The part of the nervous system that regulates the heart rate and other autonomic functions develops in late pregnancy. Now new research shows that premature birth may weaken this regulation. This may also explain why individuals born preterm may have an increased risk of cardiovascular disease later in life.
Globally, 15 million children are born prematurely each year. In addition, increasing evidence indicates that premature birth is associated with cardiovascular problems. Among other things, adults who were born preterm have higher blood pressure and poorer control of blood glucose.
The automatic functioning of the heart, which is related to the development of cardiovascular disease, is inextricably linked to the autonomic nervous system, which controls all the things we do not have to consciously think about and control. These include breathing, dilating the pupils and the functions of the organs, including regular heartbeat.
Now a new study shows that problems with the heart’s autonomic function originating from the vagus nerve and the function of the sinus node are not solely found among people born early preterm (before 34 weeks). The same applies to individuals who were born late preterm (34–36 weeks).
The discovery indicates why people born preterm might have cardiovascular problems more often in adulthood.
“Our findings suggest that being born prematurely is associated with reduced functioning of the vagus nerve in adulthood. Thus, impaired autonomic functioning of the heart may be a mechanism leading to the increased risk of cardiovascular disease among adults who were born preterm,” explains the researcher behind the new study, Risto Karvonen from the National Institute for Health and Welfare and the University of Oulu in Finland.
The new study was recently published in The Journal of Pediatrics.
Autonomic nervous system controls heart rate
The autonomic nervous system regulates people’s heart rhythm through the sympathetic and parasympathetic nervous systems.
Fibres in the sympathetic nervous system release noradrenaline into the sinus node, causing the heart to beat faster. Conversely, the vagus nerve in the parasympathetic nervous system releases acetylcholine into the heart, causing it to beat more slowly.
The interaction between these two autonomic nervous systems determines whether the heart beats appropriately.
The functioning of the heart can be altered in an undesirable direction by an impaired vagus nerve or by changes in the activity of the sympathetic nervous system.
Premature birth impairs the functioning of the vagus nerve
Risto Karvonen and colleagues examined how three groups of adults differed in the functioning of the heart: those born full term, late preterm and early preterm.
The participants were asked to sit still in a chair for 5 minutes, during which the researchers measured their heart function and looked for abnormalities in heart rhythm.
When the participants sat quietly, the vagus nerve played the greatest role in determining heart rhythm.
The study showed that both late and early preterm birth were associated with reduced functioning of the vagus nerve in adulthood.
Moreover, the research results showed that preterm-born men experience greater effects than preterm-born women.
“This result was not statistically significant, but it is interesting because it contrasts with previous studies showing that being born prematurely is most detrimental to women’s hearts,” explains Risto Karvonen.
Autonomic functioning of the heart develops in the third trimester
Risto Karvonen believes that development in the third trimester of pregnancy explains the fact that being born prematurely seems to affect the functioning of the heart for the rest of people’s lives, including the risk of developing cardiovascular disease.
The third trimester plays an important role in developing the autonomic nerve system. One thing that happens is that the vagus nerve receives most of its insulation, comprising a protective layer of myelin, in the third trimester. Further, the baroreflex function that regulates how the heart responds to changes in blood pressure develops in the third trimester.
“We believe that fetal development is vulnerable in the third trimester of pregnancy, and interrupting pregnancy may lead to long-term impairment of the heart’s autonomic functioning. Our findings indicate a possible explanation for the association between preterm birth and the risk of cardiovascular disease later in life,” says Risto Karvonen.
Exercise important for those born preterm
From a clinical perspective, the research findings emphasize that exercising is important for health, and especially for adults who were born preterm.
Demographically, this can also be very important in the future.
“Since preterm birth may increase the risk of cardiovascular disease, we should keep this in mind as people who were born preterm get older and increasingly experience heart problems,” says Risto Karvonen.
Risto Karvonen continues to study heart function among adults who were born preterm. One current project examines the heart rate recovery after exercise among those born preterm.
He is also working on a research project examining how stress affects heart functioning among people who were born preterm.
“Cardiac autonomic function in adults born preterm” has been published in The Journal of Pediatrics. In 2019, the Novo Nordisk Foundation awarded a grant to Eero Olavi Kajantie for the project Small Preterm Infants and Their Siblings – a Natural Experiment to Understand the Life-long Legacy of Early Life Environment.
“Cardiac autonomic function in adults born preterm” has been published in The Journal of Pediatrics. In 2019, the Novo Nordisk Foundation awarded a grant to Eero Olavi Kajantie for the project Small Preterm Infants and Their Siblings – a Natural Experiment to Understand the Life-long Legacy of Early Life Environment.
