Intravenous instrumentation alters the autonomic state in humans

Intravascular instrumentation may induce syncope or presyncope. It is not known whether asymptomatic subjects also have autonomic reactions, albeit concealed. We addressed this issue by studying 44 healthy young male subjects of various levels of fitness, ranging from inactivity to athletic [mean maximal oxygen uptake was 49.1 (SD 10.7) ml·kg^–1·min^–1, range 28.7–71.9 ml·kg^–1·min^–1]. The autonomic response to venous cannulation was quantified by measuring heart rate before cannulation (HR1), after cannulation (HR₂), and after complete pharmacological autonomic blockade (HR₀ = the intrinsic heart rate). The sympathovagal balance before and after cannulation was computed as HR₁/HR₀ and HR₂/HR₀, respectively. The group means of heart rate and sympathovagal balance decreased significantly (paired Student's t-test P <0.01) from 62.5 to 59.9 beats·min^–, and from 0.71 to 0.68, respectively. The maximal decrease in heart rate was 8.8 beats·min^–1, and in the sympathovagal balance was 0.11. Our study demonstrated that the asymptomatic subjects responded to intravenous instrumentation with a concealed autonomic reaction. Thus, from our findings it would seem that intravenous instrumentation interferes with measurements relating to autonomic nervous system activity.