Circulation: Heart rate, central capillary refill and peripheral perfusion
The normality of these factors is quite rightly reassuring. If outside of a reference range, these features may or may not be significant. Each of these factors can be affected by pain, fear, pyrexia and environment. Again, the extremeness of the abnormality is a consideration as is the persistence of deranged markers of circulation.
Respiration: Respiratory rate and work of breathing
Abnormal respiration is more discriminatory for SBI and sepsis, assuming that there is no other reason for being unwell and breathing abnormally (e.g. viral wheeze). The reason for this is that respiration is less prone to the physiological changes that affect circulation. Abnormal breathing may be caused by acidosis or hypoxia but is less likely to be due to a simple illness. This ties in nicely with the definition of sepsis that relates to organ dysfunction. While circulation changes may be a reaction to an uncomplicated viral illness, respiratory changes are more likely to be due to organ dysfunction.