Chapter 16

Principles of elimination management

Defaecation

Learning outcomes

Having read this chapter, the reader should be able to:

• describe the physiology of defaecation

• discuss the factors that influence defaecation

• discuss the ways in which the midwife can promote defaecation.

During pregnancy and the postnatal period, women can experience a number of changes with their bowel habits that might lead them to feel constipated or have loose stools, both of which can be uncomfortable and embarrassing. The midwife plays an important role in promoting normal bowel habits and preventing complications. This chapter focuses on defaecation for the adult: the passage of faeces through the anal sphincter. Relevant physiology, factors influencing defaecation and a discussion on how the midwife promotes defaecation are all included.

Physiology

Faeces contain 70% water and are normally semi-solid in consistency. The remaining constituents are the end-products of digestion – residue of unabsorbed food, bile pigments, epithelial cells, mucous, bacteria, cellulose, and some inorganic material. Peristalsis propels the faeces through the large intestine to the sigmoid colon while up to 2 L of water is absorbed from them (Brown et al 2011). The longer the faeces remain in the large intestine, the greater the amount of water absorbed, the harder the faeces become, and vice versa. Faeces usually stay in the sigmoid colon until the stimulus for defaecation occurs.

The presence of faeces in the sigmoid colon initiates the stimulus to defaecate as they pass into the rectum. This stimulus can vary from person to person, e.g. two to three bowel movements per day to three or four bowel movements per week (Tortora & Derrickson 2012). The rectum is very sensitive to changes in pressure and, as the faeces enter the rectum, the longitudinal rectal muscles contract which shortens the rectum and the pressure rises 2–3 mmHg (Brown et al 2011, Tortora & Derrickson 2012). As the rectal walls distend, the internal anal sphincter relaxes, reducing anal pressure and creating an awareness of the need to defaecate. If it is appropriate to defaecate, the diaphragm, abdominal and levator ani muscles contract and the glottis closes. Breath holding can assist by increasing intra-abdominal pressure up to four to five times the normal pressure (Dempsey et al 2014). This results in a rise in pressure within the rectum and a decrease in pressure exerted by the internal and external sphincters. The pelvic floor lowers as the puborectalis muscle relaxes, increasing the anorectal angle, facilitating the passage of faeces into the anal canal by peristalsis. The posture adopted can assist this process by increasing the action of the abdominal muscles and pushing the walls of the sigmoid colon and rectum inwards. If defaecation is assisted by the person ‘bearing down’ (Valsalva manoeuvre) the increased intra-abdominal and intrathoracic pressure that results causes a reduction in blood flow to the heart, temporarily reducing cardiac output. When bearing down ends, the pressure reduces and a larger than normal amount of blood returns to the heart which may cause a dangerous rise in blood pressure, particularly in a hypertensive individual (Dempsey et al 2014).

This reflex stimulus can be ignored and inhibited by adults as the external anal sphincter is under voluntary control. When ignored, the external anal sphincter contracts, increasing the anal pressure. As a result, rectal pressure decreases and the puborectalis muscle contracts. The anorectal angle is reduced as the rectum is pulled backwards and the faeces return from the anal canal to the rectum and sigmoid colon (retroperistalsis). The stimulus then disappears until the next wave of mass peristalsis moves the faeces back into the rectum (Tortora & Derrickson 2012). If the stimulus continues to be inhibited, suppression of the reflex occurs and constipation ensues.

If it is inconvenient to defaecate, impulses pass to the cerebral cortex to inhibit defaecation. In babies and young children this ability is absent, and defaecation becomes a reflex response to faeces in the rectum.

Once passed, faeces are often referred to as ‘stools’ and can vary in appearance between individuals and even within the individual. For further description of stools, refer to the Bristol stool classification (see Fig. 17.2).

Constipation

Tortora & Derrickson (2012) define constipation as infrequent or difficult defaecation caused by decreased intestinal motility and affects up to 27% of the general population (Brown et al 2011). Thibodeau & Patton (2012) advise that if the faeces remain in the large intestine for longer than 5 days more water is absorbed making the faeces hard, dry, and more difficult to pass. Prolonged constipation can lead to faecal impaction where the faeces become a large, hardened mass which are very painful to expel (Brown 2013) and faecal incontinence, which Hurnauth (2011) describes as the ‘involuntary release of bowel products or gas through the anus’. NICE (2007) suggest that 10% of the general population are affected by faecal incontinence, a condition that is poorly understood. Constipation may also indirectly predispose to haemorrhoids developing. Flatus (gas) can accumulate with reduced or absent peristalsis and can lead to abdominal distension and pain (deWit & O’Neill 2014).

Blackburn (2013) states that 10–30% of women will experience constipation in pregnancy which is worse in the first and second trimesters, although Murray & Hassell (2014) and Jefferson & Croton (2013) suggest the figure is higher at 40%. Progesterone relaxes the smooth muscle of the intestines which decreases intestinal motility and results in a prolonged transit time allowing more water and electrolytes to be absorbed. This may be compounded by progesterone inhibiting motilin release. The enlarging uterus compressing the rectosigmoid area may also contribute to constipation (Blackburn 2013).

Constipation in pregnancy causes discomfort and may result in damage to the pudendal nerve and impair the supportive function of the pelvic floor musculature (Jefferson & Croton 2013). Haemorrhoids occur in 85% of women in late pregnancy as a result of a rise in intra-abdominal pressure and the effect of the enlarging uterus impeding venous return from the lower limbs resulting in stagnation of blood and arteriovenous shunting within the compressed rectal veins (Murray & Hassell 2014). Postnatally 15–20% women experience constipation (Jackson 2011) which can be associated with haemorrhoids, perineal pain, and anal fissures.

While constipation is not life threatening, it causes pain, discomfort and distress. Other associated symptoms the woman may experience include fatigue, malaise, cramps, nausea, vomiting, confusion, restlessness, headaches and halitosis (Brown et al 2011). Constipation requires forceful abdominal compression, which if repeated can cause perineal stress (Amselem et al 2010). Straining, particularly using the Valsalva manoeuvre of forced exhalation against a closed glottis, should be avoided as it can result in an increase in both intrathoracic and central venous pressure. It may also result in a temporary reduction in vision with subconjunctival haemorrhage which may occur as a result of transient sub-clinical retinal oedema (Connor 2010). Thus it is important to treat constipation and encourage comfortable defaecation.

Factors inhibiting defaecation, predisposing to constipation

• Poor habits and constantly delaying defaecation.

• Diet, e.g. inadequate bulk, high intake of processed cheese, lean meat, eggs, and pasta (Dempsey et al 2014).

• Dehydration.

• Lack of exercise or immobility.

• Pelvic floor damage – Amselem et al (2010) found a higher incidence of pelvic floor damage amongst patients with constipation compared to those without.