When I was standing on the scales in my surgery today (I am a doctor), I noticed that the needle moved very slightly, exactly in time with my heartbeat. What could account for this fluctuation?
• Newton’s third law of motion states that for every action there is an equal and opposite reaction. With every heartbeat, the heart’s ventricles forcibly expel about 70 millilitres of blood into the pulmonary artery and the same volume into the aorta. The heart itself experiences a reaction in the opposite direction. Since the heart is well anchored in the chest, the entire body moves slightly. A sensitive scale can easily detect this movement.
Peter Bursztyn
Barrie, Ontario, Canada
Advertisement
• This phenomenon has been used in the past as a non-invasive method of measuring the stroke volume of the heart, the amount of blood ejected each time the ventricles contract. If this is calculated and multiplied by the heart rate, the cardiac output can be estimated. That is the amount of blood that the heart pumps every minute.
Michael Harrison
Wellington, New Zealand
• The effect is a result of the ejection of blood from the heart into the major vessels. This was refined to become the study known as ballistocardiography. As an electronics engineer, I became involved with its development in the early 1960s. The study went into abeyance, but there has been a recent .
Noel Meeke
Marstow, Herefordshire, UK
• Ballistocardiography has never managed to make it out of the lab into clinical practice, but it has been used to estimate the cardiac output of creatures as large as a .
Grant Hutchison
Dundee, UK
• There is another possible, though much less likely, source of the pulsation. I have a relatively mild case of essential tremor – a brain disorder that causes an involuntary tremor – and it is rare for my body to be completely still at any time. Depending how I sit or stand, sometimes this manifests itself as a twitch in time with my heartbeat. When I tried to repeat your correspondent’s results on my bathroom scales, my tremor caused a much larger variation in the reading than he observed.
Graham Legg
Andover, Hampshire, UK
We pay £25 for every answer published in ¿ìè¶ÌÊÓƵ. To answer this question – or ask a new one – email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
¿ìè¶ÌÊÓƵ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.
You can also submit answers by post to: The Last Word, ¿ìè¶ÌÊÓƵ, 25 Bedford Street, London WC2E 9ES.