Respiratory System

The timing for each developmental stage and the degree of lung development at birth vary significantly among species [8]. For dogs and cats, the initial development of the respiratory system happens well before birth and matures in the postnatal period [8–10]. In comparison to adults, the neonatal and pediatric rib cages are more compliant, intercostal muscles are weaker, work and pressures required to maintain tidal breathing are increased, and respiratory chemoreceptors are immature and less sensitive to increased CO₂ and/or decreased O₂ (Table 19.3) [10]. Young animals have higher resting respiratory rates and minute volume, increased tendency to develop atelectasis, higher oxygen demand, and lower functional residual capacity (FRC) [10]. Altogether, these changes are possible causes of respiratory fatigue and hypoxemia during anesthesia. Although there is minimal supportive evidence for the exact timing to reach complete adult alveolar development and lung maturity for dogs and cats, the maximum functional efficiency of the dog lungs occurs around 1 year of age [11]. By comparison, in humans, the same level of lung maturity is achieved at the age of 20 [11].

Because of the abovementioned physiologic alterations, oxygen supplementation prior to anesthesia is recommended for neonatal and pediatric patients. A minimum of 3 min of preinduction oxygenation via face mask, followed by intraoperative and postanesthetic oxygenation (including after extubation), prevents possible hypoxemia [12]. Importantly, a tight‐fitting face mask should be used to efficiently increase the inspiratory fraction of oxygen [13]. For example, in dogs, high flow rates of 100% flow‐by oxygen (without a mask) minimally increased the inspired concentration of oxygen from 21% to 30%, and therefore it is not considered an effective technique [13]. Further, during general anesthesia, intermittent positive pressure ventilation (IPPV) and oxygenation monitoring are recommended via pulse oximetry, capnography, and/or arterial blood gas analysis. Assisted ventilation is often recommended to maintain both normal ventilation (PCO₂ between approximately 35 and 45 mmHg) and normal oxygenation (PaO₂ higher than approximately 60 mmHg).

Cardiovascular System

The neonatal cardiovascular system undergoes dramatic changes immediately after birth to assume its own circulation and maintain homeostasis [14]. The neonatal systemic circulation is often considered a low‐resistance–high‐flow circuit because it is associated with low blood pressure, total blood volume, and peripheral vascular resistance [14–16].

To maintain normal tissue perfusion, the neonatal and pediatric patient must maintain a higher heart rate, cardiac output, plasma volume, and central venous pressure compared to the adult (Table 19.3) [15, 16]. Baroreceptors are not fully mature until approximately 12 weeks of age, leading to decreased vasomotor tone and vasoconstrictive properties; thus, heart rate drives normal cardiac output. To this end, anticholinergics are used frequently during anesthesia to maintain normal to higher heart rates and tissue perfusion. In the newborn, bradycardia is not vagally mediated and is often caused by hypoxemia [15].

Congenital heart defects are common in veterinary medicine. In a veterinary teaching hospital setting, approximately 17% of dogs and 5% of cats examined during a 10‐year period were diagnosed with congenital heart disease. Subaortic stenosis and patent ductus arteriosus in dogs and tricuspid valve dysplasia and ventricular septal defect in cats were among the most common congenital diseases [17]. Specific details of each congenital disease can be found in Chapter 1.

Anesthetic‐induced cardiovascular depression and hypotension are common cardiovascular abnormalities observed during procedures; thus, adequate venous return and fluid balance must be maintained to minimize the risk of hypotension [18]. Due to decreased cardiac reserve, fluid overload can lead to congestive heart failure and pulmonary edema. Therefore, fluid rate should be chosen based on the individual need and hydration and physical status. Cardiovascular monitoring is emphasized to detect detrimental changes to the cardiovascular system of young patients that could lead to renal and hepatic insufficiency in adult life. A possible cardiovascular insult during anesthesia while the patient is young could trigger further damage to high perfusion organs that, over time, could lead to possible renal failure during their adult life [19].

Hepatic System

The cytochrome P‐450 enzyme system is immature after birth but develops during postnatal life [20]

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