7.2 Vaporized Hydrogen Peroxide (VHP) & Hydrogen Peroxide Gas Plasma Sterilization

7.2 Vaporized Hydrogen Peroxide (VHP) & Hydrogen Peroxide Gas Plasma Sterilization

Exam Tip: Hydrogen peroxide sterilization is the most common low-temperature method replacing EtO for modern heat- and moisture-sensitive instruments. Focus heavily on material compatibility rules for the exam: NO cellulose (paper, cotton, linens) and NO liquids or moisture. The biological indicator organism is Geobacillus stearothermophilus.

Over the past two decades, Vaporized Hydrogen Peroxide (VHP) and Hydrogen Peroxide Gas Plasma systems have largely superseded Ethylene Oxide in hospital sterile processing departments. These low-temperature, dry sterilization processes offer rapid cycle times (typically 24 to 75 minutes), leave no toxic chemical residuals, require no post-sterilization aeration phase, and break down into environmentally benign byproducts: water vapor and oxygen.

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Mechanism of Action: Free Radical Oxidation

Hydrogen peroxide ($H_2O_2$) sterilizes micro-organisms via a powerful chemical oxidative process.

When liquid hydrogen peroxide (typically 59% to 94% concentrated solution) is vaporized under deep vacuum inside the sterilizer chamber, it permeates instrument lumens and surfaces. The hydrogen peroxide molecules dissociate into highly reactive hydroxyl ($OH^-$) and hydroperoxyl ($HO_2^-$) free radicals.

$\text{H}_2\text{O}_2 \xrightarrow{\text{Vacuum / Energy}} \text{OH}^\bullet + \text{OH}^\bullet$

These unstable free radicals violently attack and oxidize key cellular components:

• They disrupt lipid membrane integrity in bacterial cell walls.
• They denature structural and enzymatic proteins.
• They cleave double-stranded microbial DNA and RNA.

Through this rapid oxidation cascade, all microbial forms—including spores, mycobacteria, non-enveloped viruses, and fungi—are rendered non-viable.

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Gas Plasma vs. Vaporized Hydrogen Peroxide (VHP) Systems

While both technologies utilize liquid hydrogen peroxide as their precursor chemical, their physical cycle execution differs slightly:

The Plasma Stage Explained

In gas plasma units, after hydrogen peroxide vapor diffuses through the load, an electric radio frequency (RF) field is applied across the chamber. This energizes the vapor into a cloud of ionized gas (plasma) containing free radicals, ions, and excited atoms. When the RF power is turned off, the ionized species recombine into stable, non-toxic water vapor and oxygen molecules, leaving zero chemical residue on instruments.

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Four Sequential Cycle Phases

Standard hydrogen peroxide cycles consist of four main automated phases executed inside an airtight chamber:

1. Vacuum Phase: The chamber is evacuated to deep low pressure (sub-atmospheric pressure) to evacuate air and moisture from packaging and long narrow instrument lumens.
2. Injection Phase: A measured dose of liquid hydrogen peroxide is vaporized and injected into the chamber, vaporizing rapidly into a gas state.
3. Diffusion Phase: The $H_2O_2$ vapor permeates packaging materials and penetrates internal lumen channels, maintaining direct contact with microbial surfaces.
4. Plasma / Catalytic Evacuation Phase: RF energy is applied (in plasma systems) or deep vacuum flushes drive vapor through catalytic converters, splitting $H_2O_2$ into $O_2$ and $H_2O$. Clean air enters to return chamber to atmospheric pressure.

Critical Material Limitations & Incompatibilities

Technicians must strictly adhere to loading restrictions for hydrogen peroxide sterilizers. Operating errors involving incompatible materials are the leading cause of aborted cycles and damaged instruments.

Strictly Prohibited Materials (Absorbers & Inhibitors)

1. Cellulose and Paper Products: ABSOLUTELY PROHIBITED. Standard paper-plastic pouches, paper towels, linen surgical drapes, cotton wrappers, gauze, and wood-pulp items absorb hydrogen peroxide gas rapidly. This absorption starves the chamber atmosphere of sterilant, preventing sterilization and causing automatic cycle aborts due to low vapor concentration.
2. Liquids and Powders: Hydrogen peroxide gas cannot penetrate liquids or dry powders. Liquids boil under deep vacuum and dilute the sterilant gas.
3. Moisture / Wet Instruments: Instruments must be 100% dry prior to packaging. Residual water drops turn into liquid barriers under vacuum, diluting $H_2O_2$ and causing immediate cycle cancellations.
4. Copper, Brass, & Zinc: Heavy metals decompose hydrogen peroxide prematurely, damaging the metal and aborting the cycle.
5. Dead-End Lumens: Lumens capped at one end prevent gas flow and diffusion.

Compatible Packaging Materials

• Tyvek® Pouches: Polyethylene synthetic spun-bond pouches (plastic on one side, Tyvek on the other) allow gas penetration without absorbing sterilant.
• 100% Polyolefin Synthetic Wraps: Non-woven polypropylene wraps designed for low-temperature processing.
• Aluminum / Synthetic Rigid Containers: Anodized rigid containers fitted with non-cellulose synthetic filters (e.g., polypropylene or PTFE filters).

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Biological Monitoring & Quality Control

Quality assurance for hydrogen peroxide systems relies on chemical and biological indicators engineered specifically for oxidative environments:

• Biological Indicator Organism: Geobacillus stearothermophilus (bacterial endospores). Note that while steam sterilization also uses Geobacillus stearothermophilus, the specific test carrier and population density are formulated specifically for hydrogen peroxide resistance.
• Incubation Temperature: 55°C to 60°C (131°F to 140°F) for traditional cultures, or specialized rapid fluorescent incubators providing readouts in 24 to 60 minutes.
• Monitoring Frequency: Tested at least daily, preferably with every load, and mandatory for any load containing implantable items.
• Chemical Indicators (CIs): Chemical indicator tape and strip monitors feature chemical dyes (typically changing from magenta/red to yellow/clear) that react to hydrogen peroxide vapor concentration.