Hear That Sound? It's Feces Hitting the Fan.

[kandrathe]

In this case, it is not the prick of the needle, but the idea that the paralytic agents in the lethal injection are actually masking ridiculous amounts of pain the lethal agent itself induces.
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But...

... the first drug causes unconciousness.

Sodium Pentothal

Lethal Injection dose: 5 grams

Sodium Pentothal (also known as sodium thiopental) is an ultra-short acting barbiturate, often used for anesthesia induction and for medical induced comas. The typical anesthesia induction dose is 3-5 mg/kg (a person who weighs 200 pounds, or 91 kilograms, would get a dose of about 300 mg). Loss of consciousness is induced within 30-45 seconds at the typical dose, while a five gram dose which is fourteen times the normal dose is likely to induce unconsciousness in 5-10 seconds.

Pentothal reaches the brain within seconds and attains a peak brain concentration of about 60% of the total dose in about 30 seconds. At this level, the patient is unconscious. Within 5 to 20 minutes, the percentage in the brain falls to about 15% of the total dose since the drug redistributes to the rest of the body. At this concentration in the brain, the anesthetic effects wear off and consciousness returns.

The half-life of this drug is about 11.5 hours[2], and the concentration in the brain remains at around about 5-10% of the total dose during that time. When a 'mega-dose' is administered, as in lethal injection, the concentration in the brain during the tail phase of the distribution stays higher than the peak concentration found in the induction dose for anesthesia. This is the reason why an ultra-short acting bartbiturate, such as sodium pentothal, can be used for long-term induction of medical comas.

With a five gram dose, consciousness will probably be regained in about five to six half-lives which occurs in about 57-69 hours. The side effects of such a high dose, however, are respiratory depression (depression of the brainstem respiratory center) and vascular collapse (cardiovascular myodepression), and therefore without medical intervention such as intubation this is in itself a lethal dose.

Given that reality, I don't see how they would feel any pain other that the needle prick. The subsequent two injections insure death.