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EFFECTS OF EVOKED VS. SPONTANEOUS PAIN ON POSTOPERATIVE LUNG FUNCTION

Department of Anesthesiology, Kingston General Hospital, Queen's University, 76 Stuart Street, Kingston, Ontario, K7L 2V7

INTRODUCTION

The pathogenesis of postoperative atelectasis and pneumonia imply a role for movement-evoked pain (e.g. splinting/hypoventilation due to "pain avoidance"). However, interactions between evoked pain and lung function are poorly understood. Thus, we have examined the relationship between evoked versus spontaneous pain and postoperative pulmonary function.

METHODS

In 25 patients following hysterectomy, VAS (0-100 mm) intensity for spontaneous pain (REST), and pain during ambulation (SIT), forced expiration (BLOW), and coughing (COUGH) were measured together with oxygen saturation, oxygen requirements and peak expiratory flow (PEFR) at 8 time points during postoperative days 1 and 2 (20,24,28,32,44,48,52 and 56 hours after completion of surgery).

RESULTS

All pain measures diminished and PEFR reductions improved across the study period (p<0.05). Mean [SE] COUGH (26.1 [1.7]) and SIT (21.5 [1.5]) were more intense (p<0.05) than REST (10.5 [0.8]), and COUGH was more intense (p<0.05) than BLOW (16.8 [1.3]). Significant negative correlations between pain and PEFR were observed for COUGH, SIT, BLOW and REST at 8, 7, 4, and 2 of the 8 studied time points, respectively (p<0.05). As an illustrative example, the figure below describes the correlation between COUGH and PEFR, 20 h after completion of surgery (correlation coefficient=-0.64, p<0.05).

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Differences between COUGH and BLOW suggest that PEFR reductions reflect true changes in ventilatory capacity and are less likely due to poor performance related to pain avoidance. We hypothesize that the consistent negative correlation of cough-evoked pain with PEFR is, in part, due to avoidance of coughing which ultimately limits deep inspiration, lung re-expansion and clearance of secretions [1]. Future study should focus on understanding unique mechanisms of evoked pain and characterizing the physiological implications of evoked pain as they pertain to relevant postoperative respiratory, cardiac and thromboembolic complications.

REFERENCES

1 Clin Chest Med 2001;22:661–77.[Medline]

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