 49.4 (2000): 621-627.
Bailey, Jeffrey, and Marc J. Shapiro. "Abdominal compartment syndrome."Critical Care 4.1 (2000): 23.
Cheatham, Michael Lee. "Abdominal compartment syndrome." Current opinion in critical care 15.2 (2009): 154-162.
Maerz, Linda, and Lewis J. Kaplan. "Abdominal compartment syndrome."Critical care medicine 36.4 (2008): S212-S215.
Saggi, Bob H., et al. "Abdominal compartment syndrome." Journal of Trauma-Injury, Infection, and Critical Care 45.3 (1998): 597-609.