pH Monitoring

Definition
Description
How pH is Monitored
pH Monitoring during Heart Surgery

Definition

pH is a value that represents the balance of acidic and alkaline molecules in a given system. The human body requires a very precise balance between these in order to properly maintain homeostasis. Homeostasis, or physiological equilibrium, is the maintenance of stable conditions for bodily function. pH monitoring is the practice of keeping track of where the pH value lies in order to quickly diagnose and treat disorders that alter pH.

Description

pH monitoring is done to guard against physiological states in which the amount of acidic or alkaline molecules in the body is out of balance. Another term for acidic and alkaline is “acid and base”. Acid is represented in the body as hydrogen ions. Base is represented in the body as hydroxide ions. Too much of either one can cause serious physical harm. A pH scale is designed with the number seven falling in the middle, defined as neutral. At a pH of seven the amount of acid and base present lies in balance. Any pH greater than seven is considered more alkaline or basic. Any pH less than seven is considered more acidic. Each area of the body has its own normal range for pH. Human blood normally lies between a pH of 7.34 to 7.45. Human stomach acid normally lies between a pH of 1.5 to 2.0. Human urine normally has a pH of approximately 6.0.

A physiological state where the blood is too acidic with a low pH is called acidosis. Acidosis is caused by an imbalance in hydrogen ions present in the blood, causing the pH to fall below 7.35. On the other side of the scale, alkalosis is a physiological state where the blood pH value rises above 7.45 due an imbalance in hydroxide, and becomes too basic. The farther the pH is altered from the normal range, the more serious the patient’s condition. Even relatively small changes in blood pH can be life threatening. Blood pH values generally considered compatible with mammalian life lie between a pH of 6.8 to 7.8. For this reason, the body uses blood as a buffering system to maintain pH balance. Certain states of disease, disorders, or physical conditions sometimes overwhelm the body’s buffering systems and medical assistance becomes necessary to maintain life. In these cases, pH monitoring is a critical tool in guarding against or treating drastic alterations in body chemistry.

Monitoring pH for Acidosis

Acidosis is physiological state of excessive acid in the blood. Acidosis can be caused by an increase in acid, or simply a decrease of base, causing what is known as metabolic acidosis. Acidosis can also be caused by abnormal respiratory function. Taking in too much carbon dioxide or too little oxygen through the lungs leads to increased amounts of carbon dioxide in the blood. Carbon dioxide in the blood is changed into multiple components, resulting in the release of hydrogen ions, causing acidosis. The type of acidosis originally caused by carbon dioxide and respiratory functioning is known as respiratory acidosis.

If an increase in acid or decrease in base occurs beyond the body’s pH buffering capabilities, there are other compensatory mechanisms to restore equilibriumin pH. In response to a drop in blood pH, the brain causes a change in respiration for fast, deep breathing. This type of breathing causes an increased amount of carbon dioxide to be exhaled, drawing more and more carbon dioxide from the blood. The more carbon dioxide leaves the blood, the more hydrogen ions are removed, helping to correct acidosis. Additionally, the kidneys may increase excretion of hydrogen ions or decrease excretion of hydroxide ions in the urine. Even with these compensatory mechanisms, a severe state of acidosis often requires medical attention to correct, with pH monitoring as a critical tool for health management. Without proper medical attention the acidotic patient may experience nausea, vomiting, fatigue, confusion, weakness, shock, coma, or death. The treatment of acidosis is dependent on the cause of the disorder.

Conditions that Potentially Cause Acidosis and May Require pH Monitoring

• Ingestion of Wood Alcohol or Antifreeze (excessive acid)
• Aspirin Overdose (excessive acid)
• Poorly Controlled Diabetes (excessive acid)
• Kidney Dysfunction or Failure (cannot excrete acid normally)
• Lung Dysfunction (cannot expel carbon dioxide normally; e.g., severe pneumonia, emphysema, or asthma)
• Drugs that Disrupt Respiration (cannot expel carbon dioxide normally; e.g., narcotic pain-killer overdose)
• Ingestion of Acidic Poisons (excessive acid)
• Extreme Diarrhea (expel excessive base)

Monitoring pH for Alkalosis

Alkalosis is a physiological state of excessive base in the blood. Alkalosis can be caused by an increase in base, or a decrease of acid, causing what is known as metabolic alkalosis. Alkalosis can also be caused by abnormal respiratory function. Exhaling too much carbon dioxide leads to a decrease in carbon dioxide in the blood. A decrease in carbondioxide in the blood leads to a decrease in hydrogen ions and a relative excess of hydroxide ions. This type of alkalosis is known as respiratory alkalosis. Respiratory alkalosis can be induced by hyperventilation, a type of breathing that is very rapid and deep and leads to expulsion of large amounts of carbon dioxide.

KEY TERMS

Acidosis— A pathological state in which there is a relative excess of hydrogen ions in the arterial blood.

Alkaline— A basic substance with pH above 7.0.

Alkalosis— A pathological state in which there is a relative excess of hydroxide ions in the arterial blood.

Anxiety Attack— A disorder in which sudden feelings of dread, fear, and apprehension of danger enter a person’s mind in an overwhelming manner. Attacks may lead to a state of hyperventilation.

Arterial Blood— Blood from the arteries, the blood vessels that carry oxygen from the lungs to supply the body tissues.

Arterial Blood Gas (ABG)— A type of blood laboratory test done to check the blood for imbalances in pH or gases that affect pH.

Asthma— An inflammatory respiratory disorder in which the airway becomes obstructed and breathing is difficult.

Coronary Artery Bypass Surgery— A surgical procedure where arteries or veins from elsewhere in the patient’s body grafted into the arteries of the heart as a way to bypass damaged or narrowed heart blood vessels. Heart blood vessels may have been damaged or narrowed from fat deposition and need to be bypassed to increase the blood supply and oxygen delivered to the heart.

Corticosteroid— A type of drug used to treat inflammatory conditions that also alters blood ions and affects pH.

Diuretic— A type of drug used to treat hypertension that also alters blood ions and affects pH.

Electrode— A device used to record an electric circuit or changes in ion concentrations.

Emphysema— Lung disease that causes damage to lung tissue and breathlessness.

Heart Lung Machine— A machine that temporarily takes over the function of the heart and lungs during surgical procedures in order to maintaining blood circulation and delivery of oxygen to body tissues while the heart is being operated on.

Heart Valve Replacement Surgery— Surgery performed to repair or replace the valves in the heart that control blood flow through the heart and are responsible for the audible heartbeat.

Homeostasis— The process of maintaining balance in the normal vital life functions of a living organism.

Hydrogen Ions— Ions that contain one hydrogen atom with a positive charge. Hydrogen ions cause blood to be acidic.

Hydroxide Ions— Ions that contain one oxygen and one hydrogen atom, with a negative charge. Hydroxide ions cause blood to be alkaline.

Ischemia— A state of decreased oxygen, where body tissues may experience damage due to oxygen deprivation.

Narcotic Pain Killer— A type of pain killer often used after surgical procedures that may alter blood pH.

Physiological— Pertaining to the normal vital life functions of a living organism.

Pneumonia— An inflammatory lung disease that affects the ability of the respiratory system to function.

Radial Artery— An artery present in the wrist that is convenient for drawing blood intended for laboratory testing.

Respiratory Function— The ability of the breathing structures of the body, including the lungs, to function.

Venous Blood— Blood that carries carbon dioxide from the tissues to the heart and then the lungs to be oxygenated.

If an increase in base or decrease in acid occurs beyond the body’s pH buffering capabilities, compensatory mechanisms to restore equilibrium in pH include a change in respiration for slow, shallow breathing. This type of breathing causes a decreased amount of carbon dioxide to be exhaled, thereby increasing the amount of carbon dioxide and hydrogen ions in the blood. Additionally, the kidneys may increase excretion of hydroxide ions or decrease excretion of hydrogen ions in the urine. Similar to acidosis, a severe state of alkalosis often requires medical attention to correct, with pH monitoring as a critical tool for health management. Without proper medical attention the patient may experience irritability, muscle cramping, muscle spasms, seizures, delirium, and death. The treatment of alkalosis is dependent on the cause of the disorder.

Conditions that Potentially Cause Alkalosis and May Require pH Monitoring

• Prolonged Vomiting (loss of excessive acid)
• Ingestion of Alkaline Poisons (excessive base)
• Drugs that Cause Excessive Loss of Sodium or Potassium Ions (e.g., diuretics and corticosteroids)
• Severe Anxiety Attacks (hyperventilation)
• Pain and Fever (hyperventilation)
• Aspirin Overdose (Aspirin can cause both acidosis and alkalosis)

How pH is Monitored

pH is monitored through laboratory tests done on the blood drawn from an artery. Arterial blood is an accurate indicator of blood pH. Venous blood holds too much carbon dioxide taken from the tissues to be useful. Blood is usually taken from the radial artery located in the wrist. The blood is tested for pH, as well as levels of carbon dioxide and other pertinent blood components. The laboratory test is known as Arterial Blood Gas (ABG ). It is very important that room air is not accidentally introduced into the sample of blood drawn, as the oxygen present would make the blood unsuitable for testing. If the patient’s condition warrants it, the ABG test may be done several times a day.

pH Monitoring during Heart Surgery

Some types of heart surgery involve temporarily depriving parts of the heart of its blood supply during the procedure (while the patient is on a heart lung machine to maintain bodily function). Local blood deprivation results in decreased heart tissue oxygen (a condition called ischemia), increased tissue carbon dioxide levels, heart tissue acidosis, and possibly death of heart tissue. The level to which this actually occurs during the procedure varies from person to person. The severity of tissue acidosis has been shown to directly associate with the degree of ischemia that tissue is experiencing and consequent impact on long-term survival of the surgery patient.

Specific low pH values during different points in the surgical procedure have been identified as dangerous levels of acidosis. Allowing the heart tissue to fall below these values may be associated with post-operative heart failure and mortality. Purposefully raising the pH levels before these specific points in the procedure, prior to heart tissue ischemia and a drop in pH, may help keep the heart tissue pH above dangerous levels of acidosis and ischemia. Special intraoperative heart pH monitors have been designed to supervise the pH value of heart tissue during surgery and keep track of the surgeon’s efforts in pH modification. Electrodes designed as sterile probes that detect pH are inserted directly into the heart and used as an early warning system for dangerous changes in pH. Surgical procedures that may be benefited by pH monitors include heart valve replacement surgery and coronary artery bypass surgery. pH monitoring is an important tool for surgeons to avoid or reverse acidosis and ischemia during heart surgery, thereby improving long-term patient survival.

Resources

BOOKS

Cecil Essentials of Medicine Sixth Edition. Saunders 2004.

Harrison’s Principles of Internal Medicine Sixteenth Edition. McGraw-Hill 2005.

Merck Manual of Diagnosis and Therapy Eighteenth Edition 2006.

The Merck Manual Home Edition2004.

PERIODICALS

Khuri SF, Healey NA, Hossain M, Birjiniuk V, Crittenden MD, Josa M, Treanor PR, Najjar SF, Kumbhani DJ, Henderson WG. “Intraoperative regional myocardial acidosis and reduction in long-term survival after cardiac surgery.” Journal of Thoracic and Cardiovascular Surgery 129 (2005): 372–81.

Maria Basile, PhD