Question 8

College Answer

1. Definitions
   • 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

1. 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
2. Physiological mechanisms

Cardiac 
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.

Respiratory

• 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.

Discussion

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.

Thus:

• Decreased respiratory compliance, increased peak airway pressures- due to the pressure of the abdominal contents on the diaphragm
  • Atelectasis
  • 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.