PATHOPHYSIOLOGY

PATHOPHYSIOLOGY

Irrespective of aetiology or acuteness of onset, in dynamic (mechanical) obstruction the bowel proximal to the obstruction dilates and the bowel below the obstruction exhibits normal peristalsis and absorption until it becomes empty and collapses. Initially , proximal peristalsis is increased in an attempt to overcome the obstruction. If the obstruction is not relieved, the bowel continues to dilate; ultimately there is a reduction in peristaltic strength, resulting in flaccidity and paralysis. The distension proximal to an obstruction is caused by two factors: /uni25CF Gas : there is a significant overgrowth of both aerobic and anaerobic organisms, resulting in considerable gas produc - tion. Following the reabsorption of oxygen and carbon dioxide, the majority is made up of nitrogen (90%) and hydrogen sulphide. /uni25CF Fluid : this is made up of the various digestive juices (saliva, 500 /uni00A0 mL; bile, 500 /uni00A0 mL; pancreatic secretions, 500 /uni00A0 mL; gas - tric secretions, 1 litre; all per 24 hours). This accumulates in the gut lumen as absorption by the obstructed gut is retarded. Dehydration and electrolyte loss are therefore due to: /uni25CF reduced oral intake; /uni25CF defective intestinal absorption; /uni25CF losses as a result of vomiting; /uni25CF sequestration in the bowel lumen; /uni25CF transudation of fluid into the peritoneal cavity .

The causes of small and large bowel obstruction • The indications for surgery and other treatment options in • bowel obstruction Adhesions 40% Miscellaneous 5% Obstructed Pseudo- hernia obstruction 12% 5% Faecal impaction 8% In /f_l ammatory 15% Carcinoma 15% Figure 78.1 Pie chart showing the common causes of intestinal obstruction and relative frequencies.

It is important to appreciate that the consequences of intestinal obstruction are not immediately life-threatening unless there is superimposed strangulation. When strangulation occurs, the blood supply is compromised and the bowel becomes ischaemic. Summary box 78.2 Causes of strangulation /uni25CF /uni25CF /uni25CF /uni25CF /uni25CF Ischaemia from direct pressure on the bowel wall from a constricting band such as a hernial orifice is easy to understand. Distension of the obstructed segment of bowel results in high pressure within the bowel wall. This can happen when only part of the bowel wall is obstructed, as seen in a Richter’s hernia (see Chapter 64 ). V enous return is compr omised before the arterial supply . The resultant increase in capillary pressure leads to impaired local perfusion and, once the arterial supply is impaired, haemorrhagic infarction occurs. As the viability of the bowel is compromised, translocation and systemic expo sure to anaerobic organisms and endotoxin occurs. The morbidity and mortality associated with strangulation are largely dependent on the duration of the ischaemia and its extent. Elderly patients and those with comorbidities are more vulnerable to its e ff ects. Although in strangulated exter nal hernias the segment involved is often short, any length of ischaemic bowel can cause significant systemic e ff ects second ary to sepsis. Bowel distension and fluid sequestration proximal to the obstruction can result in significant dehydration. When bow el involvement is extensive circulatory failure is common.

Direct pressure on the bowel wall Hernial ori /f_i ces Adhesions/bands Interrupted mesenteric blood /f_l ow Volvulus Intussusception Increased intraluminal pressure Closed-loop obstruction