REGIONAL ANAESTHESIA AND TOURNIQUET PAIN CONTROVERSY
I still see people questioning about tourniquet pain and " how can patient have tourniquet pain when I have given the perfect block"
There are some misconceptions, rather misunderstandings about Tourniquet Pain and today I would like to clear this simple controversy.
1st of all, everyone is right about Tourniquet Pain (TP), so what is the controversy?
Controversy is about the understanding of this Pain. TP is unavoidable, we have to understand the TWO TYPES; TPP and TIP i.e Tourniquet Pressure Pain and Tourniquet Ischaemic Pain. I created this classification to clear this controversy.
TOURNIOUET PRESSURE PAIN (TPP) or EARLY TOURNIQUET PAIN
This pain is from HIGH PRESSURE that is applied to the limb (100mmHg above systolic pressure) to prevent arterial bleed and hence the name ARTERIAL TOURNIQUET. This pain is simple pain that is noticed by everyone when the Blood Pressure Cuff goes up during measurement of Blood Pressure, many patient feel very uncomfortable with this (observe the expression on patient’s face when the NIBP goes up for the 1st time esp in those with High BP). This is what we call TPP and this is NOT SEEN in patients under GA, you need to be awake to feel this pain. This pain originates from the cutaneous tissues and transmitted through Aδ fibres ( these get blocked by Tourniquet Pressure itself later, see under TIP).
So in patients who are going to be awake it is important to block these fibres. For e.g. in forearm surgery with tourniquet under any type of Brachial Plexus block, it is important to block the Intercostobrachial nerve as it originates from T2 and is not blocked with any of the approaches to the brachial plexus.
TOURNIOUET ISCHAEMIC PAIN (TIP)
This is the pain that can not be blocked by even the BEST Nerve/ Plexus Block. TIP can be observed even under GA or in patients undergoing surgery under SAB or Epidural Anaesthesia. Inflation of a tourniquet is followed by the development of a dull aching pain after 30-60 minutes. This manifests as an increase in heart rate (HR)/ Respiratory Rate (RR) and blood pressure ( BP) in patients under general anaesthesia (RR changes are often more evident than HR changes in patients breathing Spontaneously) .
The mechanism as to how such pain arises is thought to be due to selective pain transmission of unmyelinated, slowly conducting C fibres (0.5 – 2.0 m/sec as compared to 12-30 m/sec in A δ fibres). These fibres are continuously stimulated by skin compression, with loss of inhibition at the dorsal horn by larger myelinated Aδ fibres. With prolonged application, the conduction in larger myelinated fibres (Aδ) is inhibited by mechanical compression by the tourniquet (basis for Tourniquet Anaesthesia).
Tourniquet pain, which occurs under apparently adequate neuraxial anaesthesia, is thought to occur as C fibres are more resistant to local anaesthetic conduction block. Also, with time, as the level of the block and concentration of anaesthesia decreases, C fibres regain function faster than A fibres.
REDUCING TOURNIQUET PAIN
Various methods have been tried to decrease the incidence of tourniquet pain, but none are completely satisfactory, except tourniquet deflation.
Techniques used include:
• The addition of Opioids like Morphine, or clonidine, to the neuraxial block. The addition of morphine delays the onset of pain.
• Intravenous (IV) Ketamine 0.1 mg/kg. The N-methyl d-aspartate (NMDA) antagonist decreases central sensitisation of C fibre stimulation.
• Preoperative Gabapentin.
• IV/oral Clonidine. It may cause increased blood pressure drop after tourniquet release.
• Circumferential subcutaneous infiltration of local anaesthetic or application of EMLA Cream
• Wider cuff with lower inflation pressure.
TOURNIQUET AND LIMB ISCHEMIA
Upper limit for the duration of inflation: 30 mins - 4 hours
At 30 min, onset of Anaerobic Metabolism starts
> 1 hour of ischaemia
– Electron Microscopy shows depletion of glycogen granules in the sarcoplasm of muscle fibres
At 2 hours: Lesions associated with Acidosis are seen in the muscles
– Mitochondrial swelling
– Myelin degeneration
– Z-line lysis
Most of these changes are usually reversible with reperfusion. Reperfusion injuries is another topic on its own.
Tourniquet pain is classified into early TPP and late TIP.
Nerve blocks or cutaneous infiltration will prevent TPP but not late Ischemic pain i.e TIP. Additives can help in prolonging the onset of TIP. α Agonist as additives or as infusion are helpful in prolonging the onset of TIP but the use of IV Opioids is disappointing ( effect is transient and only helps in treating the numbers on the monitor)