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Course: Medical Surgical Nursing

Topic: Acid-Base Imbalance

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Module Goals

Learners will be able to:

Describe the significance of acid-base balance for normal function.
State the steps for arterial blood gas interpretation, including the meaning of compensation.
Compare and contrast the major acid-base disorders.
Describe the role of the respiratory and renal systems in acid-base balance.
Discuss the nursing considerations related to patients with acid-base disorders.

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Acid-Base Imbalance

Proper acid-base balance is critical to proper body function.

The lungs and kidneys work together to adjust slight imbalances.

The lungs compensate for the kidneys, and the kidneys compensate for the lungs.

Ernstmeyer & Christman, 2021

Contact info: info@nursesinternational.org

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Acid (H+)

Volatile: H2CO3

Nonvolatile:

Energy metabolism (lactic acid)
Digestion (hydrochloric acid)

Acid: A compound that can dissociate and release H+ ions

Base: A compound that can accept H+ ions

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pH

Normal value: 7.35 to 7.45

Very narrow range compatible with life

< 6.8 and > 7.8 → incompatible with life

Regulated by:

Chemical buffer systems
Lungs: Eliminate CO2
Kidneys: Eliminate H+ and reabsorb/ generate HCO3

Ernstmeyer & Christman, 2021

Contact info: info@nursesinternational.org

pH is a scale from 0-14 used to determine the acidity or alkalinity of a substance. A neutral pH is 7, which is the same pH as water. Normally, the blood has a pH between 7.35 and 7.45. A blood pH of less than 7.35 is considered acidic, and a blood pH of more than 7.45 is considered alkaline.

The pH of blood is a measure of hydrogen ion concentration. A low pH, less than 7.35, occurs in acidosis when the blood has a high hydrogen ion concentration. A high pH, greater than 7.45, occurs in alkalosis when the blood has a low hydrogen ion concentration. Hydrogen ions are by-products of the metabolism of substances such as proteins, fats, and carbohydrates. These by-products create extra hydrogen ions (H+) in the blood that need to be balanced and kept within normal range (Ernstmeyer & Christman, 2021).

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Acid-Base Balance

H+ and pH have an inverse relationship

Higher H+ = lower pH
< 7.35 is acidic
> 7.45 is alkalotic

Ernstmeyer & Christman, 2021

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Respiratory Regulation

Biga et al., 2019

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The respiratory system contributes to the body's balance of acids and bases by regulating the blood levels of carbonic acid. CO2 in the blood readily reacts with water to form carbonic acid, and the levels of CO2 and carbonic acid in the blood are in equilibrium. When the CO2 level in the blood rises (as when you hold your breath), the excess CO2 reacts with water to form additional carbonic acid, lowering blood pH. Increasing the rate and/or depth of respiration (which you might feel the “urge” to do after holding your breath) allows you to exhale more CO2. The loss of CO2 from the body reduces blood levels of carbonic acid, thereby adjusting the pH upward toward normal levels. As you might have surmised, this process also works in the opposite direction. Excessive deep and rapid breathing (as in hyperventilation) rids the blood of CO2 and reduces the level of carbonic acid, making the blood too alkaline. This brief alkalosis can be remedied by rebreathing air that has been exhaled into a paper bag. Rebreathing exhaled air will rapidly bring blood pH down toward normal (Biga et al., 2019).

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Renal Regulation

Biga et al., 2019

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The renal regulation of the body’s acid-base balance addresses the metabolic component of the buffering system. Whereas the respiratory system (together with breathing centers in the brain) controls the blood levels of carbonic acid by controlling the exhalation of CO2, the renal system controls the blood levels of bicarbonate. A decrease in blood bicarbonate can result from the inhibition of carbonic anhydrase by certain diuretics or from excessive bicarbonate loss due to diarrhea. Blood bicarbonate levels are also typically lower in people with Addison’s disease (chronic adrenal insufficiency), in which aldosterone levels are reduced, and in people with renal damage, such as chronic nephritis. Finally, low bicarbonate blood levels can result from elevated levels of ketones (common in unmanaged diabetes mellitus), which bind bicarbonate in the filtrate and prevent its conservation.

Bicarbonate ions, HCO3–, found in the filtrate, are essential to the bicarbonate buffer system, yet the cells of the tubule are not permeable to bicarbonate ions. The steps involved in supplying bicarbonate ions to the system are summarized below:

Step 1: Sodium ions are reabsorbed from the filtrate in exchange for H+ by an antiport mechanism in the apical membranes of cells lining the renal tubule.
Step 2: The cells produce bicarbonate ions that can be shunted to peritubular capillaries.
Step 3: When CO2 is available, the reaction is driven to the formation of carbonic acid, which dissociates to form a bicarbonate ion and a hydrogen ion.
Step 4: The bicarbonate ion passes into the peritubular capillaries and returns to the blood. The hydrogen ion is secreted into the filtrate, where it can become part of new water molecules and be reabsorbed as such or removed in the urine (Biga et al., 2019).

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Acidosis

pH < 7.35
Excess H+ concentration
Bicarbonate deficit
Causes:

Hypoventilation
Diabetic ketoacidosis
Shock
Excessive diarrhea

Biga et al., 2019

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Alkalosis

pH > 7.45
Excess bicarbonate concentration
Acid deficit
Causes:

Hyperventilation
Hypoxia
Excessive vomiting

Biga et al., 2019

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Symptoms of Acidosis and Alkalosis

Biga et al., 2019

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Critical Thinking Question

What would you expect? Acidosis or Alkalosis? Why?

A client with a high respiratory rate and vomiting?
What is a medical condition that might be responsible for this patient’s presentation?

What would you expect? Acidosis or Alkalosis? Why?

A client breathing 10/bpm with excessive diarrhea?
What is a medical condition that might be responsible for this patient’s presentation?

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Respiratory Acidosis

CO2 builds up in the blood, making it acidic
PaCO2 increases → pH decreases
Caused by a medical condition that decreases the exchange of oxygen and carbon dioxide at the alveolar level

Acute asthma/ CHF exacerbation
COPD

Symptoms include:

Anxiety, dyspnea, or headache
Delirium, confusion, seizures, and coma

Ernstmeyer & Christman, 2021

Contact info: info@nursesinternational.org

Respiratory acidosis develops when carbon dioxide (CO2) builds up in the body (referred to as hypercapnia), causing the blood to become increasingly acidic. Respiratory acidosis is identified when reviewing ABGs, and the pH level is below 7.35, and the PaCO2 level is above 45, indicating the cause of the acidosis is respiratory. Note that in respiratory acidosis, as the PaCO2 level increases, the pH level decreases. Respiratory acidosis is typically caused by a medical condition that decreases the exchange of oxygen and carbon dioxide at the alveolar level, such as an acute asthma exacerbation, chronic obstructive pulmonary disease (COPD), or an acute heart failure exacerbation causing pulmonary edema. It can also be caused by decreased ventilation from anesthesia, alcohol, or administration of medications such as opioids and sedatives (Ernstmeyer & Christman, 2021).

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Respiratory Alkalosis

When the body removes too much CO2 through respiration
PaCO2 decreases → pH increases
Caused by:

Hyperventilation

Symptoms include:

Shortness of breath
Lightheadedness
Chest tightness

Ernstmeyer & Christman, 2021

Contact info: info@nursesinternational.org

Respiratory alkalosis develops when the body removes too much carbon dioxide through respiration, resulting in increased pH and an alkalotic state. When reviewing ABGs, respiratory alkalosis is identified when pH levels are above 7.45 and the PaCO2 level is below 35. With respiratory alkalosis, notice that as the PaCO2 level decreases, the pH level increases.

Respiratory alkalosis is caused by hyperventilation that can occur due to anxiety, panic attacks, pain, fear, head injuries, or mechanical ventilation. Overdoses of salicylates and other toxins can also cause respiratory alkalosis initially and then often progress to metabolic acidosis in later stages. Acute asthma exacerbations, pulmonary embolisms, or other respiratory disorders can initially cause respiratory alkalosis as the lungs breath faster in an attempt to increase oxygenation, which decreases the PaCO2. After a while, however, these hypoxic disorders cause respiratory acidosis as respiratory muscles tire, breathing slows, and CO2 builds up in the blood (Ernstmeyer & Christman, 2021).

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Metabolic Acidosis

Kidneys work to excrete excess acid through urine and increase HCO3 reabsorption to keep the acid-base balance in the body.
Accumulation of acid (H+ ions) in the body and not enough bicarbonate (HCO3) to neutralize.
Caused by:

Diabetic ketoacidosis
Severe diarrhea

Symptoms include:

Confusion, hypotension, decreased LOC

Ernstmeyer & Christman, 2021

Contact info: info@nursesinternational.org

Metabolic acidosis occurs when there is an accumulation of acids (hydrogen ions) and insufficient bases (HCO3) in the body. Under normal conditions, the kidneys work to excrete acids through urine and neutralize excess acids by increasing bicarbonate (HCO3) reabsorption from the urine to maintain a normal pH. When the kidneys cannot perform this buffering function to the level required to excrete and neutralize the excess acid, metabolic acidosis is the result. Metabolic acidosis is characterized by a pH level below 7.35 and an HCO3 level below 22 when reviewing ABGs. It is important to notice that the pH and HCO3 decrease with metabolic acidosis (i.e., the pH and HCO3 move in the same downward direction).

A common cause of metabolic acidosis is diabetic ketoacidosis, where acids called ketones build up in the blood when blood sugar is extremely elevated. Another common cause of metabolic acidosis in hospitalized patients is lactic acidosis, which can be caused by impaired tissue oxygenation. Metabolic acidosis can also be caused by the increased loss of bicarbonate due to severe diarrhea or renal disease that causes decreased acid elimination. Additionally, toxins such as salicylate excess can cause metabolic acidosis (Ernstmeyer & Christman, 2021).

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Metabolic Alkalosis

When there is too much bicarbonate in the body OR an excessive loss of acid from the body
Both pH and HCO3 are elevated
Caused by:

Prolonged periods of vomiting or nasogastric suctioning

Symptoms include:

Confusion
Hypotension
Cardiac dysfunction

Ernstmeyer & Christman, 2021

Contact info: info@nursesinternational.org

Metabolic alkalosis occurs when there is too much bicarbonate (HCO3) in the body or an excessive loss of acid (H+ ions).

Metabolic alkalosis is defined by a pH above 7.45 and an HCO3 level above 26 on ABG results. Note that both pH and HCO3 are elevated in metabolic alkalosis.

Metabolic alkalosis can be caused by gastrointestinal loss of hydrogen ions, excessive urine loss, excessive levels of bicarbonate, or a shift of hydrogen ions from the bloodstream into cells.

Prolonged vomiting or nasogastric suctioning can also cause metabolic alkalosis. Gastric secretions have high levels of hydrogen ions (H+), so as acid is lost, the pH level of the bloodstream increases.

Excessive urinary loss (due to diuretics or excessive mineralocorticoids) can cause metabolic alkalosis due to the loss of hydrogen ions in the urine. Intravenous administration of sodium bicarbonate can also cause metabolic alkalosis due to increased levels of bases introduced into the body. Although it was once thought that excessive intake of calcium antacids could cause metabolic alkalosis, it has been found that this only occurs if they are administered concurrently with Kayexalate (Ernstmeyer & Christman, 2021).

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Acid-Base Balance Compensation

If the imbalance is metabolic, the lungs will compensate.

In metabolic acidosis, the lungs will blow off excess CO2.
In metabolic alkalosis, the lungs will retain CO2.

If the imbalance is respiratory, the kidneys will compensate.

In respiratory acidosis, the kidneys will excrete more H+ and increase bicarbonate reabsorption.
In respiratory alkalosis, the kidneys will retain more H+ ions and decrease the absorption of bicarbonate.

Biga et al., 2019

Contact info: info@nursesinternational.org

Respiratory compensation for metabolic acidosis increases the respiratory rate to drive off CO2 and readjust the bicarbonate to carbonic acid ratio to the 20:1 level. This adjustment can occur within minutes. The normal response of the respiratory system to elevated pH is to increase the amount of CO2 in the blood by decreasing the respiratory rate to conserve CO2. There is a limit to the decrease in respiration, however, that the body can tolerate. Hence, the respiratory route is less efficient at compensating for metabolic alkalosis than acidosis.

Metabolic and renal compensation for respiratory diseases that can create acidosis revolves around the conservation of bicarbonate ions. In cases of respiratory acidosis, the kidney increases the conservation of bicarbonate and secretion of H+. These processes increase the concentration of bicarbonate in the blood, reestablishing the proper relative concentrations of bicarbonate and carbonic acid. In cases of respiratory alkalosis, the kidneys decrease the production of bicarbonate and reabsorb H+ from the tubular fluid. These processes can be limited by the exchange of potassium by the renal cells, which use a K+-H+ exchange mechanism (antiporter) (Biga et al., 2019).

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Case Study/ Critical Thinking Question/ What Would the Nurse Do?

A client is diagnosed with Diabetic Ketoacidosis.

What symptoms would the nurse observe that are the body’s natural mechanisms to adjust the acid/base imbalance?

Is this a metabolic acid/base imbalance or a respiratory acid/base imbalance?

Which system will compensate?

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Arterial Blood Gases (ABG)

Arterial blood gases (ABG) are drawn from an artery rather than a vein

Usually radial artery

ABGs measure blood levels of:

Amount of hydrogen ions (pH)
Arterial oxygen (PaO2)
Arterial carbon dioxide (PaCO2)
Arterial bicarbonate (HCO3)
Oxygen saturation (SaO2)

Ernstmeyer & Christman, 2021

Contact info: info@nursesinternational.org

An ABG is a blood sample typically drawn from the radial artery by a respiratory therapist, emergency or critical care nurse, or health care provider.

ABG results evaluate oxygen, carbon dioxide, pH, and bicarbonate levels:

The partial pressure of oxygen in the blood is referred to as PaO2. The normal PaO2 level of a healthy adult is 80 to 100 mmHg.

The PaO2 reading is more accurate than a SpO2 reading because it is not affected by hemoglobin levels. The PaCO2 level is the partial pressure of carbon dioxide in the blood. The normal PaCO2 level of a healthy adult is 35-45 mmHg.

The normal range of pH level for arterial blood is 7.35-7.45
The normal range for the bicarbonate (HCO3) level is 22-26.
The SaO2 level is also obtained, which is the calculated arterial oxygen saturation level (Ernstmeyer & Christman, 2021).

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Normal Blood Gas Parameters

Ernstmeyer & Christman, 2021

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Arterial Blood Gases (ABG) Interpretation

Look at each of the values
Determine if pH is acidotic or alkalotic
Determine the cause (respiratory or metabolic)

Analyze PaCO2 for respiratory
Analyze HCO3 for metabolic

Determine if the patient is compensating
Assess the PaO2 and O2 saturation for hypoxia

Ernstmeyer & Christman, 2021

Contact info: info@nursesinternational.org

To interpret the results of an ABG, perform the following six steps:

Look at each of the values. If the pH is between 7.35 and 7.45, and the CO2, HCO3−, and PaO2 are within normal limits, the ABGs are normal. If any value is out of normal, then continue.
Determine if the pH is acidotic or alkalotic. Values less than 7.35 are acidotic, and values greater than 7.45 as alkalotic. A normal pH may indicate normal acid-base status, compensation is occurring, or a mixed disorder is present.
Determine the cause. First, analyze the PaCO2 to see if the patient has respiratory acidosis or alkalosis. Since the lungs control PaCO2, it is the respiratory component of the ABG. Because CO2 forms carbonic acid when dissolved in the blood, high PaCO2 levels decrease pH and indicate respiratory acidosis. Low PaCO2 levels increase pH and indicate respiratory alkalosis. If the pH value moves in the appropriate direction for the PaCO2 change (e.g., ↓ pH with ↑ PaCO2; ↑ pH with ↓ PaCO2), a respiratory problem is the primary disturbance.
Analyze the HCO3− level to see if the patient has metabolic acidosis or alkalosis. Since the kidneys primarily control HCO3−, it is the metabolic component of the ABG. Because HCO3− is a base, high levels of HCO3− increase pH and result in metabolic alkalosis. Low levels decrease pH and result in metabolic acidosis. If the pH value moves in the appropriate direction for the change (e.g., ↓ pH with ↓ HCO3−; ↑ pH with ↑ HCO3−), a metabolic problem is the primary disturbance.
Determine if the patient is compensating or if a mixed disorder is present. Look at the component (CO2 or HCO3−) that is not the cause of the primary disturbance. If the component that does not match the pH is moving in the opposite direction, the body is attempting to compensate. For example, if the pH is slightly acidotic (7.33), the CO2 is high (55 mm Hg), and the HCO3− is high (36 mEq/L), the CO2 is the parameter that matches the acidotic pH. The patient's underlying acid-base imbalance is respiratory acidosis. The HCO3− level is alkalotic. Since this is in the opposite direction of respiratory acidosis, compensation is occurring. If both parameters match the pH, consider the possibility that a combined respiratory or metabolic acidosis or alkalosis is present. For example, if the pH is acidotic (7.28), the CO2 is high (55 mm Hg), and the HCO3− is low (16 mEq/L), the patient's underlying acid-base imbalance is combined respiratory-metabolic acidosis.
Assess the PaO2 and O2 saturation. If these values are abnormal, hypoxemia is present (Ernstmeyer & Christman, 2021).

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Arterial Blood Gases (ABG) Interpretation

A simple way to interpret ABGs is the ROME method

R - respiratory O - opposite

Respiratory component (PaCO2) moves in the opposite direction of the pH if the lungs are causing the imbalance

M - metabolic E - equal

Metabolic component (HCO3) moves in the same direction as the pH if the kidneys are causing the imbalance

Ernstmeyer & Christman, 2021

Contact info: info@nursesinternational.org

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Critical Thinking Question

A client reports severe vomiting that has lasted for 4 days. They state they have been trying to take antacids to help, but they continue to vomit. They are now very fatigued and mildly disoriented.

Their ABG is as follows:

Ph 7.48
PaCO2 38
HCO₃31

How would you interpret this ABG data? What type of Acid-Base imbalance is this client exhibiting? How did they get this way? At what rate would you be expecting them to breathe to help compensate, fast or slow?

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Partial Compensation

Before the body is fully compensated, you will be able to see that compensation is taking place
All components will be abnormal

pH, pCO2, and HCO3 will all be abnormal

When components match in the same column, you can tell if it is acidosis or alkalosis and which system is causing the imbalance

e.g. pH and CO2 are both acidic and HCO3 is alkalotic → partially compensated respiratory acidosis.

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Full Compensation

The goal is for pH to be normal so fully compensated results will have a normal pH and abnormal pCO2 and HCO3

pH of 7.35 to 7.39 → acidosis
pH of 7.4 to 7.45 → alkalosis

Eg: pH = 7.38, pCO2 = 32, HCO3 = 20

pH of 7.38 → normal; toward acidosis
pCO2 of 32 → respiratory alkalosis
HCO3 of 20 → metabolic acidosis

= Fully compensated metabolic acidosis

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Critical Thinking Question

How would you interpret the following ABG reading?

Ph 7.42
PaCO2 28
HCO₃19

Is this ABG demonstrating compensated or uncompensated acid/base imbalance?

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Nursing Considerations: Subjective Assessment

Includes:

Age
History of chronic disease
Surgeries and/or traumas
Dietary intake
Activity level
Prescribed medications
Medication compliance
Pain
Bowel and bladder functioning

Ernstmeyer & Christman, 2021

Contact info: info@nursesinternational.org

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Nursing Considerations: Objective Assessment

A complete head-to-toe assessment should be performed to avoid missing clues to the patient’s condition:

Daily weight
Accurate intake and output
Vital signs
Observe for:

New mental status changes such as confusion or decreased level of consciousness
Cardiac arrhythmias

Ernstmeyer & Christman, 2021

Contact info: info@nursesinternational.org

Objective assessment data is information that the nurse directly observes. This data is obtained through a physical examination using inspection, auscultation, and palpation. A complete head-to-toe assessment should be performed to avoid missing clues to the patient’s condition.

Accurate daily weights can provide important clues to fluid balance. Weights must be taken on the same scale, at the same time of day, with the patient wearing similar clothing in order to be accurate. A one kilogram change in weight in 24 hours is considered significant because this represents a one liter fluid gain or loss and should be reported to the provider.
Accurate measurement of 24-hour intake and output helps validate weight findings. Averaged urine output of less than 30 mL/hour or 0.5mL/hr/kg of concentrated urine should be reported to the provider.
Vital signs should be analyzed. An elevated blood pressure and bounding pulses are often seen with fluid volume excess. Decreased blood pressure with an elevated heart rate and a weak or thready pulse are hallmark signs of fluid volume deficit. Systolic blood pressure less than 100 mm Hg in adults, unless other parameters are provided, should be reported to the health care provider.
Lung crackles can signify fluid volume excess and are often first auscultated in the lower posterior lung fields.
Tight, edematous, shiny skin indicates fluid volume excess. See Figure 15.15[1] for an image of edema. Conversely, skin tenting, dry mucous membranes, or dry skin indicate fluid volume deficit.
New mental status changes such as confusion or decreased level of consciousness can indicate fluid, electrolyte, or acid-base imbalance, especially hypo- or hypernatremia, acid-base imbalances, or fluid volume deficit.
Cardiac arrhythmias can be seen with acid-base imbalances and electrolyte imbalances, especially with hypo- or hyperkalemia and alkalosis. See Table 15.6a for a comparison of expected and unexpected findings and those that require notification of a health care provider (Ernstmeyer & Christman, 2021).

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Red Flags

Shortness of breath
Decreased level of consciousness
Confusion
Lightheadedness
Chest tightness
Anxiety

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Cultural Considerations

Religion, culture, beliefs, and ethnic customs can influence how families understand and use health concepts:

Health beliefs: In some cultures talking about a possible poor health outcome will cause that outcome to occur.
Health customs: In some cultures family members play a large role in health care decision-making.
Ethnic customs: Differing gender roles may determine who makes decisions about accepting & following treatment recommendations.

AHRQ, 2020

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Cultural Considerations (Continued)

Religion, culture, beliefs, and ethnic customs can influence how families understand and use health concepts:

Religious beliefs: Faith and spiritual beliefs may affect health seeking behavior and willingness to accept treatment.
Dietary customs: Dietary advice may be difficult to follow if it does not fit the foods or cooking methods of the family
Interpersonal customs: Eye contact or physical touch may be ok in some cultures but inappropriate or offensive in others.

AHRQ, 2020

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References

Agency for Healthcare Research. (2020). Health Literacy Universal Precautions Toolkit, 2nd Edition. https://www.ahrq.gov/health-literacy/improve/precautions/tool10.html

Biga, L. et al. (2019). Anatomy and Physiology. https://open.oregonstate.education/aandp/chapter/26-5-disorders-of-acid-base-balance/

Ernstmeyer, K., & Christman, E. (Eds.). (2021). Open RN Nursing Fundamentals. https://wtcs.pressbooks.pub/nursingfundamentals/chapter/15-5-acid-base-balance

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