Maintaining Acid-Base Balance In Preterm Newborns

Maintaining a normal acid-base level in preterm newborns is a complex task due to their unique physiology and metabolism. Preterm infants are at a higher risk of developing acidosis due to immature kidney function and gastrointestinal issues. The interpretation of blood gases in these newborns is crucial for guiding ventilation strategies and ensuring adequate oxygenation. Blood gas measurements provide insights into the acid-base balance, with parameters such as pH, PaCO2, PaO2, and bicarbonate levels being monitored. Accepting slightly lower pH values and aiming for specific carbon dioxide partial pressure ranges are recommended for preterm newborns on invasive ventilation. The choice of blood sample type, such as arterial, venous, or capillary, also impacts the accuracy of the results. Understanding these factors is essential for providing optimal care to preterm infants.

Monitor blood gases to evaluate acid-base balance

Blood gas measurement is the gold standard for evaluating acid-base balance in ventilated preterm newborns. It gives a clear picture of the baby's acid-base balance and oxygenation.

Blood Gas Parameters

• PH: The normal range for serum pH is 7.35–7.45. A decrease in pH from 7.0 to 6.0 represents a tenfold increase in [H+] (hydrogen ions). pH < 7.35 indicates acidosis, while pH > 7.45 indicates alkalosis.
• PaCO2: Partial pressure of arterial carbon dioxide. The normal range is 4.6–6.0 kPa. It measures the amount of CO2 dissolved in the blood. Insufficient ventilation or impaired gas exchange can cause abnormal PaCO2 levels.
• PaO2: Partial pressure of oxygen dissolved within the plasma. The normal range is 7.0–12.0 kPa in term newborns and 6.5–10.5 kPa in preterm newborns. Only arterial blood gives an accurate measure of PaO2.
• Bicarbonate (HCO3): The normal range is 22–26 mEq/L in term newborns and 20–24 mEq/L in preterm newborns. A decrease in bicarbonate concentration can indicate metabolic acidosis.
• Base Excess: The normal range is -2 to +2. It indicates the amount of acid or base needed to return the pH to a normal level of 7.4.
• Lactate: The normal range is < 2.0 mmol/L. Elevated lactate levels may indicate inadequate oxygen levels and can suggest underlying conditions such as sepsis, tissue hypoxia, or shock.

Blood Gas Sampling

Blood gases can be obtained from cord, arterial, venous, or capillary specimens. Arterial blood samples are the most accurate for monitoring ventilated babies and are usually taken from an arterial line. Capillary blood gases are easily affected by cold extremities, poor perfusion, and squeezing, which can lead to inaccurate results.

Interpreting Blood Gas Results

The interpretation of blood gases in preterm newborns can be challenging due to their unique physiology and metabolism. The following steps can be used to evaluate acid-base balance:

• Evaluate the pH to determine if acidemia or alkalemia is present.
• Assess the respiratory parameter (PaCO2) and the metabolic parameter (HCO3) to determine the origin of the acidemia or alkalemia.
• Determine the compensation status.
• Complete the acid-base classification.

It is important to consider the clinical picture, previous blood gas values, and treatment measures to understand the underlying cause of abnormal values. Additionally, other factors such as clinical presentation, haemodynamics, and imaging should also be considered when making ventilation decisions.

Understand the complex interactions between respiratory and kidney function

The acid-base balance in preterm newborns is maintained through complex interactions between the respiratory and kidney function. The lungs and kidneys work together to regulate pH levels by controlling the excretion of carbon dioxide and bicarbonate, respectively.

The lungs play a crucial role in maintaining acid-base balance by regulating carbon dioxide (CO2) levels. CO2 is produced as a byproduct of metabolism and needs to be excreted to prevent a build-up of acid in the body. The lungs achieve this by converting CO2 back into gas during exhalation. Insufficient ventilation or impaired gas exchange can lead to abnormal CO2 levels, resulting in respiratory acidosis or alkalosis.

The kidneys, on the other hand, are responsible for maintaining the body's acid-base balance by regulating bicarbonate levels. Bicarbonate is an important buffer that helps neutralize excess acids in the body. Preterm newborns are at a higher risk of developing metabolic acidosis due to immature kidney function, which can result in a loss of bicarbonate. Additionally, the kidneys can compensate for respiratory acidosis by increasing the reabsorption of bicarbonate.

The interaction between the respiratory and kidney function becomes evident when the body attempts to compensate for disturbances in acid-base balance. For example, in respiratory acidosis, the kidneys respond by increasing the reabsorption of bicarbonate to counteract the increased CO2 levels. This compensation process can take several days if not addressed through ventilation therapy.

Furthermore, the lungs and kidneys work together to maintain the appropriate pH level, which is crucial for cellular function. While buffer mechanisms can help restore pH levels, maintaining the optimal pH is essential to ensure proper cellular function.

In summary, the complex interactions between the respiratory and kidney function involve the regulation of CO2 and bicarbonate levels, compensation for disturbances in acid-base balance, and the maintenance of optimal pH levels to support cellular function. These interactions are particularly important in preterm newborns, who are more susceptible to acid-base imbalances due to their immature organ systems.

Recognise the importance of protein for growth and development

Preterm infants require a lot more protein to achieve normal intrauterine growth rates than they are usually fed in their first few days after birth. It often takes several weeks to start providing enough protein to maintain normal growth rates. Most very preterm infants do not receive the protein necessary to produce the 2-3 kilograms of body mass over a 12-16 week period of NICU care and, as a result, end up growth-restricted by term, with a deficit in lean body mass.

Protein requirements at 24-30 weeks' gestation are as high as 4 g/kg/day, decreasing to 2-3 g/kg/day by term. Individual amino acids are important not just as building blocks for protein synthesis and net protein balance, but also as essential signalling molecules for normal cellular function.

Brain growth and later life cognitive function are directly related to protein intake during the neonatal period in preterm infants. Data show that successful increases in protein balance in preterm infants can be achieved with higher-than-usual rates of amino acid and protein nutrition, noting that positive protein balance requires at least 1.5 g/kg/day, but there is still an increased protein balance up to 4 g/kg/day.

Further research is needed to determine optimal amounts and mixtures of protein and amino acids for both intravenous and enteral feeding to improve growth, development, and functional capacity in preterm infants.

Be aware of the risk of gastrointestinal disturbances

Gastrointestinal disturbances are a significant risk for preterm newborns, as their immature digestive and absorptive processes, as well as their gut motility, can lead to poor extrauterine growth and critical complications.

Preterm infants have an immature gastrointestinal tract, which is a major challenge in neonatal care. Their digestive and absorption capacities are reduced, and they experience prolonged gastric emptying times and limited intestinal motility compared to term infants. This can lead to a nutritional crisis and alter the preterm infant's response to orally administered therapeutic agents.

The disorders of gastrointestinal ontogenesis, along with other factors such as early postnatal stress, microbiota alterations induced by infections, or early antimicrobial use in the Neonatal Intensive Care Unit (NICU), result in an impaired activation of intestinal peristalsis and of the gut-brain axis. Disturbances of physiological inflammatory responses further contribute to this impairment.

Newborns need the structural and functional maturation of the gastrointestinal tract for digestion and absorption of nutrients from colostrum and breast milk. They also need a complete development of intestinal motor function, which includes suck-swallow coordination, continence of the gastroesophageal sphincter tone, adequate gastric emptying, and intestinal peristalsis.

Preterm infants are at a higher risk of developing gastrointestinal disturbances such as gastroesophageal reflux, gastric residuals, and constipation due to delayed gastric emptying, prolonged bowel transit, abdominal distension, and delayed passage of meconium.

• Provide nutritional support that meets the high and challenging nutritional needs of preterm infants.
• Monitor and manage feeding intolerance, which is a major problem in the NICU and can lead to food intolerance.
• Optimize the nutritional management of preterm infants by understanding the main developmental stages of digestion and absorption processes, as well as the maturational phases of gastrointestinal motility.
• Promote the development of a healthy gut microbiome, as preterm infants are more susceptible to an abnormal microbiome due to their immature gastrointestinal tract and naïve immune systems.
• Minimize the use of antibiotics, as they can alter the normal development of the gut microbiome and increase the risk of antibiotic resistance.
• Provide supportive care and closely monitor preterm infants for signs of gastrointestinal disturbances, such as feeding intolerance, abdominal distension, and delayed passage of meconium.

Understand the role of the kidneys in restoring acid-base balance

The kidneys play a crucial role in restoring acid-base balance and maintaining homeostasis. They do this primarily by reabsorbing bicarbonate from urine back into the blood and excreting hydrogen ions into the urine.

The kidneys filter blood continuously by distributing it to millions of tiny functional units called nephrons. Each nephron is made up of a glomerulus, or a cluster of capillaries, where blood filtration begins. As blood passes through the glomerulus, about one-fifth of the plasma moves into the renal tubule.

The renal tubular system is responsible for reabsorbing water and electrolytes, while waste products and acid are left behind. The renal tubule is structured into several segments: the proximal convoluted tubule, the U-shaped loop of Henle, and the distal convoluted tubule, which eventually empties into the collecting duct.

Bicarbonate reabsorption occurs as the filtrate leaves the glomerulus and enters the proximal convoluted tubule. Here, bicarbonate binds to hydrogen ions secreted by the tubule cells, forming carbonic acid. An enzyme called carbonic anhydrase then splits the carbonic acid into water and carbon dioxide. This reaction allows bicarbonate to be reabsorbed into the blood, helping to maintain the acid-base balance.

In addition to reabsorbing bicarbonate, the kidneys also generate new bicarbonate through ammoniagenesis and the excretion of titratable acids. This process occurs primarily in the proximal tubule, where glutamine is metabolized to produce alpha-ketoglutarate, which is involved in gluconeogenesis. This process ultimately results in the formation of two molecules of bicarbonate and ammonia per glutamine molecule.

The kidneys' ability to regulate acid-base balance is essential for maintaining normal human physiology. By excreting acids and bases and controlling bicarbonate concentrations, the kidneys work alongside the lungs to maintain the body's pH within a narrow range that is compatible with life.

Frequently asked questions