CO₂ Below the Diaphragm: When the Problem Isn't the CO₂, but the Delivery Circuit

CO₂ below the diaphragm is a practical clinical option for contrast-sparing and Zero Contrast strategies, particularly in patients with CKD (Chronic Kidney Disease), diabetes, or at risk of contrast-induced AKI (Acute Kidney Injury). However, when images appear "noisy," inconsistent, or difficult to interpret, the issue is rarely the CO₂ itself—it is almost always the delivery circuit (contamination, setup, consumables, air contamination). This article is intended for interventional radiologists and vascular surgeons who want to make CO₂ injections more standardized, audit-ready, and aligned with patient safety.

Why CO₂ images become inconsistent: the role of ambient air and circuit contamination

In infra-diaphragmatic angiography, image quality depends on the actual fraction of CO₂ delivered and on how the gas reaches the patient. If ambient air or uncontrolled micro-volumes enter the circuit (through connectors, syringes, stopcocks, or filling procedures), the apparent density, bubble pattern, and continuity of the CO₂ bolus are altered. The clinical consequences are well known: uneven vessel opacification, inconsistent runs, the need for repeated acquisitions, and often a return to iodinated contrast as a "rescue" option in the most fragile patients.

This variability is typically operator-dependent because manual injection introduces differences in purge timing, filling technique, the presence of micro-leaks, the sequence of procedural steps, and the consistency of injection parameters from one run to the next. In patients at high renal risk, this is not merely an imaging issue—it becomes a matter of clinical governance, making it more difficult to demonstrate that the procedure was performed in a standardized and safe manner.

CO₂ below the diaphragm: the essential checks for repeatable, audit-ready injections

A reliable CO₂ angiography circuit is a complete system consisting of the gas source, circuit components (tubing, valves, connectors), filtration, air contamination management, and appropriate single-use consumables. In practice, vessel opacification is the final outcome of a chain of controls: if even one element is weak, variability increases and image quality declines.

To make injections repeatable and defensible during audits, these controls should be explicit, verifiable, and documented. In particular, technical barriers against air entry, measures to minimize circuit contamination, and consistent delivery parameters (volume, pressure, and timing) are essential. The goal is not theoretical perfection, but a level of standardization that ensures comparable angiographic runs and allows the team to reconstruct who performed each step, how it was performed, and under which settings.

• Gas purity and traceability: use medical-grade CO₂ and maintain clear documentation of the gas supply according to institutional policies.
• Closed circuit with single-use components where indicated: reduces air contamination and variability caused by reused or inconsistently assembled components.
• Filtration and protection against particulates and contaminants: helps preserve circuit integrity and ensures more consistent gas delivery.
• Air trap management: ensure the presence and correct positioning of a dedicated air trap together with standardized purge procedures.
• Connection integrity: verify that connectors are secure, micro-leaks are absent, and stopcocks are correctly positioned. Micro-leaks are a common cause of otherwise unexplained inconsistent angiographic runs.
• Standardized delivery parameters: establish protocols for each vascular territory (e.g., iliac, femoropopliteal, tibial) including volume, pressure, timing, and repeat criteria.

Why manual CO₂ injection increases variability—and when it becomes a patient safety issue

Manual injection can be effective in experienced hands, but it is inherently vulnerable because every discretionary step introduces variability. Differences in filling, the interval between filling and injection, connection and disconnection procedures, and inconsistent control of residual air all contribute to images that vary from one run to the next. In practice, two operators with the same clinical objective may achieve different results despite both "using CO₂."

In patients with impaired renal function, this instability comes at a clinical cost: repeated attempts to obtain adequate imaging may prolong the procedure, increase the number of acquisitions, and ultimately lead to the use of iodinated contrast. Furthermore, the limited traceability of a manual setup makes it difficult to answer common internal audit questions, such as which circuit was used, which safety checks were performed, and which delivery parameters were selected.

Automated, traceable workflows: improving image quality, repeatability, and audit readiness

Automated workflows are designed to reduce operator-dependent variability by standardizing critical steps, including circuit preparation, purge procedures, air trap management, delivery parameters, and the use of dedicated consumables. When these parameters are consistently applied, CO₂ delivery becomes more predictable, resulting in more consistent images across runs, operators, and procedures.

Traceability is what distinguishes a perceived sense of control from demonstrable control. Recording system settings and key procedural steps allows teams to reconstruct how the procedure was performed, support clinical reviews, and make CO₂ use more defensible within a Clinical Governance framework. This is one reason why solutions such as the Angiodroid automatic CO₂ injector and its digital workflow are increasingly considered by centers aiming to implement scalable contrast-sparing protocols, rather than performing only occasional CO₂ cases.

From frustration to standardization: building a consistent infra-diaphragmatic CO₂ protocol

Standardization begins with a clear decision: defining CO₂ as a first-line imaging option for selected infra-diaphragmatic indications and identifying priority patient groups, such as those with CKD, iodinated contrast allergy, or increased AKI risk. This should be supported by a controlled circuit, predefined image quality criteria (when to repeat an acquisition versus when to change strategy), and standardized baseline parameters for each vascular territory. Finally, team training should cover not only the principles of CO₂ angiography but also the discipline of proper circuit setup.

When CO₂ is managed as a technology with its own dedicated workflow rather than as an improvised alternative contrast agent, variability decreases and vessel opacification becomes more predictable. The clinical message is straightforward: CO₂ below the diaphragm performs best when the delivery circuit is specifically designed to eliminate avoidable sources of variability.

Frequently Asked Questions

Clinical note: This content is intended for educational purposes only and does not replace local protocols, device instructions for use, or the clinical judgment of the treating team. In Europe, users should always follow hospital policies, MDR requirements, and the manufacturer's instructions regarding the delivery circuit, consumables, and the use of medical CO₂.