Effect of atropine on the biomechanical properties of the oesophageal wall in humans

Abstract

Recently, we reported a novel ultrasound technique to assess biomechanical properties of the oesophagus in human subjects. In the present study, we use the technique, in combination with atropine, to determine the active and passive biomechanical properties of the oesophagus in normal healthy humans. A manometric catheter equipped with a high-compliance bag and a high-frequency intraluminal ultrasonography probe was used to record pressure and oesophageal geometry. Oesophageal distensions with either isovolumic (5-20 ml water) or with isobaric (10-60 mmHg) technique were performed. Intra-bag pressure and ultrasound images of the oesophagus were recorded simultaneously. Following injection of atropine (15 microg kg-1, I.V.), the oesophageal distensions were repeated. The oesophageal wall compliance, circumferential wall tension, stress, strain and elastic modulus were calculated. Atropine resulted in an increase in the oesophageal wall compliance during isobaric distension, but no change in compliance was observed during isovolumic distension. The stress-strain relationship was found to be linear during both types of distension, before as well as after atropine. The Young's modulus, which is the slope of a linear stress-strain relationship, was significantly higher after atropine in the isovolumic study but not in the isobaric study. The stress-strain relationship of the active component (muscle contraction) was different during isovolumic and isobaric distensions but the passive components were similar. The passive and active stress-strain relationships of the human oesophagus resemble those of other soft biological tissues. Furthermore, the method of oesophageal distension has significant influence on the active but not the passive biomechanical properties due to a strain-rate effect.

Figure 1. Isovolumic distensions

A, in the isovolumic study, intra-bag pressure (P) increased linearly with bag volume (V) before (▪) and after (□) atropine. B, in the isovolumic study, luminal CSA increased linearly with bag volume (V) before (▪) and after (□) atropine. Luminal CSA after atropine was only significantly lower at the higher distension volumes (17.5 ml and 20 ml), as denoted by the asterisk. C, the relationship between intra-bag pressure and luminal CSA in the isovolumic study is also linear before (▪) and after (□) atropine. Compliance tended to be lower than before atropine (P= 0.076).

Figure 2. Isobaric distension

The relationship between intra-bag pressure (P) and luminal cross sectional area in the isobaric study is linear before (▪) and after (□) atropine. Compliance after atropine was significantly higher than before atropine (P= 0.0099). The distensibility did not show a statistically significant difference before and after atropine.

Figure 3. Relationship between stress and strain during isovolumic and isobaric distensions

A, the relationship between Green strain (ɛ) and Kirchhoff stress (σ) before (▪) and after (□) atropine in the isovolumic study. The data are fitted by a least squares fit: σ= 4.9ɛ+ 6.4 (R²= 0.94) and σ= 11.1ɛ+ 0.9 (R²= 0.93), before and after atropine, respectively. Young's elastic modulus, given by the slope, is significantly higher after atropine (P= 0.037). B, relationship between Green strain (ɛ) and Kirchhoff stress (σ) before (▪) and after (□) atropine in the isobaric study. The data are fitted by a least squares fit: σ= 13.6ɛ+ 1.0 (R²= 0.95) and σ= 14.2ɛ+ 2.6 (R²= 0.98), before and after atropine, respectively. There is no statistical difference between Young's elastic modulus before and after atropine.

Figure 4. Relationship between tension and stretch ratio during isovolumic and isobaric distensions

A, relationship between stretch ratio (λ) and tension (T) before (▪) and after (□) atropine in isovolumic study. The data are fitted to a 3rd polynomial equation before and after atropine; T_b= 60.4λ³− 254.3λ²+ 458.3λ− 264 (R²= 0.99) and T_a= 49.7λ³− 129.4λ²+ 155.8λ− 76.0 (R²= 0.99), respectively. B, relationship between stretch ratio (λ) and tension (T) before (▪) and after (□) atropine in the isobaric study. The data are fitted to a 3rd polynomial equation before and after atropine: T_b= 4.6λ³+ 104.6λ²− 224.9λ+ 115.4 (R²= 0.99) and T_a= 62.1λ³− 122.8λ²+ 102.2λ− 41.4 (R²= 0.99), respectively.

Figure 5. Relationship between stress and strain for the contractile elements of the oesophageal wall during isovolumic and isobaric distensions

The active components of stress–strain relationship in the isovolumic (continuous line) and in the isobaric (dotted line) study.

Figure 6. Relationship between stress and strain for the passive (visco-elastic) elements of the oesophageal wall during isovolumic and isobaric distensions

The passive components of stress–strain relationship in the isovolumic (continuous line) and in the isobaric (dotted line) study.