After this lecture/discussion, the learner should be able to:
1. Describe the mechanisms that maintain fluid, electrolyte and acid-base balance.
2. Compare the mechanisms and effects of fluid deficit and excess.
3. Discuss the mechanisms and effects of deficits and excess.
4. Describe the mechanisms that maintain acid-base balance.
5. Differentiate between metabolic and respiratory acidosis and alkalosis.
6. Apply the pathophysiologic principles of acid-base balance to the interpretation of ABG measurements.
7. Analyze the components of ABGs to identify the type of acid-base balance.
8. Describe the causes and effects of each type of acid-base balance.
9. Use ABG findings in formulating the care of the patient with an acid-base imbalance.
10. Describe the management of patients with a fluid, electrolyte, or acid-base imbalance.
HOW IMPORTANT IS WATER?
Between 50% and 60% of the human body by weight is water
Water provides a medium for transporting nutrients to cells and wastes from cells and for transporting substances such as hormones, enzymes, blood platelets, and red and white blood cells
Water facilitates cellular metabolism and proper cellular chemical functioning
Water acts as a solvent for electrolytes and nonelectrolytes
Helps maintain normal body temperature
Facilitates digestion and promotes elimination
Acts as a tissue lubricant
VARIATIONS IN FLUID CONTENT BODY FAT Because fat cells contain little water and lean tissue is rich in water, the more obese the person, the smaller the percentage of total body water compared with body weight. This is also true between sexes because females tend to have proportionally more body fat than males. There is also an increase in fat cells in older people
VARIATIONS IN FLUID CONTENT AGE 77% Infants 60% Adult Male 50% Adult Female 45% Elderly
AVENUES BY WHICH WATER ENTERS AND LEAVES THE BODY
ANTIDIURETIC HORMONE REGULATION MECHANISMS Blood volume or BP Volume receptor Atria and great veins Hypothalamus Posterior pituitary gland Osmoreceptors in hypothalamus Osmolarity ADH Kidney tubules H2O reabsorption vascular volume and osmolarity Narcotics, Stress, Anesthetic agents, Heat, Nicotine, Antineoplastic agents, Surgery
ALDOSTERONE-RENIN-ANGIOTENSIN SYSTEM Juxtaglomerular cells-kidney Serum Sodium Blood volume Angiotensin I Kidney tubules Angiotensin II Adrenal Cortex Sodium resorption (H2O resorbed with sodium); Blood volume Angiotensinogen in plasma RENIN Angiotensin-converting enzyme ALDOSTERONE Intestine, sweat glands, Salivary glands Via vasoconstriction of arterial smooth muscle
The desirable amount of fluid intake and loss in adults ranges from 1500 to 3500 mL each 24 hours. Ave= 2500 mL
Normally INTAKE = OUTPUT
Changes in ECF volume = alterations in sodium balance
Change in sodium/water ratio = either hypoosmolarity or hyperosmolarity
Fluid excess or deficit = loss of fluid balance
As with all clinical problems, the same pathophysiologic change is not of equal significance to all people
For example, consider two persons who have the same viral syndrome with associated nausea and vomiting
ECF Volume Depletion No Significant E+ disturbances Man with renal failure Life threatening E+ disturbances
May occur as a result of:
Reduced fluid intake
Loss of body fluids
Sequestration (compartmentalizing) of body fluids
Pathophysiology and Clinical Manifestations DECREASED FLUID VOLUME Stimulation of thirst center in hypothalamus Person complains of thirst ADH Secretion Water resorption Urine Output Renin-Angiotensin-Aldosterone System Activation Sodium and Water Resorption Urine specific gravity except with osmotic diuresis
Pathophysiology and Clinical Manifestations UNTREATED FLUID VOLUME DEFICIT Depletion of fluids available BODY TEMPERATURE Dry mucous membranes Difficulty with speech Cells become unable to continue providing water to replace ECF losses Signs of circulatory collapse blood pressure heart rate respiratory rate Restlessness and Apprehension
Collaborative Care Management Identification of vulnerable patients and risk factors: * Compromised mental state * Physical limitations * Disease states * Limited access to adequate food and fluids Development of a plan of care Family members should be educated about the importance of fluid and nutrition intake Collaboration with the nurse, patient, family members, and other health care providers for continued assessment and treatment of problems Ongoing assessment and detailed action plan of fluid and serum electrolyte balance. Factors such as medications (particularly diuretics), hyperventilation, fever, burns, diarrhea, and diabetes with appropriate referral
Collaborative Care Key Points
1 Liter of water = 1 kg of water by weight
Fluid replacement are calculated according to this ratio plus 1.5 L to fulfill the current daily needs
For example, JUAN, a one-year-old, lost 1 kg of water from diarrhea as weighed from his diaper over the last 24 hours. Therefore, since 1 kg=1 L, fluid replacement therapy for him will involve 1 L of fluids + 1500 L.
Oral fluid resuscitation is preferable but if the patient is unable to tolerate fluids, IV Therapy may be ordered
Vital signs should be assessed regularly
Postural hypotension is common for postural persons with fluid volume deficit. How do we assess this?
For example, in the care of LOIDA, a 31 year old with severe DHN, you take her blood pressure (130/80) and pulse (75) while shes lying down. Then you ask her to sit at the edge of bed. When you take her blood pressure again, you get 115/80 and when you take her pulse, you get 80. This is consistent with intravascular volume depletion.
Daily weighing is also useful to monitor fluid and electrolyte balance
Laboratory results should be reviewed for various fluid and electrolyte disturbances so that appropriate adjustments to therapy can be initiated
FLUID EXCESS/HYPERVOLEMIA Psychiatric Disorders, SIADH, Certain head injuries Dietary Sodium Indiscretion Renal and endocrine disturbances, malignancies, adenomas Overhydration Excessive Sodium Intake Failure of renal or hormonal regulatory functions FLUID VOLUME EXCESS/HYPERVOLEMIA Sodium Normal Extracellular fluid Normal sodium concentration ADH Renal Tubules ADH ADH ADH ADH ADH Extracellular volume expands Fluid becomes progressively hyponatremic
Since ECF becomes hypoosmolar, fluid moves into the cells to equalize the concentration on both sides of the cell membrane
Thus there, is an increase in intracellular fluid
The brain cells are particularly sensitive to the increase of intracellular water, the most common signs of hypoosmolar overhydration are changes in mental status. Confusion, ataxia, and convulsions may also occur.
Other clinical manifestations include: hyperventilation, sudden weight gain, warm, moist skin, increased ICP: slow bounding pulse with an increase in systolic and decrease in diastolic pressue and peripheral edema, usually not marked
WHAT DO ELECTROLYTES DO? Promote neuromuscular irritability Maintain body fluid volume and osmolarity Distribute body water between fluid compartments Regulate acid-base balance
Controls and regulates volume of body fluids
Its concentration is the major determinant of ECF volume
Is the chief electrolyte of ECF
Influence ICF Volume
Participates in the generation and transmission of nerve impulses
Is an essential electrolyte in the sodium-potassium pump
RDA: not known precisely. 500 mg
Eliminated primarily by the kidneys, smaller in feces and perspiration
Salt intake affects sodium concentrations
Sodium is conserved through reabsorption in the kidneys, a process stimulated by aldosterone