What are the age related changes that occur in the nervous system,pulmonary and cardiovascular system of geriatric population and how does the renal changes affect the anesthetic management?
Nervous System
With aging memory decline occurs in greater than 40% of individuals older than age 60 years.
Cerebral atrophy occurs with aging with a decrease in the volume of gray and white matter.The decrease in gray matter volume is due to neuronal shrinkage but there is 15% loss of white matter with aging.Such loss results in gyral atrophy and increased ventricular size. Shrinkage in the subcortical white matter and the hippocampus may be accelerated by hypertension and vascular disease.
Significant regional reductions are seen in the neurotransmitters dopamine, acetylcholine, norepinephrine, and serotonin with aging. Levels of glutamate, the primary neurotransmitter in cortex, do not seem to be affected. Coupling of cerebral electric activity, cerebral metabolic rate, and cerebral blood flow remains intact in elderly individuals.
Decreases in brain reserve are manifested by decreases in functional ADL, increased sensitivity to anesthetic medications, increased risk for perioperative delirium, and increased risk for postoperative cognitive dysfunction.
Neuraxial changes include a reduction of the area of the epidural space, increased permeability of the dura, and decreased volume of cerebrospinal fluid. The diameter and number of myelinated fibers in the dorsal and ventral nerve roots are decreased in elderly individuals. In peripheral nerves, inter–Schwann cell distance is decreased, as is conduction.These changes tend to make elderly individuals more sensitive to neuraxial and peripheral nerve block.
Cardiovascular System
As the heart ages, changes occur in its morphilogy which results functionally, in decreased contractility, increased myocardial stiffness and ventricular filling pressures, and decreased β-adrenergic sensitivity.Vascular stiffness increases with advancing age related to breakdown of elastin and collagen.
Increased vascular stiffness causes increase in cardiac load.Alterations in left ventricular afterload can lead to left ventricular wall thickening, hypertrophy, and impaired diastolic filling.
Decreased ventricular compliance and increased afterload combine to cause a compensatory prolongation of myocardial contraction. This occurs at the expense of a decreased early diastolic filling time. Under these conditions, the contribution of atrial contraction to late ventricular filling becomes more important and explains why cardiac rhythm other than sinus is often poorly tolerated in elderly individuals.
Changes in the autonomic system with aging include a decrease in response to β-receptor stimulation and an increase in sympathetic nervous system activity,the attenuated β-receptor response in elderly individuals during exercise or stress is associated with decreased maximal heart rate and decreased peak ejection fraction. This response causes the increased peripheral flow demand to be met primarily by preload reserve, making the heart more susceptible to cardiac failure.
Some pathological conditions associated with aging heart are Impairment of diastolic relaxation leads to diastolic dysfunction in the aging heart. In its severest form, diastolic dysfunction may manifest as diastolic heart failure. Predisposing disease states for this condition include hypertension with left ventricular hypertrophy, ischemic heart disease, hypertrophic cardiomyopathies, and valvular heart disease.
Diastolic dysfunction or failure is often related to systemic blood pressure and does not imply volume overload. Echocardiography is the diagnostic modality of choice,Classically, echocardiography shows preserved or hyperdynamic left ventricular systolic function and characteristic changes of flow velocity at the mitral valve.
Respiratory System.
Structural changes in the lung with aging include the loss of recoil which combined with altered surfactant production leads to an increase in lung compliance. Increased compliance leads to limited maximal expiratory flow and a decreased ventilatory response to exercise.Loss of elastic elements within the lung is associated with enlargement of the respiratory bronchioles and alveolar ducts, and a tendency for early collapse of the small airways on exhalation causes increased anatomic dead space, decreased diffusing capacity, and increased closing capacity all leading to impaired gas exchange.
Loss of height and calcification of the vertebral column and rib cage lead to a typical barrel chest appearance with diaphragmatic flattening so the chest wall becomes less compliant, and work of breathing is increased.
alterations in lung volumes with aging are residual volume increases by 5% to 10% per decade. Vital capacity decreases. Closing capacity increases with age. Change in the relationship between functional residual capacity and closing capacity cause an increased ventilation-perfusion mismatch and represent the most important mechanism for the increase in alveolar-arterial gradient for oxygen observed in aging In younger individuals, closing capacity is below functional residual capacity. At 44 years of age, closing capacity equals functional residual capacity in the supine position, and at 66 years of age, closing capacity equals functional residual capacity in the upright position.When closing capacity encroaches on tidal breathing, ventilation-perfusion mismatch occurs. When functional residual capacity is below closing capacity, shunt increases, and arterial oxygenation decreases. This results in impairment of preoxygenation. Increased closing capacity in concert with depletion of muscle mass causes a progressive decrease in forced expiratory volume in 1 second by 6% to 8% per decade.
Increases in pulmonary vascular resistance and pulmonary arterial pressure occur with age and may be secondary to decreases in cross-sectional area of the pulmonary capillary bed.Hypoxic pulmonary vasoconstriction is blunted in elderly individuals and may cause difficulty with one-lung ventilation.
Renal changes that affect the anesthetic management
Renal cortical mass also decreases by 20 to 25% with age, but the most prominent effect of aging is the loss of up to half of the glomeruli by age 80.The decrease in the glomerular filtration rate of approximately 1 mL/min/yr after age 40 typically reduces renal excretion of drugs to a level where drug dosage adjustment becomes a progressively important consideration beginning at approximately age 60. Nevertheless, the degree of decline in glomerular filtration rate is highly variable and is likely to be much less than predicted in many individuals, especially those who avoid excessive dietary protein.
The aged kidney does not eliminate excess sodium or retain sodium when necessary as effectively as that of a young adult.Part of the failure to conserve sodium when appropriate may be because of reduced aldosterone secretion. Similarly, the aged kidney does not retain or eliminate free water as rapidly as young kidneys when challenged by water deprivation or free water excess. Lastly, the sensation of thirst declines with age. Fluid and electrolyte homeostasis is more vulnerable in the older patient.Renal blood flow seems to decrease about 10% per decade. There is a progressive decline in creatinine clearance with age, yet with normal aging, serum creatinine remains relatively unchanged. This occurs because muscle mass also decreases with aging. Serum creatinine is a poor predictor of renal function in elderly patients. This concept is important in proper dosage adjustment for medications that are excreted by the kidneys.
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