The adult cardiovascular physiology is well known to clinical science, the vascular and cardiac response to different type of drugs is well defined. Outcomes in the short and long term have been extensively researched as well as the response of the body in the presence of a number of comorbidities, many factors have been accounted for and studied given that cardiovascular diseases are the number one killer in the developed world.
The story is different regarding neonates, for these patients have an ongoing maturation of the cardiovascular physiology from day to day and hour to hour, and their response to different types of drugs entirely depend on the receptor expression, organelle development and autoregulation capabilities which can be very different from patient to patient. The etiology of short-term and long-term complications secondary to treating hypotension either aggressively or conservatively are not fully understood, differentiation among patients that would benefit from conservative treatment versus those that would need the use of inotropes together with vasopressors and other drugs is a question that remains unanswered. The lack of information collected is proof that we need more studies to determine the appropriate therapeutic line to follow with the aim to recover from the hypotensive state without developing comorbidities. In order to standardize a treatment, to create a protocol, to make the appropriate treatment choice we are in need to specify what exactly classifies as hypotension, now the “standard” value for a term babies during the first 12h of life are: systolic blood pressure 60-85mmHg and diastolic blood pressure 45-55mmHg with a mean blood pressure 50-65.
To simplify this, this assay will discuss three, very different points of view regarding the approach to hypotension: a population-based, pathophysiology-based and outcome-based. Physiology of blood pressure regulation in the neonate Blood pressure is the pressure that the blood exerts against the vessel walls, this can be further broken up into systolic, diastolic and mean blood pressure. To regulate this pressure, the human body can regulate different factors and modify them to obtain better perfusion. The main modifiable factors are cardiac output and vascular resistance.
Adequate blood pressure will usually be related to adequate perfusion of the vital and non-vital organs, ensuring an acceptable concentration of oxygen delivered to them. However, demands are different in the developing organism where there is a 40% increase in demand for fat and proteins. When blood flow decreases, different mechanisms are devoted to avoiding ischemia and tissue damage. The neonate can obtain more O2 by increasing its extraction, and the venous O2 saturation lowers from 65-70% to 40-50% which implies an end-capillary pressure of 3-4kPa, this tension is the minimum needed in order to drive the diffusion of O2 from the capillary to the mitochondria
Smooth muscle cells within the arterial wall react to changes in vascular tone. This type of regulation takes place in the precapillary muscular vessels known as arterioles. When there is an increase of pressure the cells will contract to compensate the passive stretching, opposite to this when there is a decrease in pressure the vessels will increase in calibre. The vessel changes as a result of pressure are called autoregulation. Autoregulation aims to maintain an adequate and mostly stable blood flow within a range of different blood pressures. From this point on, the vessels will successfully maintain blood flow as the pressure is still increasing. When a critical point is reached (150 mmHg in the above image) autoregulation is no longer able to maintain the adequate value and will be overwhelmed leading to a dangerous increase in pressure. It should be noted that the inverse process it is equally dangerous, lower pressures lead to lower CBF and hypoxia.
It is of importance that in the newborn the size of great arteries is small enough to have an impact on vascular resistance. It has been proven that the great vessels are more reactive to hypoxia . This changes can be understood by looking at Poiseuille’s law where it is clear that blood flow is greatly influenced by the radius for a change in the caliber of the vessel will lead to a change to the power of four in the blood flow. Population-based definition of hypotension in neonates The exact definition of hypotension is, as of today, a mystery in the field of neonatology. It seems clear that maturation plays a critical role in order to choose an adequate value for blood pressure at a given time, several individual factors play a critical role that cannot be established as a constant in time for everybody if we look at the individual. Statistical analysis, use of percentiles and distribution of the global population has been a gold standard to orientate normal ranges such as body weight and height for the normal development of a healthy child.
This classification style can offer a better overview of the values in which most newborns have a CBF that could be considered within reasonable limits, always assuming that achieving the right CBF is the target of the blood pressure modification. Just like in adult medicine, blood pressure limits do not always clinically correlate with symptomatology, a patient may come to the consult with a blood pressure of 100/70 mmHg without any difficulty, and upon further examination over a period of time establish that those numbers might, in fact, be the patients normal blood pressure parameters and sporadic headaches might develop at “normal ranges”, unlike a hypertensive patient who could feel signs and symptoms of hypotension such as dizziness, shortness of breath and tachycardia when they are brought to the so-called “normal parameters”.
Establishing normal parameters would have to start with a significant amount of data recollection, some of this data would not be complicated to obtain, for measuring blood pressure is relatively simple, however determining the CBF could prove challenging, even with the use of a surrogate such as TDC (Transcranial Doppler) or MTT (mean transit time) Once parameters are stablished for a term newborn it comes into question the variables that can affect the blood pressure control, the main one being maturation at birth and others are sex and weight. The value should, undoubtedly, change over time which rises then another question, do physicians monitor by the hour, day, week? Whatever the result, population-based definition is a good method for the creation guidelines such as PALS guidelines 2015 in which the definition of hypotension in the neonate is SBP<60mmHg
This method, however, it is not reliable in a Neonatal Intensive Care Unit setting for treatment must be tailored to the needs of a very specific patient whose many characteristics may not fit the general rule, hence these values should not be taken into consideration with caution. Moreover; careful, personal and close management is the appropriate course of action. For very specific cases, other approaches relying on different characteristics, might be more suitable. Pathophysiology-based definition of hypotension in neonates In neonates, the main physiological changes that affect blood pressure regulation are cardiac output, neural control, humoral control, autoregulation and chemical factors.
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