Arterial Blood Gas (ABG) Sampling: Procedure & Guide

This section details key physiological differences between arteries and veins. Arteries carry blood under high pressure directly from the heart, causing forceful bleeding when cut, while veins carry blood under low pressure with slower flow. Arteries have smaller diameters to maintain pressure, while veins have larger diameters. Valves in veins prevent backflow since blood returns slowly after circulating through the body. Blood volume distribution shows 15% in arteries, 65% in veins, 5% in capillaries, 10% in lungs, and 10% in the heart. The misconception that arteries are red and veins are blue due to oxygen content is completely incorrect.

This section covers the key physiological differences between arteries and veins. In systemic circulation, arteries carry oxygen-rich blood away from the heart while veins carry oxygen-poor blood back. In pulmonary circulation, this pattern reverses. Blood pressure is high in arteries (especially the aorta) due to the left ventricle's forceful contraction, while pressure is low in veins. At any given moment, approximately 65% of total blood volume is in systemic veins, 15% in systemic arteries, 5% in capillaries, and 10% in pulmonary circulation.

Arterial blood (carrying oxygen-rich blood from the lungs) has high oxygen partial pressure (100 mmHg) and low carbon dioxide partial pressure (40 mmHg). Venous blood (returning from tissues) has low oxygen partial pressure (40 mmHg) and high carbon dioxide partial pressure (46 mmHg). These differences drive gas exchange in the lungs and tissues. The pressure gradients for oxygen and carbon dioxide are maintained by ventilation and perfusion.

Arteries have high pressure because they are being pumped directly by the heart, while veins have low pressure because blood flows sluggishly back to the heart without direct pumping. This pressure difference explains why arterial bleeding is more severe and spurts forcefully compared to venous bleeding.

The heart pumps blood through arteries at very high pressure, while blood moves through veins at lower pressure. This fundamental difference explains most other differences between arteries and veins. High pressure in arteries requires thicker walls to prevent rupture, while low pressure in veins allows for thinner walls. Blood flow through veins is facilitated by smooth muscle contractions and physical movement of the body.

This comprehensive section covers the foundational principles of acid-base chemistry and the body's buffer systems. pH is defined as the negative logarithm of hydrogen ion concentration, with normal blood pH at 7.4. Strong acids completely dissociate while weak acids only partially dissociate. The pKa value indicates the pH at which an acid is half-dissociated. Buffer solutions resist pH changes when small amounts of acid or base are added. Normal blood pH is maintained between 7.35 and 7.45, with acidosis below 7.35 and alkalosis above 7.45. The body employs three lines of defense: blood buffers as the first line, respiratory regulation as the second, and renal regulation as the third. The bicarbonate-carbonic acid system is the most important plasma buffer, with normal bicarbonate at 24 mEq/L and carbonic acid at 1.2 mEq/L. The Henderson-Hasselbalch equation (pH = pKa + log([HCO3-]/[H2CO3])) describes the relationship between pH and buffer components, with a 20:1 ratio producing pH 7.4. The phosphate buffer system operates intracellularly with a pKa of 6.8 closest to physiological pH. Protein buffers, particularly hemoglobin in red blood cells, provide significant buffering capacity through amino acid side chains.

Three primary buffer systems maintain acid-base homeostasis. The bicarbonate-carbonic acid system (pK=6.1) dominates extracellular fluid; pH = 6.1 + log([HCO3-]/(0.03×PCO2)). Phosphate buffers serve urinary and intracellular roles, handling excess H+ beyond bicarbonate capacity. Protein buffers, especially hemoglobin, function intracellularly during CO2 transport. Hemoglobin binds H+ in tissues and releases it in lungs, demonstrating dynamic buffering during gas exchange. Together, these systems rapidly neutralize pH disturbances before renal compensation occurs.

This comprehensive section covers the fundamental principles of acid-base chemistry and the body's buffering mechanisms. An acid releases H+ ions while a base accepts them. pH is defined as the negative logarithm of H+ concentration, meaning small pH changes represent large concentration changes. Physiological pH in extracellular fluid is 7.40 (40 nmol/L H+), while intracellular pH is slightly lower at 7.05. Human survival requires extracellular pH between 6.9 and 7.7. The body faces constant acid invasion from volatile acids (producing CO2 from oxidation, ~1300 mmol/day) and fixed acids (from diet and metabolism, ~1000 mmol/kg/day). Buffer systems are the first line of defense, consisting of conjugate acid-base pairs that accept or release H+ ions. The bicarbonate buffer system is the most important extracellular buffer, involving the equilibrium between carbonic acid and bicarbonate. Intracellular buffering involves proteins, hemoglobin, and phosphate systems.

Blood pH must remain within the narrow range of 7.35-7.45 for proper enzyme function. The body maintains this balance through buffer systems that resist pH changes when acids or bases are added. A buffer consists of a weak acid and its conjugate base salt, which can neutralize both acids and bases. The bicarbonate buffer system (H2CO3/NaHCO3) is most abundant in plasma, existing in a 1:20 ratio. When strong acid enters, bicarbonate reacts with it; when strong base enters, carbonic acid reacts with it. The body contains three main buffer types: bicarbonate, phosphate, and protein buffers. Hemoglobin functions as a dual-purpose buffer, transporting oxygen while buffering CO2. This integrated system allows rapid pH correction through multiple complementary mechanisms.

Acid-base balance is essential for normal enzymatic function, with normal H+ concentration at 40 mEq/L corresponding to pH 7.4. Acids donate H+ ions while bases accept them. The body maintains pH through three systems: immediate chemical buffers (bicarbonate, phosphate, hemoglobin), rapid respiratory compensation (within minutes), and slower renal mechanisms (hours to days). The bicarbonate buffer system is primary, converting excess H+ to carbonic acid which splits into CO2 and water for exhalation. Phosphate serves as a secondary buffer. Understanding these foundational principles is essential for interpreting acid-base disturbances.

Standard precautions are universal infection control measures applied to all patients at all times. These include hand hygiene (the most important measure), use of personal protective equipment (gloves, masks, gowns, caps) depending on the procedure, and proper use of sharps containers. Hand hygiene should be performed before and after glove use. Used needles must never be capped and must be disposed of immediately in a sharps container. When disposing of devices like razors, the cap should be removed before disposal to prevent accidental injury. The fewer times healthcare workers handle sharps, the lower their exposure risk. Modern medical devices increasingly incorporate safety mechanisms that reduce direct contact with sharp materials, but proper training in their use remains essential.

Performing aseptic procedures requires following a systematic protocol: (1) Explain the procedure to the patient and obtain consent, (2) Perform hand hygiene, (3) Wear appropriate PPE (gown, mask, goggles/glasses, gloves), (4) Prepare a sterile tray with instruments, (5) Open packaging without contaminating the sterile parts, (6) Clean the injection site using alcohol pads in circular outward motion, (7) Perform the procedure, (8) Dispose of waste properly, (9) Remove gloves, and (10) Perform hand hygiene again. This prevents cross-infection between patients and healthcare workers.

Standard precautions evolved from isolated communities (1700) through infectious hospitals (1800), hospital sectors (1960), hospital categories (1975), disease-specific precautions (1983), universal precautions (1985), body substance precautions (1990), to current standard precautions (1995). Standard precautions include hand washing, personal protective equipment (gowns, masks, face shields), gloves for fluid contact, and precautions for contaminated sharps. These must be applied to all patients regardless of isolation status. Safe sharps handling requires protecting oneself first, maintaining attention, never recapping needles (use Kelly or mosquito forceps), and disposing of contaminated materials in impermeable, puncture-resistant receptacles.

Key do's of sharp safety include: always use sharps only when absolutely necessary for a procedure; dispose of sharps immediately after use in an approved puncture-resistant sharps container; use safety engineered devices such as retractable needles or needleless systems whenever available; activate the safety feature on sharps right after use; keep eyes on sharps at all times while using or disposing of them; place sharps containers within arms reach, ideally at eye level, and not overfilled; report all sharps injuries to your instructor or supervisor immediately; use proper body positioning and support during injections or blood draws to avoid slips; wear gloves as part of standard precautions; inspect all sharps for defects before use for things like broken seals, bent needles, or other defects that may cause injury; follow facility protocol for disposal, reporting, and cleanup of sharps incidents; participate in safety training and stay up-to-date on best practices for sharp safety.

Aseptic technique must be used when handling, preparing, and storing medications and injection equipment. Key elements include using new sterile syringes and needles, preventing contact with non-sterile environments, proper hand hygiene, alcohol disinfection of rubber septums, scrubbing access ports with antiseptic (waiting 10 seconds for drying), and accessing ports only with sterile devices. Medication preparation should occur in designated clean areas separate from patient care areas, with proper lighting, refrigeration logs, hand hygiene capability, and limited access. USP standards mandate a one-hour time limit from IV bag preparation/spiking until administration begins. All clinician-prepared syringes must be labeled unless administered immediately at bedside. Joint Commission National Patient Safety Goals require labeling all medications and containers including syringes, medicine caps, basins, and solutions on and off sterile fields. Sharps and medications should be stored centrally to minimize reuse risk. Sharps must be disposed of only in approved containers, never recap or bend needles before disposal, replace containers at fill lines, secure them to prevent spilling, and keep them visible and accessible at use locations. OSHA standards require functional containers (durable, puncture-resistant, leak-resistant, labeled) accommodating the largest sharps used. CDC estimates nearly 600,000 annual percutaneous injuries from contaminated sharps. Needle stick injuries occur 27% of the time during insertion/removal of hollow bore needles, requiring extra caution. Drug diversion prevention involves three categories: prevention (systems guarding against theft, staff protocols minimizing unauthorized access), detection (monitoring pharmacy records, camera surveillance, barcode tracking, secure delivery), and response (assessing harm, consulting public health officials, reporting to enforcement agencies).